23 files changed, 823 insertions, 220 deletions

diff --git a/gfx/CMakeLists.txt b/gfx/CMakeLists.txt
index d60a59f..8a9bc8f 100644
--- a/gfx/CMakeLists.txt
+++ b/gfx/CMakeLists.txt
@@ -16,6 +16,8 @@ add_shader_library(shaders
   shaders/cubemap_filtering.vert
   shaders/debug3d.frag
   shaders/debug3d.vert
+  shaders/immediate_mode.frag
+  shaders/immediate_mode.vert
   shaders/irradiance_map.frag
   shaders/prefiltered_environment_map.frag
   shaders/quad.vert
@@ -39,6 +41,7 @@ add_library(gfx
   src/render/shader_program.c
   src/render/shader.c
   src/render/texture.c
+  src/renderer/imm_renderer.c
   src/renderer/renderer.c
   src/scene/animation.c
   src/scene/camera.c
diff --git a/gfx/include/gfx/gfx.h b/gfx/include/gfx/gfx.h
index b9b7183..303ddcd 100644
--- a/gfx/include/gfx/gfx.h
+++ b/gfx/include/gfx/gfx.h
@@ -1,9 +1,10 @@
 #pragma once
 
+typedef struct ImmRenderer   ImmRenderer;
 typedef struct RenderBackend RenderBackend;
-typedef struct Renderer Renderer;
-typedef struct Scene Scene;
-typedef struct SceneCamera SceneCamera;
+typedef struct Renderer      Renderer;
+typedef struct Scene         Scene;
+typedef struct SceneCamera   SceneCamera;
 
 typedef struct Gfx Gfx;
 
@@ -19,6 +20,9 @@ RenderBackend* gfx_get_render_backend(Gfx*);
 /// Get the renderer.
 Renderer* gfx_get_renderer(Gfx*);
 
+/// Get the immediate mode renderer.
+ImmRenderer* gfx_get_imm_renderer(Gfx*);
+
 /// Create a new scene.
 Scene* gfx_make_scene(Gfx*);
 
diff --git a/gfx/include/gfx/render_backend.h b/gfx/include/gfx/render_backend.h
index ef6e4e9..b07cf16 100644
--- a/gfx/include/gfx/render_backend.h
+++ b/gfx/include/gfx/render_backend.h
@@ -6,6 +6,7 @@
 
 #include "sizes.h"
 
+#include <math/aabb3.h>
 #include <math/fwd.h>
 #include <math/mat4.h>
 #include <math/vec4.h>
@@ -47,10 +48,24 @@ typedef enum BufferType {
   Buffer2d,
   Buffer3d,
   Buffer4d,
+  BufferFloat,
   BufferU8,
   BufferU16
 } BufferType;
 
+/// Buffer data descriptor.
+typedef struct BufferDataDesc {
+  union {
+    const void*     data;
+    const vec2*     vec2s;
+    const vec3*     vec3s;
+    const float*    floats;
+    const uint8_t*  u8s;
+    const uint16_t* u16s;
+  };
+  size_t count;
+} BufferDataDesc;
+
 /// Buffer descriptor.
 ///
 /// 'count' is the number of elements in the array. For untyped buffers, this is
@@ -64,27 +79,20 @@ typedef enum BufferType {
 /// Typed buffers don't work well with interleaved vertex attributes. Not sure
 /// this is really worth it.
 typedef struct BufferDesc {
-  BufferUsage usage;
-  BufferType  type;
-  union {
-    const void*     data;
-    const vec2*     vec2s;
-    const vec3*     vec3s;
-    const uint8_t*  u8s;
-    const uint16_t* u16s;
-  };
-  size_t count;
+  BufferUsage    usage;
+  BufferType     type;
+  BufferDataDesc data;
 } BufferDesc;
 
 /// A buffer view for vertex data (attributes or indices).
 /// Either 'data' or 'buffer' must be set.
 #define MAKE_BUFFER_VIEW(NAME, TYPE) \
   typedef struct NAME {              \
-    const TYPE*   data;              \
-    const Buffer* buffer;            \
-    size_t        offset_bytes;      \
-    size_t        size_bytes;        \
-    size_t        stride_bytes;      \
+    const TYPE* data;                \
+    Buffer*     buffer;              \
+    size_t      offset_bytes;        \
+    size_t      size_bytes;          \
+    size_t      stride_bytes;        \
   } NAME;
 
 /// A buffer view for untyped data.
@@ -116,6 +124,14 @@ MAKE_BUFFER_VIEW(BufferViewIdx16, uint16_t)
 
 /// Describes a piece of geometry.
 ///
+/// Buffer views may point to either already-existing GPU buffers or to data in
+/// host memory.
+///
+/// If the buffer views do not already point to GPU buffers, GPU buffers are
+/// created for the geometry. The 'buffer_usage' field specifies the usage for
+/// the created buffers. Use BufferStatic for static geometry and BufferDynamic
+/// for dynamic geometry.
+///
 /// Currently we support only up to 16-bit vertex indices. Might have to change
 /// this to support a larger variety of 3D models.
 typedef struct GeometryDesc {
@@ -138,6 +154,8 @@ typedef struct GeometryDesc {
   VertexCount     num_verts;
   size_t          num_indices;
   PrimitiveType   type;
+  BufferUsage     buffer_usage;
+  aabb3           aabb;
 } GeometryDesc;
 
 /// Shader compiler define.
@@ -298,6 +316,18 @@ void gfx_set_viewport(RenderBackend*, int width, int height);
 /// Get the render backend's viewport dimensions.
 void gfx_get_viewport(RenderBackend*, int* width, int* height);
 
+/// Set blending state.
+void gfx_set_blending(RenderBackend*, bool enable);
+
+/// Set depth mask.
+void gfx_set_depth_mask(RenderBackend*, bool enable);
+
+/// Set cull mode.
+void gfx_set_culling(RenderBackend*, bool enable);
+
+/// Set polygon offset.
+void gfx_set_polygon_offset(RenderBackend*, float scale, float bias);
+
 // -----------------------------------------------------------------------------
 // Buffers.
 // -----------------------------------------------------------------------------
@@ -308,6 +338,9 @@ Buffer* gfx_make_buffer(RenderBackend*, const BufferDesc*);
 /// Destroy the buffer.
 void gfx_destroy_buffer(RenderBackend*, Buffer**);
 
+/// Update the buffer's data.
+void gfx_update_buffer(RenderBackend*, Buffer*, const BufferDataDesc*);
+
 // -----------------------------------------------------------------------------
 // Geometry.
 // -----------------------------------------------------------------------------
@@ -321,20 +354,27 @@ void gfx_destroy_geometry(RenderBackend*, Geometry**);
 /// Upload new vertex data for the geometry.
 ///
 /// This is similar to gfx_make_geometry(), but the geometry need not be
-/// entirely specified. Only the vertex attributes set in the descriptor are
-/// updated.
+/// entirely specified.
+///
+/// Only the vertex attributes, vertex count, and index count set in the
+/// descriptor are updated. Index data, primitive type, and other properties of
+/// the geometry are not updated.
 ///
-/// This function only updates vertex attributes, not indices or primitive type.
+/// New data must be given as arrays in host memory. That is, the buffer views
+/// in the descriptor must point to CPU arrays, not GPU buffers.
 ///
-/// Note that the descriptor cannot specify more vertex attributes than the
-/// geometry was created with. If the size or any other attribute not handled
-/// by this update function needs to be changed, then a new geometry must be
-/// created.
+/// Note that the descriptor cannot specify a larger vertex or index count than
+/// what the geometry was created with. If the geometry size or any other
+/// attribute not handled by this update function needs to be changed, then a
+/// new geometry must be created.
 void gfx_update_geometry(Geometry*, const GeometryDesc*);
 
 /// Render the geometry.
 void gfx_render_geometry(const Geometry*);
 
+/// Return the geometry's bounding box.
+aabb3 gfx_get_geometry_aabb(const Geometry*);
+
 // -----------------------------------------------------------------------------
 // Textures.
 // -----------------------------------------------------------------------------
diff --git a/gfx/include/gfx/renderer.h b/gfx/include/gfx/renderer.h
index d82fab6..1517af6 100644
--- a/gfx/include/gfx/renderer.h
+++ b/gfx/include/gfx/renderer.h
@@ -1,11 +1,54 @@
 #pragma once
 
