5 files changed, 501 insertions, 72 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 537c00d..ec4d307 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,10 +23,10 @@ target_compile_options(swgfx PRIVATE -Wall -Wextra)
 # Tests.
 
 add_executable(swgfx-test
-    test/test.c)
+    test/test.c
+    test/test.h)
 
 target_link_libraries(swgfx-test
     swgfx)
 
-target_compile_options(swgfx-test PRIVATE -Wall -Wextra)
-
+target_compile_options(swgfx-test PRIVATE  -DUNIT_TEST -DNDEBUG -Wall -Wextra)
diff --git a/include/swgfx.h b/include/swgfx.h
index b6dc769..94eb359 100644
--- a/include/swgfx.h
+++ b/include/swgfx.h
@@ -1,7 +1,7 @@
 /*
 Software rendering library.
 
-Cooridnate systems:
+Coordinate systems:
 - Pixel coordinates (i,j) refer to the center of the pixel.
   Thus, real-valued coordinates (x,y) with no fractional part point at the pixel center.
 - Viewport origin is the top-left corner of the screen.
@@ -30,6 +30,10 @@ typedef struct sgQuad  { sgVec2  p0, p1; }     sgQuad;
 typedef struct sgTri2  { sgVec2  p0, p1, p2; } sgTri2;
 typedef struct sgTri3  { sgVec3  p0, p1, p2; } sgTri3;
 
+typedef uint16_t sgIdx;
+typedef struct   sgVert   { sgIdx position, normal, texcoord; } sgVert;
+typedef struct   sgTriIdx { sgVert v0, v1, v2; } sgTriIdx;
+
 // TODO: Should we use real-valued colours?
 typedef struct sgPixel { uint8_t r, g, b, a; } sgPixel;
 
@@ -43,7 +47,9 @@ void   sgDel(swgfx**);
 void sgColourBuffer(swgfx*, sgVec2i dimensions, sgPixel* buffer);
 void sgPresent     (swgfx*, sgVec2i dimensions, sgPixel* screen);
 
-void sgCam        (swgfx*, sgVec3 position, sgVec3 forward);
+void sgModelId    (swgfx*);
+void sgModel      (swgfx*, sgVec3 position, sgVec3 right, sgVec3 up, sgVec3 forward);
+void sgView       (swgfx*, sgVec3 position, sgVec3 forward);
 void sgOrtho      (swgfx*, R left, R right, R top, R bottom, R near, R far);
 void sgPerspective(swgfx*, R fovy, R aspect, R near, R far);
 void sgViewport   (swgfx*, int x0, int y0, int width, int height);
@@ -56,6 +62,8 @@ void sgTriangles2    (swgfx*, size_t count, const sgTri2*);
 void sgTriangleStrip2(swgfx*, size_t count, const sgVec2*);
 void sgTriangles     (swgfx*, size_t count, const sgTri3*, const sgNormal*);
 void sgTriangleStrip (swgfx*, size_t count, const sgVec3*, const sgNormal*);
+void sgTrianglesIndexed(swgfx*, size_t numIndices, const sgIdx* indices, const sgVec3* positions);
+void sgTrianglesIndexedNonUniform(swgfx*, size_t numTris, const sgTriIdx* tris, const sgVec3* positions);
 
 void sgCheck(swgfx*);
 
diff --git a/src/swgfx.c b/src/swgfx.c
index 772a691..3b9ce29 100644
--- a/src/swgfx.c
+++ b/src/swgfx.c
@@ -6,6 +6,11 @@ Matrices:
 Coordinate systems:
   - Right-handed.
   - NDC in [-1, +1].
+  - Viewport goes up and to the right.
+  - Window goes down and to the right.
+  - (x,y) is the center of a pixel.
+    - Top-left:     (x - 1/2, y - 1/2)
+    - Bottom-right: (x + 1/2, y + 1/2)
 */
 #include <swgfx.h>
 
