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diff --git a/src/llr/llr_impl.h b/src/llr/llr_impl.h
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+++ b/src/llr/llr_impl.h
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+#pragma once
+
+#include <gfx/llr/llr.h>
+#include <gfx/sizes.h>
+
+#include <math/mat4.h>
+#include <math/vec3.h>
+
+#include <stdbool.h>
+#include <stddef.h>
+
+typedef struct Geometry      Geometry;
+typedef struct GfxCore       GfxCore;
+typedef struct IBL           IBL;
+typedef struct Material      Material;
+typedef struct ShaderProgram ShaderProgram;
+typedef struct Texture       Texture;
+
+/// Immediate mode renderer.
+///
+/// The renderer caches state changes in memory and only programs the underlying
+/// shader program when a draw call is issued and if anything has changed. This
+/// keeps the number of graphics API calls to a minimum, but requires tracking
+/// state changes. The 'changed' booleans below fulfill this purpose, and
+/// indicate whether a given state has changed since the last draw call.
+///
+/// The renderer must combine state changes accordingly. For example, if only
+/// the lights have changed, then it is sufficient to update light uniforms in
+/// the current shader program. On the other hand, if the shader program has
+/// changed, then the renderer must reconfigure it from scratch and set light
+/// uniforms, camera uniforms, etc.
+///
+/// Note that the shader program API has its own level of caching as well, so
+/// reconfiguration at the level of the renderer does not result in the
+/// worst-case set of graphics API calls.
+///
+/// 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.
+/// TODO: Flush the buffer when it reaches its maximum size to remove this
+///  constraint.
+typedef struct ImmRenderer {
+  GfxCore* gfxcore;
+
+  vec3 camera_position;
+  mat4 view;           // Camera view matrix.
+  mat4 projection;     // Camera projection matrix.
+  bool camera_changed; // Whether the camera parameters have changed.
+
+  // -------------------------------------------
+  // Immediate-mode rendering of scene elements.
+
+  IBL*     ibl;
+  Texture* brdf_integration_map;
+
+  ShaderProgram* shader;         // Active shader. Not owned.
+  bool           shader_changed; // Whether the shader has changed.
+
+  // Lights are not const because environment lights store lazily-computed
+  // irradiance maps.
+  Light* lights[IMM_MAX_NUM_LIGHTS]; // Lights stack.
+  int    num_lights;   // Number of lights enabled at a given point in time. It
+                       // points to one past the top of the stack.
+  bool lights_changed; // Whether the lights have changed.
+
+  bool   skeleton_changed;
+  size_t num_joints;
+  mat4   joint_matrices[GFX_MAX_NUM_JOINTS];
+
+  // ---------------------------------------
+  // Immediate-mode rendering of primitives.
+
+  ShaderProgram* imm_shader; // Immediate-mode shader program for primitives.
+  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];
+
+  // -------------
+  // Matrix stack.
+
+  // The matrix stack contains pre-multiplied matrices.
+  // It is also never empty. The top of the stack is an identity matrix when the
+  // stack is "empty" from the user's perspective.
+  mat4 matrix_stack[IMM_MAX_NUM_MATRICES];
+  int  stack_pointer; // Points to the top of the stack.
+} ImmRenderer;
+
+/// Create a new immediate mode renderer.
+bool gfx_imm_make(ImmRenderer*, GfxCore*);
+
+/// Destroy the immediate mode renderer.
+void gfx_imm_destroy(ImmRenderer*);
+
+/// Flush draw commands.
+void gfx_imm_flush(ImmRenderer*);