+#include <math/aabb3.h>
+#include <math/camera.h>
+#include <math/mat4.h>
+#include <math/vec3.h>
+#include <math/vec4.h>
+
 typedef struct RenderBackend RenderBackend;
-typedef struct Scene Scene;
-typedef struct SceneCamera SceneCamera;
+typedef struct Scene         Scene;
+typedef struct SceneCamera   SceneCamera;
+
+typedef struct ImmRenderer ImmRenderer;
+typedef struct Renderer    Renderer;
 
-typedef struct Renderer Renderer;
+// -----------------------------------------------------------------------------
+// Main Renderer.
+// -----------------------------------------------------------------------------
 
 /// Render the scene.
-void gfx_render_scene(Renderer*, RenderBackend*, const Scene*,
-                      const SceneCamera*);
+void gfx_render_scene(Renderer*, const Scene*, const SceneCamera*);
+
+// -----------------------------------------------------------------------------
+// Immediate Mode Renderer.
+// -----------------------------------------------------------------------------
+
+/// Prepare the graphics systems for immediate-mode rendering.
+///
+/// Call this before issuing any immediate-mode rendering draws.
+void gfx_imm_start(ImmRenderer*);
+
+/// End immediate mode rendering.
+///
+/// Call this after issuing immediate-mode rendering draws and before swapping
+/// buffers.
+void gfx_imm_end(ImmRenderer*);
+
+/// Draw a set of triangles.
+void gfx_imm_draw_triangles(ImmRenderer*, const vec3[], size_t num_triangles);
+
+/// Draw a triangle.
+void gfx_imm_draw_triangle(ImmRenderer*, const vec3[3]);
+
+/// Draw a bounding box.
+void gfx_imm_draw_aabb(ImmRenderer*, aabb3);
+
+/// Set the camera.
+void gfx_imm_set_camera(ImmRenderer*, const Camera*);
+
+/// Set the model matrix.
+void gfx_imm_set_model_matrix(ImmRenderer*, const mat4*);
+
+/// Set the render colour.
+void gfx_imm_set_colour(ImmRenderer*, vec4 colour);
diff --git a/gfx/include/gfx/scene/object.h b/gfx/include/gfx/scene/object.h
index 59372c5..ccc9999 100644
--- a/gfx/include/gfx/scene/object.h
+++ b/gfx/include/gfx/scene/object.h
@@ -2,6 +2,8 @@
 
 #include <math/fwd.h>
 
+#include <math/aabb3.h>
+
 typedef struct Mesh      Mesh;
 typedef struct SceneNode SceneNode;
 typedef struct Skeleton  Skeleton;
@@ -28,3 +30,8 @@ void gfx_remove_object_mesh(SceneObject*, Mesh*);
 
 /// Set the object's skeleton.
 void gfx_set_object_skeleton(SceneObject*, const Skeleton*);
+
+/// Computes the object's bounding box.
+///
+/// The object's bounding box is the bounding box of its mesh geometries.
+aabb3 gfx_calc_object_aabb(const SceneObject*);
diff --git a/gfx/include/gfx/sizes.h b/gfx/include/gfx/sizes.h
index 17e7c7d..9f13e30 100644
--- a/gfx/include/gfx/sizes.h
+++ b/gfx/include/gfx/sizes.h
@@ -74,6 +74,12 @@
 /// Maximum number of compiler defines in a Shader.
 #define GFX_MAX_SHADER_COMPILER_DEFINES 16
 
-// Gfx
+// Renderer.
+
+/// Maximum number of triangles that the immediate-mode renderer can draw in a
+/// frame.
+#define IMM_MAX_NUM_TRIANGLES 1024
+
+// Gfx.
 
 #define GFX_MAX_NUM_SCENES 4
diff --git a/gfx/include/gfx/util/shader.h b/gfx/include/gfx/util/shader.h
index cadca55..9bde8cf 100644
--- a/gfx/include/gfx/util/shader.h
+++ b/gfx/include/gfx/util/shader.h
@@ -21,6 +21,9 @@ ShaderProgram* gfx_make_cook_torrance_shader_perm(
 /// Create a 3D debugging shader.
 ShaderProgram* gfx_make_debug3d_shader(RenderBackend*);
 
+/// Create a shader for drawing in immediate mode.
+ShaderProgram* gfx_make_immediate_mode_shader(RenderBackend*);
+
 /// Create a shader for computing irradiance maps from cube maps.
 ShaderProgram* gfx_make_irradiance_map_shader(RenderBackend*);
 
diff --git a/gfx/shaders/immediate_mode.frag b/gfx/shaders/immediate_mode.frag
new file mode 100644
index 0000000..ac23b5c
--- /dev/null
+++ b/gfx/shaders/immediate_mode.frag
@@ -0,0 +1,10 @@
+precision highp float;
+
+uniform vec4 Colour;
+
+out vec4 FragColour;
+
+void main()
+{
+  FragColour = vec4(pow(Colour.rgb, vec3(1.0/2.2)), Colour.a);
+}
diff --git a/gfx/shaders/immediate_mode.vert b/gfx/shaders/immediate_mode.vert
new file mode 100644
index 0000000..65070bb
--- /dev/null
+++ b/gfx/shaders/immediate_mode.vert
@@ -0,0 +1,11 @@
+precision highp float;
+
+uniform mat4 Model;
+uniform mat4 ViewProjection;
+
+layout (location = 0) in vec3 vPosition;
+
+void main()
+{
+  gl_Position = ViewProjection * Model * vec4(vPosition, 1.0);
+}
diff --git a/gfx/src/gfx.c b/gfx/src/gfx.c
index 4640a52..27312b2 100644
--- a/gfx/src/gfx.c
+++ b/gfx/src/gfx.c
@@ -13,9 +13,10 @@
 #include <stdlib.h>
 
 typedef struct Gfx {
+  scene_idx     scene; // First child scene.
   RenderBackend render_backend;
   Renderer      renderer;
-  scene_idx     scene; // First child scene.
+  ImmRenderer   imm_renderer;
 } Gfx;
 
 Gfx* gfx_init() {
@@ -33,6 +34,12 @@ Gfx* gfx_init() {
     gfx_destroy(&gfx);
     return 0;
   }
+  if (!imm_renderer_make(&gfx->imm_renderer, &gfx->render_backend)) {
+    // TODO: Add error logs to the initialization failure cases here and inside
+    // the renderers.
+    gfx_destroy(&gfx);
+    return 0;
+  }
   scene_mem_init();
   return gfx;
 }
@@ -43,7 +50,8 @@ void gfx_destroy(Gfx** gfx) {
     return;
   }
   scene_mem_destroy();
-  renderer_destroy(&(*gfx)->renderer, &(*gfx)->render_backend);
+  renderer_destroy(&(*gfx)->renderer);
+  imm_renderer_destroy(&(*gfx)->imm_renderer);
   gfx_del_render_backend(&(*gfx)->render_backend);
   free(*gfx);
   *gfx = 0;
@@ -59,6 +67,11 @@ Renderer* gfx_get_renderer(Gfx* gfx) {
   return &gfx->renderer;
 }
 
+ImmRenderer* gfx_get_imm_renderer(Gfx* gfx) {
+  assert(gfx);
+  return &gfx->imm_renderer;
+}
+
 Scene* gfx_make_scene(Gfx* gfx) {
   Scene* scene = mem_alloc_scene();
   if (!scene) {
diff --git a/gfx/src/render/buffer.c b/gfx/src/render/buffer.c
index 392777c..3c0c794 100644
--- a/gfx/src/render/buffer.c
+++ b/gfx/src/render/buffer.c
@@ -6,8 +6,9 @@
 #include <math/vec3.h>
 #include <math/vec4.h>
 