@@ -15,7 +20,7 @@ Coordinate systems:
 #include <stdlib.h>
 #include <string.h>
 
-static const sgVec3 Up3 = (sgVec3){0,1,0};
+static constexpr sgVec3 Up3 = (sgVec3){0,1,0};
 
 typedef struct sgViewport_t { int x0, y0, width, height; } sgViewport_t;
 typedef struct sgTri2       { sgVec2 p0, p1, p2; }         sgTri2;
@@ -30,8 +35,18 @@ typedef struct swgfx {
   sgVec2i      dims;     // Colour buffer dimensions.
   sgPixel*     colour;   // Colour buffer.
   sgViewport_t viewport;
+  sgMat4       model;    // Model matrix.
   sgMat4       view;     // View matrix.
   sgMat4       proj;     // Projection matrix.
+  // Pre-multiplied matrices.
+  // The model matrix changes once per object, more frequently than view or
+  // projection. View and projection are expected to change infrequently, maybe
+  // once per frame.
+  // Make it so that changing the model matrix only requires one matrix
+  // multiplication (mvp = model * viewProj) and not two (mvp = model * view * projection)
+  // before rendering the model's triangles.
+  sgMat4       viewProj; // View-projection matrix.
+  sgMat4       mvp;      // Model-view-projection matrix.
 } swgfx;
 
 static inline sgVec3 neg3(sgVec3 v) { return (sgVec3){-v.x, -v.y, -v.z}; }
@@ -40,6 +55,10 @@ static inline sgVec3 sub3(sgVec3 a, sgVec3 b) {
   return (sgVec3){a.x - b.x, a.y - b.y, a.z - b.z};
 }
 
+static inline R dot3(sgVec3 a, sgVec3 b) {
+  return a.x * b.x + a.y * b.y + a.z * b.z;
+}
+
 static inline sgVec3 cross3(sgVec3 a, sgVec3 b) {
   return (sgVec3) {
     a.y * b.z - a.z * b.y,
@@ -48,8 +67,7 @@ static inline sgVec3 cross3(sgVec3 a, sgVec3 b) {
 }
 
 static inline R normsq3(sgVec3 v) { return v.x * v.x + v.y * v.y + v.z * v.z; }
-
-static inline R norm3(sgVec3 v) { return sqrt(normsq3(v)); }
+static inline R norm3  (sgVec3 v) { return (R)sqrt(normsq3(v)); }
 
 static inline sgVec3 normalize3(sgVec3 v) {
   const R n = norm3(v);
@@ -57,6 +75,10 @@ static inline sgVec3 normalize3(sgVec3 v) {
   return (sgVec3){v.x / n, v.y / n, v.z / n};
 }
 
+static inline sgVec4 Vec4FromVec3(sgVec3 v, R w) {
+  return (sgVec4){v.x, v.y, v.z, w};
+}
+
 static inline sgMat4 Mat4(
     R m00, R m01, R m02, R m03,   // v0.x v1.x v2.x v3.x
     R m10, R m11, R m12, R m13,   // v0.y v1.y v2.y v3.y
@@ -78,6 +100,10 @@ static inline sgMat4 Mat4FromVec3(sgVec3 right, sgVec3 up, sgVec3 forward, sgVec
 }
 
 static inline R Mat4At(sgMat4 m, int row, int col) { return m.val[col][row]; }
+static inline sgVec3 Mat4v0(sgMat4 m) { return *((sgVec3*)m.val[0]); }
+static inline sgVec3 Mat4v1(sgMat4 m) { return *((sgVec3*)m.val[1]); }
+static inline sgVec3 Mat4v2(sgMat4 m) { return *((sgVec3*)m.val[2]); }
+static inline sgVec3 Mat4v3(sgMat4 m) { return *((sgVec3*)m.val[3]); }
 
 static inline sgMat4 Mat4Mul(sgMat4 A, sgMat4 B) {
   R m00 = Mat4At(A, 0, 0) * Mat4At(B, 0, 0) +
@@ -162,6 +188,31 @@ static inline sgVec3 Mat4MulVec3(sgMat4 m, sgVec3 v, R w) {
     .z = Mat4At(m, 2, 0) * v.x + Mat4At(m, 2, 1) * v.y + Mat4At(m, 2, 2) * v.z + Mat4At(m, 2, 3) * w};
 }
 