-static size_t get_buffer_size_bytes(const BufferDesc* desc) {
-  switch (desc->type) {
+static size_t get_buffer_size_bytes(
+    BufferType type, const BufferDataDesc* desc) {
+  switch (type) {
   case BufferUntyped:
     return desc->count;
   case Buffer2d:
@@ -16,11 +17,15 @@ static size_t get_buffer_size_bytes(const BufferDesc* desc) {
     return desc->count * sizeof(vec3);
   case Buffer4d:
     return desc->count * sizeof(vec4);
+  case BufferFloat:
+    return desc->count * sizeof(float);
   case BufferU8:
     return desc->count * sizeof(uint8_t);
   case BufferU16:
     return desc->count * sizeof(uint16_t);
   }
+  assert(false);
+  return 0;
 }
 
 static GLenum get_buffer_usage(BufferUsage usage) {
@@ -36,11 +41,13 @@ static GLenum get_buffer_usage(BufferUsage usage) {
 
 bool gfx_init_buffer(Buffer* buffer, const BufferDesc* desc) {
   assert(buffer);
-  buffer->size_bytes = get_buffer_size_bytes(desc);
+  buffer->type       = desc->type;
+  buffer->usage      = desc->usage;
+  buffer->size_bytes = get_buffer_size_bytes(desc->type, &desc->data);
   const GLenum usage = get_buffer_usage(desc->usage);
   glGenBuffers(1, &buffer->vbo);
   glBindBuffer(GL_ARRAY_BUFFER, buffer->vbo);
-  glBufferData(GL_ARRAY_BUFFER, buffer->size_bytes, desc->data, usage);
+  glBufferData(GL_ARRAY_BUFFER, buffer->size_bytes, desc->data.data, usage);
   glBindBuffer(GL_ARRAY_BUFFER, 0);
   ASSERT_GL;
   return true;
@@ -53,3 +60,20 @@ void gfx_del_buffer(Buffer* buffer) {
     buffer->vbo = 0;
   }
 }
+
+void gfx_update_buffer(
+    RenderBackend* render_backend, Buffer* buffer, const BufferDataDesc* desc) {
+  assert(render_backend);
+  assert(buffer);
+  assert(desc);
+  // OpenGL allows updating static buffers, but it is not optimal for
+  // performance, so we enforce data in static buffers remain static.
+  assert(buffer->usage == BufferDynamic);
+
+  const size_t update_size_bytes = get_buffer_size_bytes(buffer->type, desc);
+  assert(update_size_bytes <= buffer->size_bytes);
+
+  glBindBuffer(GL_ARRAY_BUFFER, buffer->vbo);
+  glBufferSubData(GL_ARRAY_BUFFER, 0, update_size_bytes, desc->data);
+  glBindBuffer(GL_ARRAY_BUFFER, 0);
+}
diff --git a/gfx/src/render/buffer.h b/gfx/src/render/buffer.h
index 575fbb9..0c81e7b 100644
--- a/gfx/src/render/buffer.h
+++ b/gfx/src/render/buffer.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <gfx/render_backend.h>
+
 #include "gl_util.h"
 
 #include <math/fwd.h>
@@ -7,11 +9,11 @@
 #include <stdbool.h>
 #include <stddef.h>
 
-typedef struct BufferDesc BufferDesc;
-
 typedef struct Buffer {
-  GLuint vbo;
-  size_t size_bytes;
+  GLuint      vbo;
+  BufferType  type;
+  BufferUsage usage;
+  size_t      size_bytes;
 } Buffer;
 
 /// Create a buffer from raw data.
diff --git a/gfx/src/render/geometry.c b/gfx/src/render/geometry.c
index 076a956..e9d3ae5 100644
--- a/gfx/src/render/geometry.c
+++ b/gfx/src/render/geometry.c
@@ -6,7 +6,11 @@
 #include <math/vec2.h>
 #include <math/vec3.h>
 
-#define view_is_populated(BUFFER_VIEW) (BUFFER_VIEW.data || BUFFER_VIEW.buffer)
+/// Determines whether a view is populated.
+///
+/// Note that views are allowed to have no data, in which case a buffer of the
+/// specified size is created.
+#define view_is_populated(BUFFER_VIEW) (BUFFER_VIEW.size_bytes > 0)
 
 static GLenum primitive_type_to_gl(PrimitiveType type) {
   switch (type) {
@@ -22,23 +26,18 @@ static GLenum primitive_type_to_gl(PrimitiveType type) {
   }
 }
 
-/// Create a buffer for the view.
-///
-/// If the view already has a buffer, return the buffer. Otherwise create a
-/// buffer from the view's data and assign it to the view.
-static const Buffer* get_or_make_view_buffer(
-    RenderBackend* render_backend, BufferView* view) {
-  if (view->buffer) {
-    return view->buffer;
-  } else {
-    return view->buffer = gfx_make_buffer(
-               render_backend, &(BufferDesc){
-                                   .usage = BufferStatic,
-                                   .type  = BufferUntyped,
-                                   .data  = view->data,
-                                   .count = view->size_bytes});
-  }
-}
+/// Create a buffer for the buffer view if the view does not already point to
+/// a buffer.
+#define INIT_VIEW_BUFFER(render_backend, view, buffer_type, buffer_usage) \
+  if (!view.buffer) {                                                     \
+    view.buffer = (__typeof__(view.buffer))gfx_make_buffer(               \
+        render_backend, &(BufferDesc){                                    \
+                            .usage      = buffer_usage,                   \
+                            .type       = buffer_type,                    \
+                            .data.data  = view.data,                      \
+                            .data.count = view.size_bytes});              \
+  }                                                                       \
+  assert(view.size_bytes <= view.buffer->size_bytes);
 
 /// Create a buffer for the view, then configure it in the VAO.
 static bool configure_buffer(
@@ -48,15 +47,13 @@ static bool configure_buffer(
   assert(render_backend);
   assert(desc);
   assert(view);
+  assert(view->buffer);
   assert(
       desc->num_verts <=
       view->size_bytes / (num_components * component_size_bytes));
-  const Buffer* buffer = get_or_make_view_buffer(render_backend, view);
-  if (!buffer) {
-    return false;
-  }
-  assert(view->size_bytes <= buffer->size_bytes);
-  glBindBuffer(GL_ARRAY_BUFFER, buffer->vbo);
+  assert(view->size_bytes <= view->buffer->size_bytes);
+
+  glBindBuffer(GL_ARRAY_BUFFER, view->buffer->vbo);
   glEnableVertexAttribArray(channel);
   if ((component_type == GL_FLOAT) || normalized) {
     glVertexAttribPointer(
@@ -72,6 +69,8 @@ static bool configure_buffer(
         channel, num_components, component_type, view->stride_bytes,
         (const void*)view->offset_bytes);
   }
+  glBindBuffer(GL_ARRAY_BUFFER, 0);
+
   return true;
 }
 
@@ -80,74 +79,138 @@ static bool configure_vertex_attributes(
   assert(render_backend);
   assert(desc);
 
-  bool result = true;
-
   if (view_is_populated(desc->positions3d)) {
-    result =
-        result && configure_buffer(
-                      render_backend, desc, (BufferView*)&desc->positions3d, 3,
-                      sizeof(float), GL_FLOAT, GL_FALSE, GFX_POSITION_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->positions3d, Buffer3d, desc->buffer_usage);
+    if (!desc->positions3d.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->positions3d, 3,
+            sizeof(float), GL_FLOAT, GL_FALSE, GFX_POSITION_CHANNEL)) {
+      return false;
+    }
   } else if (view_is_populated(desc->positions2d)) {
-    result =
-        result && configure_buffer(
-                      render_backend, desc, (BufferView*)&desc->positions2d, 2,
-                      sizeof(float), GL_FLOAT, GL_FALSE, GFX_POSITION_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->positions2d, Buffer2d, desc->buffer_usage);
+    if (!desc->positions2d.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->positions2d, 2,
+            sizeof(float), GL_FLOAT, GL_FALSE, GFX_POSITION_CHANNEL)) {
+      return false;
+    }
   }
-
   if (view_is_populated(desc->normals)) {
-    result =
-        result && configure_buffer(
-                      render_backend, desc, (BufferView*)&desc->normals, 3,
-                      sizeof(float), GL_FLOAT, GL_FALSE, GFX_NORMAL_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->normals, Buffer3d, desc->buffer_usage);
+    if (!desc->normals.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->normals, 3, sizeof(float),
+            GL_FLOAT, GL_FALSE, GFX_NORMAL_CHANNEL)) {
+      return false;
+    }
   }
-
   if (view_is_populated(desc->tangents)) {
-    result =
-        result && configure_buffer(
-                      render_backend, desc, (BufferView*)&desc->tangents, 4,
-                      sizeof(float), GL_FLOAT, GL_FALSE, GFX_TANGENT_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->tangents, Buffer4d, desc->buffer_usage);
+    if (!desc->tangents.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->tangents, 4,
+            sizeof(float), GL_FLOAT, GL_FALSE, GFX_TANGENT_CHANNEL)) {
+      return false;
+    }
   }
-
   if (view_is_populated(desc->texcoords)) {
-    result =
-        result && configure_buffer(
-                      render_backend, desc, (BufferView*)&desc->texcoords, 2,
-                      sizeof(float), GL_FLOAT, GL_FALSE, GFX_TEXCOORDS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->texcoords, Buffer2d, desc->buffer_usage);
+    if (!desc->texcoords.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->texcoords, 2,
+            sizeof(float), GL_FLOAT, GL_FALSE, GFX_TEXCOORDS_CHANNEL)) {
+      return false;
+    }
   }
-
   if (view_is_populated(desc->joints.u8)) {
-    result = result && configure_buffer(
-                           render_backend, desc, (BufferView*)&desc->joints.u8,
-                           4, sizeof(uint8_t), GL_UNSIGNED_BYTE, GL_FALSE,
-                           GFX_JOINTS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->joints.u8, BufferU8, desc->buffer_usage);
+    if (!desc->joints.u8.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->joints.u8, 4,
+            sizeof(uint8_t), GL_UNSIGNED_BYTE, GL_FALSE, GFX_JOINTS_CHANNEL)) {
+      return false;
+    }
   } else if (view_is_populated(desc->joints.u16)) {
-    result = result && configure_buffer(
-                           render_backend, desc, (BufferView*)&desc->joints.u16,
-                           4, sizeof(uint16_t), GL_UNSIGNED_SHORT, GL_FALSE,
-                           GFX_JOINTS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->joints.u16, BufferU16, desc->buffer_usage);
+    if (!desc->joints.u16.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->joints.u16, 4,
+            sizeof(uint16_t), GL_UNSIGNED_SHORT, GL_FALSE,
+            GFX_JOINTS_CHANNEL)) {
+      return false;
+    }
   }
 
   // If weights are given as unsigned integers, then they are normalized when
   // read by the shader.
   if (view_is_populated(desc->weights.u8)) {
-    result = result && configure_buffer(
-                           render_backend, desc, (BufferView*)&desc->weights.u8,
-                           4, sizeof(uint8_t), GL_UNSIGNED_BYTE, GL_TRUE,
-                           GFX_WEIGHTS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->weights.u8, BufferU8, desc->buffer_usage);
+    if (!desc->weights.u8.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->weights.u8, 4,
+            sizeof(uint8_t), GL_UNSIGNED_BYTE, GL_TRUE, GFX_WEIGHTS_CHANNEL)) {
+      return false;
+    }
   } else if (view_is_populated(desc->weights.u16)) {
-    result = result && configure_buffer(
-                           render_backend, desc,
-                           (BufferView*)&desc->weights.u16, 4, sizeof(uint16_t),
-                           GL_UNSIGNED_SHORT, GL_TRUE, GFX_WEIGHTS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->weights.u16, BufferU16, desc->buffer_usage);
+    if (!desc->weights.u16.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->weights.u16, 4,
+            sizeof(uint16_t), GL_UNSIGNED_SHORT, GL_TRUE,
+            GFX_WEIGHTS_CHANNEL)) {
+      return false;
+    }
   } else if (view_is_populated(desc->weights.floats)) {
-    result = result &&
-             configure_buffer(
-                 render_backend, desc, (BufferView*)&desc->weights.floats, 4,
-                 sizeof(float), GL_FLOAT, GL_FALSE, GFX_WEIGHTS_CHANNEL);
+    INIT_VIEW_BUFFER(
+        render_backend, desc->weights.floats, BufferFloat, desc->buffer_usage);
+    if (!desc->weights.floats.buffer ||
+        !configure_buffer(
+            render_backend, desc, (BufferView*)&desc->weights.floats, 4,
+            sizeof(float), GL_FLOAT, GL_FALSE, GFX_WEIGHTS_CHANNEL)) {
+      return false;
+    }
   }
 
-  glBindBuffer(GL_ARRAY_BUFFER, 0);
-  return result;
+  return true;
+}
+
+static bool configure_indices(
+    RenderBackend* render_backend, GeometryDesc* desc) {
+  assert(render_backend);
+  assert(desc);
+
+  if (view_is_populated(desc->indices8)) {
+    assert(desc->num_indices > 0);
+    assert(
+        desc->num_indices <= desc->indices8.size_bytes / sizeof(VertexIndex8));
+    INIT_VIEW_BUFFER(
+        render_backend, desc->indices8, BufferU8, desc->buffer_usage);
+    if (!desc->indices8.buffer) {
+      return false;
+    }
+  } else if (view_is_populated(desc->indices16)) {
+    assert(desc->num_indices > 0);
+    assert(
+        desc->num_indices <=
+        desc->indices16.size_bytes / sizeof(VertexIndex16));
+    INIT_VIEW_BUFFER(
+        render_backend, desc->indices16, BufferU16, desc->buffer_usage);
+    if (!desc->indices16.buffer) {
+      return false;
+    }
+  }
+
+  return true;
 }
 
 bool gfx_init_geometry(
@@ -163,47 +226,29 @@ bool gfx_init_geometry(
 
   geometry->mode           = primitive_type_to_gl(input_desc->type);
   geometry->desc           = *input_desc;
+  geometry->num_verts      = input_desc->num_verts;
   geometry->render_backend = render_backend;
 
-  // We manipulate the descriptor copy below. Create a shorter name for it.
+  // The geometry's copy of the descriptor is manipulated below. Create a
+  // shorter name for it.
   GeometryDesc* desc = &geometry->desc;
 
   glGenVertexArrays(1, &geometry->vao);
   glBindVertexArray(geometry->vao);
-
   if (!configure_vertex_attributes(render_backend, desc)) {
-    gfx_del_geometry(geometry);
-    return false;
+    goto cleanup;
   }
-
-  if (view_is_populated(desc->indices8)) {
-    assert(desc->num_indices > 0);
-    assert(
-        desc->num_indices <= desc->indices8.size_bytes / sizeof(VertexIndex8));
-    const Buffer* buffer =
-        get_or_make_view_buffer(render_backend, (BufferView*)&desc->indices8);
-    if (!buffer) {
-      gfx_del_geometry(geometry);
-      return false;
-    }
-    assert(desc->indices8.size_bytes <= buffer->size_bytes);
-  } else if (view_is_populated(desc->indices16)) {
-    assert(desc->num_indices > 0);
-    assert(
-        desc->num_indices <=
-        desc->indices16.size_bytes / sizeof(VertexIndex16));
-    const Buffer* buffer =
-        get_or_make_view_buffer(render_backend, (BufferView*)&desc->indices16);
-    if (!buffer) {
-      gfx_del_geometry(geometry);
-      return false;
-    }
-    assert(desc->indices16.size_bytes <= buffer->size_bytes);
+  if (!configure_indices(render_backend, desc)) {
+    goto cleanup;
   }
-
   glBindVertexArray(0);
   ASSERT_GL;
-  return geometry;
+
+  return true;
+
+cleanup:
+  gfx_del_geometry(geometry);
+  return 0;
 }
 
 void gfx_del_geometry(Geometry* geometry) {
@@ -217,6 +262,7 @@ void gfx_del_geometry(Geometry* geometry) {
 void gfx_update_geometry(Geometry* geometry, const GeometryDesc* desc) {
   assert(geometry);
   assert(desc);
+  // New geometry size cannot exceed original size.
   assert(desc->positions3d.size_bytes <= geometry->desc.positions3d.size_bytes);
   assert(desc->positions2d.size_bytes <= geometry->desc.positions2d.size_bytes);
   assert(desc->normals.size_bytes <= geometry->desc.normals.size_bytes);
@@ -230,22 +276,32 @@ void gfx_update_geometry(Geometry* geometry, const GeometryDesc* desc) {
       desc->weights.floats.size_bytes <=
       geometry->desc.weights.floats.size_bytes);
 
-  geometry->desc = *desc;
+  if (desc->positions3d.data) {
+    // The geometry must already have an underlying GPU buffer.
+    assert(geometry->desc.positions3d.buffer);
+    gfx_update_buffer(
+        geometry->render_backend, geometry->desc.positions3d.buffer,
+        &(BufferDataDesc){
+            .vec3s = desc->positions3d.data,
+            .count = desc->positions3d.size_bytes / sizeof(vec3)});
+  }
+  // TODO: more
+  else {
+    assert(false && "TODO: gfx_update_geometry() - handle other buffer types");
+  }
 
-  glBindVertexArray(geometry->vao);
-  bool result =
-      configure_vertex_attributes(geometry->render_backend, &geometry->desc);
-  // Shouldn't fail since we're just uploading buffer data, not creating new
-  // buffers.
-  assert(result);
-  glBindVertexArray(0);
+  if (desc->num_verts != 0) {
+    geometry->num_verts = desc->num_verts;
+  }
 }
 
 void gfx_render_geometry(const Geometry* geometry) {
   assert(geometry);
   assert(geometry->vao);
+
   const GeometryDesc* desc = &geometry->desc;
   glBindVertexArray(geometry->vao);
+
   if (desc->indices8.buffer) {
     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, desc->indices8.buffer->vbo);
     glDrawElements(
@@ -257,7 +313,13 @@ void gfx_render_geometry(const Geometry* geometry) {
         geometry->mode, desc->num_indices, GL_UNSIGNED_SHORT,
         (const void*)desc->indices16.offset_bytes);
   } else {
-    glDrawArrays(geometry->mode, 0, desc->num_verts);
+    glDrawArrays(geometry->mode, 0, geometry->num_verts);
   }
+
   glBindVertexArray(0);
 }
+
+aabb3 gfx_get_geometry_aabb(const Geometry* geometry) {
+  assert(geometry);
+  return geometry->desc.aabb;
+}
diff --git a/gfx/src/render/geometry.h b/gfx/src/render/geometry.h
index 8fb36da..484934d 100644
--- a/gfx/src/render/geometry.h
+++ b/gfx/src/render/geometry.h
@@ -13,9 +13,11 @@
 /// the renderer assumes ownership of all rendering resources, which simplifies
 /// their management.
 typedef struct Geometry {
-  GLuint         vao;
-  GLenum         mode;
-  GeometryDesc   desc;
+  GLuint       vao;
+  GLenum       mode;
+  GeometryDesc desc;
+  size_t num_verts; // May differ from the initial value in the descriptor if
+                    // the geometry is updated.
   RenderBackend* render_backend;
 } Geometry;
 
diff --git a/gfx/src/render/render_backend.c b/gfx/src/render/render_backend.c
index 8feefab..defc164 100644
--- a/gfx/src/render/render_backend.c
+++ b/gfx/src/render/render_backend.c
@@ -96,6 +96,41 @@ void gfx_get_viewport(RenderBackend* render_backend, int* width, int* height) {
   *height = render_backend->viewport.height;
 }
 
+void gfx_set_blending(RenderBackend* render_backend, bool enable) {
+  assert(render_backend);
+  if (enable) {
+    glEnable(GL_BLEND);
+    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+  } else {
+    glDisable(GL_BLEND);
+  }
+}
+
+void gfx_set_depth_mask(RenderBackend* render_backend, bool enable) {
+  assert(render_backend);
+  glDepthMask(enable ? GL_TRUE : GL_FALSE);
+}
+
+void gfx_set_culling(RenderBackend* render_backend, bool enable) {
+  assert(render_backend);
+  if (enable) {
+    glEnable(GL_CULL_FACE);
+  } else {
+    glDisable(GL_CULL_FACE);
+  }
+}
+
+void gfx_set_polygon_offset(
+    RenderBackend* render_backend, float scale, float bias) {
+  assert(render_backend);
+  if ((scale != 0.0f) || (bias != 0.0f)) {
+    glEnable(GL_POLYGON_OFFSET_FILL);
+  } else {
+    glDisable(GL_POLYGON_OFFSET_FILL);
+  }
+  glPolygonOffset(scale, bias);
+}
+
 // -----------------------------------------------------------------------------
 // Buffers.
 // -----------------------------------------------------------------------------
diff --git a/gfx/src/renderer/imm_renderer.c b/gfx/src/renderer/imm_renderer.c
new file mode 100644
index 0000000..e9f98f8
--- /dev/null
+++ b/gfx/src/renderer/imm_renderer.c
@@ -0,0 +1,181 @@
+#include "renderer_impl.h"
+
+#include <gfx/render_backend.h>
+#include <gfx/util/shader.h>
+
+#include <math/aabb3.h>
+
+#include <assert.h>
+#include <string.h> // memcpy
+
+#include <log/log.h> // TODO: remove
+
+bool imm_renderer_make(ImmRenderer* renderer, RenderBackend* render_backend) {
+  assert(renderer);
+  assert(render_backend);
+
+  const size_t num_triangle_verts = IMM_MAX_NUM_TRIANGLES * 3;
+
+  renderer->render_backend = render_backend;
+
+  renderer->triangles = gfx_make_geometry(
+      render_backend,
+      &(GeometryDesc){
+          .type         = Triangles,
+          .buffer_usage = BufferDynamic,
+          .num_verts    = num_triangle_verts,
+          .positions3d =
+              (BufferView3d){.size_bytes = num_triangle_verts * sizeof(vec3)}});
+  if (!renderer->triangles) {
+    goto cleanup;
+  }
+
+  renderer->shader = gfx_make_immediate_mode_shader(render_backend);
+  if (!renderer->shader) {
+    goto cleanup;
+  }
+
+  gfx_imm_set_colour(renderer, vec4_make(0.0, 0.0, 0.0, 1.0));
+
+  return true;
+
+cleanup:
+  imm_renderer_destroy(renderer);
+  return false;
+}
+
+void imm_renderer_destroy(ImmRenderer* renderer) {
+  assert(renderer);
+  assert(renderer->render_backend);
+
+  if (renderer->triangles) {
+    gfx_destroy_geometry(renderer->render_backend, &renderer->triangles);
+    // TODO: Could also destroy the geometry's buffers here.
+  }
+
+  if (renderer->shader) {
+    gfx_destroy_shader_program(renderer->render_backend, &renderer->shader);
+  }
+}
+
+void imm_renderer_flush(ImmRenderer* renderer) {
+  assert(renderer);
+
+  if (renderer->num_triangle_verts > 0) {
+    gfx_update_geometry(
+        renderer->triangles,
+        &(GeometryDesc){
+            .num_verts   = renderer->num_triangle_verts,
+            .positions3d = (BufferView3d){
+                                          .data       = renderer->triangle_verts,
+                                          .size_bytes = renderer->num_triangle_verts * sizeof(vec3)}
+    });
+
+    gfx_apply_uniforms(renderer->shader);
+    gfx_render_geometry(renderer->triangles);
+
+    renderer->num_triangle_verts = 0;
+  }
+}
+
+void gfx_imm_start(ImmRenderer* renderer) {
+  assert(renderer);
+
+  ShaderProgram* shader = renderer->shader;
+  gfx_activate_shader_program(shader);
+  // TODO: Having to apply manually is annoying. Maybe just apply implicitly
+  // when the program is activated?
+  // Note that then we'll need to be able to just gfx_apply_uniforms() in
+  // flush().
+  // gfx_set_culling(renderer->render_backend, false);
+  gfx_apply_uniforms(shader);
+  gfx_set_blending(renderer->render_backend, true);
+  gfx_set_depth_mask(renderer->render_backend, false);
+  gfx_set_polygon_offset(renderer->render_backend, 0.5f, 0.5f);
+}
+
+void gfx_imm_end(ImmRenderer* renderer) {
+  assert(renderer);
+
+  imm_renderer_flush(renderer);
+
+  gfx_set_polygon_offset(renderer->render_backend, 0.0f, 0.0f);
+  gfx_set_depth_mask(renderer->render_backend, true);
+  gfx_set_blending(renderer->render_backend, false);
+  // gfx_set_culling(renderer->render_backend, true);
+  gfx_deactivate_shader_program(renderer->shader);
+}
+
+void gfx_imm_draw_triangles(
+    ImmRenderer* renderer, const vec3 verts[], size_t num_triangles) {
+  assert(renderer);
+  assert(verts);
+  const size_t new_verts = num_triangles * 3;
+  assert(
+      renderer->num_triangle_verts + new_verts < (IMM_MAX_NUM_TRIANGLES * 3));
+
+  memcpy(
+      renderer->triangle_verts + renderer->num_triangle_verts, verts,
+      new_verts * sizeof(vec3));
+
+  renderer->num_triangle_verts += new_verts;
+}
+
+void gfx_imm_draw_triangle(ImmRenderer* renderer, const vec3 verts[3]) {
+  gfx_imm_draw_triangles(renderer, verts, 1);
+}
+
+void gfx_imm_draw_aabb(ImmRenderer* renderer, aabb3 box) {
+  assert(renderer);
+
+  const vec3 verts[8] = {
+      box.min,                                    //     2 ----- 6
+      vec3_make(box.min.x, box.min.y, box.max.z), //    /       /|
+      vec3_make(box.min.x, box.max.y, box.min.z), //   3 ----- 7 |
+      vec3_make(box.min.x, box.max.y, box.max.z), //   |       | |
+      vec3_make(box.max.x, box.min.y, box.min.z), //   | 0 ----- 4
+      vec3_make(box.max.x, box.min.y, box.max.z), //   |/      |/
+      vec3_make(box.max.x, box.max.y, box.min.z), //   1 ----- 5
+      box.max};
+
+#define tri(i0, i1, i2) verts[i0], verts[i1], verts[i2]
+
+  // TODO: Use vertex indices in Geometry.
+  const vec3 tris[36] = {// Front.
+                         tri(1, 5, 7), tri(1, 7, 3),
+                         // Right.
+                         tri(5, 4, 6), tri(5, 6, 7),
+                         // Back.
+                         tri(4, 0, 2), tri(4, 2, 6),
+                         // Left.
+                         tri(0, 1, 3), tri(0, 3, 2),
+                         // Top.
+                         tri(3, 7, 6), tri(3, 6, 2),
+                         // Bottom.
+                         tri(0, 4, 5), tri(0, 5, 1)};
+
+  gfx_imm_draw_triangles(renderer, tris, 12);
+}
+
+void gfx_imm_set_camera(ImmRenderer* renderer, const Camera* camera) {
+  assert(renderer);
+  assert(renderer->shader);
+  imm_renderer_flush(renderer);
+  const mat4 modelview = spatial3_inverse_transform(&camera->spatial);
+  const mat4 mvp       = mat4_mul(camera->projection, modelview);
+  gfx_set_mat4_uniform(renderer->shader, "ViewProjection", &mvp);
+}
+
+void gfx_imm_set_model_matrix(ImmRenderer* renderer, const mat4* model) {
+  assert(renderer);
+  assert(model);
+  imm_renderer_flush(renderer);
+  gfx_set_mat4_uniform(renderer->shader, "Model", model);
+}
+
+void gfx_imm_set_colour(ImmRenderer* renderer, vec4 colour) {
+  assert(renderer);
+  assert(renderer->shader);
+  imm_renderer_flush(renderer);
+  gfx_set_vec4_uniform(renderer->shader, "Colour", colour);
+}
diff --git a/gfx/src/renderer/renderer.c b/gfx/src/renderer/renderer.c
index b0bef33..5d88ae6 100644
--- a/gfx/src/renderer/renderer.c
+++ b/gfx/src/renderer/renderer.c
@@ -18,6 +18,7 @@
 
 #include <assert.h>
 
+// TODO: Move to a header like "constants.h".
 static const int IRRADIANCE_MAP_WIDTH               = 1024;
 static const int IRRADIANCE_MAP_HEIGHT              = 1024;
 static const int PREFILTERED_ENVIRONMENT_MAP_WIDTH  = 128;
@@ -29,29 +30,31 @@ bool renderer_make(Renderer* renderer, RenderBackend* render_backend) {
   assert(renderer);
   assert(render_backend);
 
+  renderer->render_backend = render_backend;
+
   // TODO: Load the IBL stuff lazily.
   if (!(renderer->ibl = gfx_make_ibl(render_backend))) {
-    renderer_destroy(renderer, render_backend);
+    renderer_destroy(renderer);
     return false;
   }
 
   if (!(renderer->brdf_integration_map = gfx_make_brdf_integration_map(
             renderer->ibl, render_backend, BRDF_INTEGRATION_MAP_WIDTH,
             BRDF_INTEGRATION_MAP_HEIGHT))) {
-    renderer_destroy(renderer, render_backend);
+    renderer_destroy(renderer);
     return false;
   }
 
   return true;
 }
 
-void renderer_destroy(Renderer* renderer, RenderBackend* render_backend) {
+void renderer_destroy(Renderer* renderer) {
   if (!renderer) {
     return;
   }
-  assert(render_backend);
+  assert(renderer->render_backend);
   if (renderer->ibl) {
-    gfx_destroy_ibl(render_backend, &renderer->ibl);
+    gfx_destroy_ibl(renderer->render_backend, &renderer->ibl);
   }
 }
 
@@ -168,6 +171,8 @@ static void draw_recursively(
     const SceneObject* object = mem_get_object(node->object);
     assert(object);
 
+    // TODO: Here we would frustum-cull the object.
+
     // TODO: Avoid computing matrices like Modelview or MVP if the shader does
     // not use them.
     const mat4 model_matrix = mat4_mul(node_transform, object->transform);
@@ -189,6 +194,11 @@ static void draw_recursively(
       }
       assert(mesh->geometry);
       assert(mesh->material);
+
+      // TODO: Here we would frustum-cull the mesh. The AABB would have to be
+      // transformed by the model matrix. Rotation would make the AABB
+      // relatively large, but still, the culling would be conservative.
+
       // Apply common shader uniforms not captured by materials.
       ShaderProgram* shader = mesh->shader;
       gfx_set_mat4_uniform(shader, "ModelMatrix", &model_matrix);
@@ -240,14 +250,12 @@ static void draw_recursively(
 }
 
 void gfx_render_scene(
-    Renderer* renderer, RenderBackend* render_backend, const Scene* scene,
-    const SceneCamera* camera) {
+    Renderer* renderer, const Scene* scene, const SceneCamera* camera) {
   assert(renderer);
-  assert(render_backend);
+  assert(scene);
+  assert(camera);
 
-  if (!scene) {
-    return;
-  }
+  RenderBackend* render_backend = renderer->render_backend;
 
   const mat4 projection = camera ? camera->camera.projection : mat4_id();
   const mat4 camera_rotation =
diff --git a/gfx/src/renderer/renderer_impl.h b/gfx/src/renderer/renderer_impl.h
index ce8ebe7..833025a 100644
--- a/gfx/src/renderer/renderer_impl.h
+++ b/gfx/src/renderer/renderer_impl.h
@@ -1,18 +1,52 @@
 #pragma once
 
 #include <gfx/renderer.h>
-
+#include <gfx/sizes.h>
 #include <gfx/util/ibl.h>
 
+#include <math/vec3.h>
+#include <math/vec4.h>
+
 #include <stdbool.h>
+#include <stddef.h>
+
+// Currently, the immediate mode renderer can only draw up to a maximum number
+// of primitives per frame. It does not adjust this number dynamically. Keeps
+// things simple while the extra complexity is not needed.
+
+typedef struct Buffer        Buffer;
+typedef struct Geometry      Geometry;
+typedef struct ShaderProgram ShaderProgram;
 
 typedef struct Renderer {
-  IBL* ibl;
-  Texture* brdf_integration_map;
+  RenderBackend* render_backend;
+  IBL*           ibl;
+  Texture*       brdf_integration_map;
 } Renderer;
 
+typedef struct ImmRenderer {
+  RenderBackend* render_backend;
+  ShaderProgram* shader;
+  Geometry*      triangles;
+  size_t         num_triangle_verts; // Number of triangle verts this frame.
+  // TODO: wireframe rendering.
+  struct {
+    bool wireframe : 1;
+  } flags;
+  vec3 triangle_verts[IMM_MAX_NUM_TRIANGLES * 3];
+} ImmRenderer;
+
 /// Create a new renderer.
 bool renderer_make(Renderer*, RenderBackend*);
 
 /// Destroy the renderer.
-void renderer_destroy(Renderer*, RenderBackend*);
+void renderer_destroy(Renderer*);
+
+/// Create a new immediate mode renderer.
+bool imm_renderer_make(ImmRenderer*, RenderBackend*);
+
+/// Destroy the immediate mode renderer.
+void imm_renderer_destroy(ImmRenderer*);
+
+/// Flush draw commands.
+void imm_renderer_flush(ImmRenderer*);
diff --git a/gfx/src/scene/object.c b/gfx/src/scene/object.c
index 47d2f25..64bb5a6 100644
--- a/gfx/src/scene/object.c
+++ b/gfx/src/scene/object.c
@@ -1,5 +1,8 @@
 #include "object_impl.h"
 
+#include <gfx/render_backend.h>
+
+#include "mesh_impl.h"
 #include "node_impl.h"
 #include "scene_memory.h"
 
@@ -75,3 +78,32 @@ void gfx_set_object_skeleton(SceneObject* object, const Skeleton* skeleton) {
   assert(skeleton);
   object->skeleton = mem_get_skeleton_index(skeleton);
 }
+
+// TODO: Could compute just once if we changed the Object API to require an
+// object descriptor up front instead of allowing the client to add meshes
+// and skeletons dynamically.
+aabb3 gfx_calc_object_aabb(const SceneObject* object) {
+  assert(object);
+
+  bool  first = true;
+  aabb3 box;
+
+  mesh_link_idx ml = object->mesh_link;
+  while (ml.val) {
+    const MeshLink* mesh_link = mem_get_mesh_link(ml);
+    const mesh_idx  mi        = mesh_link->mesh;
+    if (mi.val) {
+      const Mesh* mesh     = mem_get_mesh(mi);
+      const aabb3 mesh_box = gfx_get_geometry_aabb(mesh->geometry);
+      if (first) {
+        box   = mesh_box;
+        first = false;
+      } else {
+        box = aabb3_sum(box, mesh_box);
+      }
+    }
+    ml = mesh_link->next;
+  }
+
+  return box;
+}
diff --git a/gfx/src/scene/scene_memory.c b/gfx/src/scene/scene_memory.c
index 62e9401..217c719 100644
--- a/gfx/src/scene/scene_memory.c
+++ b/gfx/src/scene/scene_memory.c
@@ -182,6 +182,9 @@ void mem_free_skeleton(Skeleton** skeleton) {
 }
 
 // Query by index.
+//
+// TODO: Check for 0 index and return nullptr? Otherwise this can accidentally
+// return a pointer to the dummy objects.
 Anima* mem_get_anima(anima_idx index) {
   return mempool_get_block(&mem.animas, index.val);
 }
diff --git a/gfx/src/util/scene.c b/gfx/src/util/scene.c
index e8dd6b1..dc55d5a 100644
--- a/gfx/src/util/scene.c
+++ b/gfx/src/util/scene.c
@@ -373,23 +373,31 @@ static float read_float(const void* data, const cgltf_accessor* accessor) {
 #define ACCESSOR_FOREACH_VEC(dimensions, accessor, body)                       \
   {                                                                            \
     assert((1 <= dimensions) && (dimensions <= 4));                            \
-    assert(!(dimensions == 1) || (accessor->type == cgltf_type_scalar));       \
-    assert(!(dimensions == 2) || (accessor->type == cgltf_type_vec2));         \
-    assert(!(dimensions == 3) || (accessor->type == cgltf_type_vec3));         \
-    assert(!(dimensions == 4) || (accessor->type == cgltf_type_vec4));         \
+    assert(                                                                    \
+        ((dimensions == 1) && (accessor->type == cgltf_type_scalar)) ||        \
+        ((dimensions == 2) && (accessor->type == cgltf_type_vec2)) ||          \
+        ((dimensions == 3) && (accessor->type == cgltf_type_vec3)) ||          \
+        ((dimensions == 4) && (accessor->type == cgltf_type_vec4)));           \
     const cgltf_buffer_view* view   = accessor->buffer_view;                   \
     const cgltf_buffer*      buffer = view->buffer;                            \
     const cgltf_size         offset = accessor->offset + view->offset;         \
+    const uint8_t*           bytes  = (const uint8_t*)buffer->data + offset;   \
+    /* Component size in bytes. */                                             \
     const cgltf_size comp_size = get_component_size(accessor->component_type); \
-    const uint8_t*   bytes     = (const uint8_t*)buffer->data + offset;        \
+    /* Element size in bytes. */                                               \
+    const cgltf_size elem_size = dimensions * comp_size;                       \
+    /* Stride in bytes. If the view stride is 0, then the elements are tightly \
+     * packed. */                                                              \
+    const cgltf_size stride = view->stride != 0 ? view->stride : elem_size;    \
+    /* There isn't an accessor stride in the spec, but cgltf still specifies   \
+     * one. */                                                                 \
+    assert(accessor->stride == elem_size);                                     \
+    /* Accessor data must fit inside the buffer. */                            \
     assert(                                                                    \
-        (offset + accessor->count * dimensions * comp_size) < buffer->size);   \
-    /* From the spec: */                                                       \
-    /*  "Buffer views with other types of data MUST NOT not define */          \
-    /*   byteStride (unless such layout is explicitly enabled by an */         \
-    /*   extension)."*/                                                        \
-    assert(view->stride == 0);                                                 \
-    assert(accessor->stride == dimensions * comp_size);                        \
+        (offset + (accessor->count * elem_size) +                              \
+         ((accessor->count - 1) * view->stride)) <= buffer->size);             \
+    /* Accessor data must fit inside the view. */                              \
+    assert(accessor->count * accessor->stride <= view->size);                  \
     cgltf_float x = 0, y = 0, z = 0, w = 0;                                    \
     /* Silence unused variable warnings. */                                    \
     (void)y;                                                                   \
@@ -401,58 +409,63 @@ static float read_float(const void* data, const cgltf_accessor* accessor) {
      types, we take the performance hit and perform checks and conversions     \
      inside the loop for simplicity. */                                        \
     if (accessor->component_type == cgltf_component_type_r_32f) {              \
-      const cgltf_float* floats = (const cgltf_float*)bytes;                   \
       switch (dimensions) {                                                    \
       case 1:                                                                  \
         assert(accessor->type == cgltf_type_scalar);                           \
-        for (cgltf_size i = 0; i < accessor->count; ++i) {                     \
-          x = *floats++;                                                       \
+        for (cgltf_size i = 0; i < accessor->count; ++i, bytes += stride) {    \
+          const cgltf_float* floats = (const cgltf_float*)bytes;               \
+          x                         = *floats;                                 \
           body;                                                                \
         }                                                                      \
         break;                                                                 \
       case 2:                                                                  \
         assert(accessor->type == cgltf_type_vec2);                             \
-        for (cgltf_size i = 0; i < accessor->count; ++i) {                     \
-          x = *floats++;                                                       \
-          y = *floats++;                                                       \
+        for (cgltf_size i = 0; i < accessor->count; ++i, bytes += stride) {    \
+          const cgltf_float* floats = (const cgltf_float*)bytes;               \
+          x                         = *floats++;                               \
+          y                         = *floats;                                 \
           body;                                                                \
         }                                                                      \
         break;                                                                 \
       case 3:                                                                  \
         assert(accessor->type == cgltf_type_vec3);                             \
-        for (cgltf_size i = 0; i < accessor->count; ++i) {                     \
-          x = *floats++;                                                       \
-          y = *floats++;                                                       \
-          z = *floats++;                                                       \
+        for (cgltf_size i = 0; i < accessor->count; ++i, bytes += stride) {    \
+          const cgltf_float* floats = (const cgltf_float*)bytes;               \
+          x                         = *floats++;                               \
+          y                         = *floats++;                               \
+          z                         = *floats;                                 \
           body;                                                                \
         }                                                                      \
         break;                                                                 \
       case 4:                                                                  \
         assert(accessor->type == cgltf_type_vec4);                             \
-        for (cgltf_size i = 0; i < accessor->count; ++i) {                     \
-          x = *floats++;                                                       \
-          y = *floats++;                                                       \
-          z = *floats++;                                                       \
-          w = *floats++;                                                       \
+        for (cgltf_size i = 0; i < accessor->count; ++i, bytes += stride) {    \
+          const cgltf_float* floats = (const cgltf_float*)bytes;               \
+          x                         = *floats++;                               \
+          y                         = *floats++;                               \
+          z                         = *floats++;                               \
+          w                         = *floats;                                 \
           body;                                                                \
         }                                                                      \
         break;                                                                 \
       }                                                                        \
     } else {                                                                   \
-      for (cgltf_size i = 0; i < accessor->count; ++i) {                       \
-        x = read_float(bytes, accessor);                                       \
-        bytes += accessor->stride;                                             \
+      for (cgltf_size i = 0; i < accessor->count; ++i, bytes += stride) {      \
+        const uint8_t* component = bytes;                                      \
+                                                                               \
+        x = read_float(component, accessor);                                   \
+        component += comp_size;                                                \
         if (dimensions > 1) {                                                  \
-          y = read_float(bytes, accessor);                                     \
-          bytes += accessor->stride;                                           \
+          y = read_float(component, accessor);                                 \
+          component += comp_size;                                              \
         }                                                                      \
         if (dimensions > 2) {                                                  \
-          z = read_float(bytes, accessor);                                     \
-          bytes += accessor->stride;                                           \
+          z = read_float(component, accessor);                                 \
+          component += comp_size;                                              \
         }                                                                      \
         if (dimensions > 3) {                                                  \
-          w = read_float(bytes, accessor);                                     \
-          bytes += accessor->stride;                                           \
+          w = read_float(component, accessor);                                 \
+          component += comp_size;                                              \
         }                                                                      \
         body;                                                                  \
       }                                                                        \
@@ -538,10 +551,10 @@ static bool load_buffers(
     assert(buffer->data);
     buffers[i] = gfx_make_buffer(
         render_backend, &(BufferDesc){
-                            .usage = BufferStatic,
-                            .type  = BufferUntyped,
-                            .data  = buffer->data,
-                            .count = buffer->size});
+                            .usage      = BufferStatic,
+                            .type       = BufferUntyped,
+                            .data.data  = buffer->data,
+                            .data.count = buffer->size});
     if (!buffers[i]) {
       return false;
     }
@@ -564,10 +577,10 @@ static bool load_tangent_buffers(
     assert(buffer->data);
     tangent_buffers[i] = gfx_make_buffer(
         render_backend, &(BufferDesc){
-                            .usage = BufferStatic,
-                            .type  = BufferUntyped,
-                            .data  = buffer->data,
-                            .count = buffer->size_bytes});
+                            .usage      = BufferStatic,
+                            .type       = BufferUntyped,
+                            .data.data  = buffer->data,
+                            .data.count = buffer->size_bytes});
     if (!tangent_buffers[i]) {
       return false;
     }
@@ -843,6 +856,26 @@ static Material* make_default_material() {
   return gfx_make_material(&desc);
 }
 
+/// Compute the bounding box of the vertices pointed to by the accessor.
+/// 'dim' is the dimension of the vertices (2D or 3D).
+aabb3 compute_aabb(const cgltf_accessor* accessor, int dim) {
+  aabb3 box = {0};
+  if (accessor->has_min && accessor->has_max) {
+    box = aabb3_make(
+        vec3_from_array(accessor->min), vec3_from_array(accessor->max));
+  } else {
+    ACCESSOR_FOREACH_VEC(dim, accessor, {
+      const vec3 p = vec3_make(x, y, z);
+      if (i == 0) {
+        box = aabb3_make(p, p);
+      } else {
+        box = aabb3_add(box, p);
+      }
+    });
+  }
+  return box;
+}
+
 /// Load all meshes from the glTF scene.
 static bool load_meshes(
     const cgltf_data* data, Gfx* gfx, Buffer** buffers,
@@ -910,7 +943,8 @@ static bool load_meshes(
       }
 
       GeometryDesc geometry_desc = {
-          .type = from_gltf_primitive_type(prim->type)};
+          .type         = from_gltf_primitive_type(prim->type),
+          .buffer_usage = BufferStatic};
 
       // Vertex indices.
       if (prim->indices) {
@@ -969,11 +1003,13 @@ static bool load_meshes(
           switch (accessor->type) {
           case cgltf_type_vec2:
             assert(geometry_desc.positions3d.buffer == 0);
-            buffer_view_2d = &geometry_desc.positions2d;
+            buffer_view_2d     = &geometry_desc.positions2d;
+            geometry_desc.aabb = compute_aabb(accessor, 2);
             break;
           case cgltf_type_vec3:
             assert(geometry_desc.positions2d.buffer == 0);
-            buffer_view_3d = &geometry_desc.positions3d;
+            buffer_view_3d     = &geometry_desc.positions3d;
+            geometry_desc.aabb = compute_aabb(accessor, 3);
             break;
           default:
             LOGE(
@@ -1478,6 +1514,8 @@ static SceneNode* load_scene(
     scene_nodes[i] = gfx_make_node();
   }
 
+  // TODO: If the scene does not have animations, then a top-level LogicalNode
+  // would make more sense than an AnimaNode.
   anima_node = gfx_make_node();
   load_skins(data, buffers, scene_nodes, anima_desc->skeletons);
   load_animations(data, scene_nodes, anima_desc->animations);
diff --git a/gfx/src/util/shader.c b/gfx/src/util/shader.c
index fdfb1d3..ed81f79 100644
--- a/gfx/src/util/shader.c
+++ b/gfx/src/util/shader.c
@@ -7,6 +7,8 @@
 #include <shaders/cubemap_filtering.vert.h>
 #include <shaders/debug3d.frag.h>
 #include <shaders/debug3d.vert.h>
+#include <shaders/immediate_mode.frag.h>
+#include <shaders/immediate_mode.vert.h>
 #include <shaders/irradiance_map.frag.h>
 #include <shaders/prefiltered_environment_map.frag.h>
 #include <shaders/quad.vert.h>
@@ -94,6 +96,11 @@ ShaderProgram* gfx_make_cook_torrance_shader_perm(
       num_defines);
 }
 
+ShaderProgram* gfx_make_immediate_mode_shader(RenderBackend* render_backend) {
+  return make_shader_program(
+      render_backend, immediate_mode_vert, immediate_mode_frag, 0, 0);
+}
+
 ShaderProgram* gfx_make_irradiance_map_shader(RenderBackend* render_backend) {
   return make_shader_program(
       render_backend, cubemap_filtering_vert, irradiance_map_frag, 0, 0);
diff --git a/gltfview/src/game.c b/gltfview/src/game.c
index f822b08..662272c 100644
--- a/gltfview/src/game.c
+++ b/gltfview/src/game.c
@@ -2,6 +2,7 @@
 
 #include <gfx/error.h>
 #include <gfx/render_backend.h>
+#include <gfx/scene/camera.h>
 #include <gfx/scene/light.h>
 #include <gfx/scene/material.h>
 #include <gfx/scene/mesh.h>
@@ -32,8 +33,10 @@ static const char* FLIGHT_HELMET =
     "/assets/glTF-Sample-Models/2.0/FlightHelmet/glTF/FlightHelmet.gltf";
 static const char* DAMAGED_HELMET =
     "/assets/glTF-Sample-Models/2.0/DamagedHelmet/glTF/DamagedHelmet.gltf";
+static const char* GIRL =
+    "/home/jeanne/Nextcloud/assets/models/girl/girl-with-ground.gltf";
 
-#define DEFAULT_SCENE_FILE DAMAGED_HELMET
+#define DEFAULT_SCENE_FILE GIRL
 
 static const char* CLOUDS1_TEXTURE = "/assets/skybox/clouds1/clouds1_west.bmp";
 
@@ -189,6 +192,26 @@ static bool load_texture_debugger_scene(Game* game) {
   return true;
 }
 
+/// Render the bounding boxes of all scene objects.
+static void render_bounding_boxes(ImmRenderer* imm, const SceneNode* node) {
+  if (gfx_get_node_type(node) == ObjectNode) {
+    // TODO: Look at the scene log. The JointNodes are detached from the
+    // ObjectNodes. This is why the boxes are not being transformed as expected
+    // here. Anima needs to animate boxes? Use OOBB in addition to AABB?
+    const mat4         model = gfx_get_node_global_transform(node);
+    const SceneObject* obj   = gfx_get_node_object(node);
+    const aabb3        box   = gfx_calc_object_aabb(obj);
+    gfx_imm_set_model_matrix(imm, &model);
+    gfx_imm_draw_aabb(imm, box);
+  }
+
+  // Render children's boxes.
+  for (NodeIter it = gfx_get_node_child(node); it;
+       it          = gfx_get_next_child(it)) {
+    render_bounding_boxes(imm, gfx_get_iter_node(it));
+  }
+}
+
 bool game_new(Game* game, int argc, const char** argv) {
   // TODO: getopt() to implement proper argument parsing.
   const char* scene_filepath = argc > 1 ? argv[1] : DEFAULT_SCENE_FILE;
@@ -239,6 +262,8 @@ void game_end(Game* game) { gfx_destroy(&game->gfx); }
 void game_update(Game* game, double t, double dt) {
   // LOGD("Tick");
 
+  // TODO: Animation should be handled by Gfx instead. Descend through the scene
+  // looking for animas and animate them. gfx_animate(t).
   Anima* anima = gfx_get_node_anima(game->root_node);
   gfx_update_animation(anima, t);
 
@@ -263,8 +288,18 @@ void game_update(Game* game, double t, double dt) {
 }
 
 void game_render(const Game* game) {
-  gfx_render_scene(
-      game->renderer, game->render_backend, game->scene, game->camera);
+  gfx_render_scene(game->renderer, game->scene, game->camera);
+
+  ImmRenderer* imm = gfx_get_imm_renderer(game->gfx);
+  assert(imm);
+  gfx_imm_start(imm);
+  gfx_imm_set_camera(imm, gfx_get_camera_camera(game->camera));
+  gfx_imm_set_colour(imm, vec4_make(0.2, 0.2, 1.0, 0.3));
+  // DEBUG
+  // const aabb3 box = aabb3_make(vec3_make(0, 0, 0), vec3_make(1, 1, 1));
+  // gfx_imm_draw_aabb(imm, box);
+  render_bounding_boxes(imm, gfx_get_scene_root(game->scene));
+  gfx_imm_end(imm);
 }
 
 void game_set_viewport(Game* game, int width, int height) {