+static inline sgVec4 Mat4MulVec4(sgMat4 m, sgVec4 v) {
+  sgVec4 u;
+  u.x = Mat4At(m, 0, 0) * v.x + Mat4At(m, 0, 1) * v.y +
+        Mat4At(m, 0, 2) * v.z + Mat4At(m, 0, 3) * v.w;
+  u.y = Mat4At(m, 1, 0) * v.x + Mat4At(m, 1, 1) * v.y +
+        Mat4At(m, 1, 2) * v.z + Mat4At(m, 1, 3) * v.w;
+  u.z = Mat4At(m, 2, 0) * v.x + Mat4At(m, 2, 1) * v.y +
+        Mat4At(m, 2, 2) * v.z + Mat4At(m, 2, 3) * v.w;
+  u.w = Mat4At(m, 3, 0) * v.x + Mat4At(m, 3, 1) * v.y +
+        Mat4At(m, 3, 2) * v.z + Mat4At(m, 3, 3) * v.w;
+  return u;
+}
+
+static inline sgMat4 Mat4InverseTransform(sgMat4 m) {
+  const sgVec3 r = Mat4v0(m);
+  const sgVec3 u = Mat4v1(m);
+  const sgVec3 f = Mat4v2(m);
+  const sgVec3 t = Mat4v3(m);
+  return Mat4(
+      r.x, r.y, r.z, -dot3(r, t),
+      u.x, u.y, u.z, -dot3(u, t),
+      f.x, f.y, f.z, -dot3(f, t),
+      0.f, 0.f, 0.f, 1.f);
+}
+
 static inline sgMat4 Mat4Look(sgVec3 position, sgVec3 forward, sgVec3 up) {
   const sgVec3 right = normalize3(cross3(forward, up));
   up                 = normalize3(cross3(right, forward));
@@ -169,43 +220,59 @@ static inline sgMat4 Mat4Look(sgVec3 position, sgVec3 forward, sgVec3 up) {
 }
 
 static inline sgMat4 Mat4Perspective(R fovy, R aspect, R near, R far) {
-  R f = tan(fovy / 2.0);
+  R f = (R)tan(fovy / 2.0);
   assert(f > 0.0);
-  f = 1.0 / f;
+  f = 1.f / f;
   const R a = near - far;
   return Mat4(
-    f / aspect, 0, 0, 0,
-    0, f, 0, 0,
-    0, 0, (far + near) / a, (2 * far * near / a),
-    0, 0, -1, 0);
+    f / aspect, 0,                 0,                    0,
+    0,          f,                 0,                    0,
+    0,          0,  (far + near) / a, (2 * far * near / a),
+    0,          0,                 -1,                  0);
 }
 
-static inline sgPixel* PixelRow(sgPixel* image, int width, int y) {
-  return image + (y * width);
+#ifndef _NDEBUG
+static bool InBounds(int width, int height, sgVec2i p) {
+  return (0 <= p.x) && (p.x < width) &&
+         (0 <= p.y) && (p.y < height);
 }
+#endif // _NDEBUG
 
-static inline sgPixel* Pixel(sgPixel* image, int width, int x, int y) {
+static inline sgPixel* Pixel(sgPixel* image, int width, int height, int x, int y) {
+  assert(InBounds(width, height, (sgVec2i){x,y}));
   return image + (y * width) + x;
 }
 
-#define XY(X,Y) Pixel(gfx->colour, gfx->dims.x, X, Y)
-
-static inline R rmin(R a, R b) { return (a <= b) ? a : b; }
-static inline R rmax(R a, R b) { return (a >= b) ? a : b; }
-
+static inline R   rmin(R   a, R   b) { return (a <= b) ? a : b; }
+static inline R   rmax(R   a, R   b) { return (a >= b) ? a : b; }
+static inline int imin(int a, int b) { return (a <= b) ? a : b; }
+static inline int imax(int a, int b) { return (a >= b) ? a : b; }
 static inline sgVec2 min2(sgVec2 a, sgVec2 b) {
   return (sgVec2){.x = rmin(a.x, b.x), .y = rmin(a.y, b.y) };
 }
-
 static inline sgVec2 max2(sgVec2 a, sgVec2 b) {
   return (sgVec2){.x = rmax(a.x, b.x), .y = rmax(a.y, b.y) };
 }
+static inline sgVec2i min2i(sgVec2i a, sgVec2i b) {
+  return (sgVec2i){.x = imin(a.x, b.x), .y = imin(a.y, b.y) };
+}
+static inline sgVec2i max2i(sgVec2i a, sgVec2i b) {
+  return (sgVec2i){.x = imax(a.x, b.x), .y = imax(a.y, b.y) };
+}
 
 static inline sgAABB2 TriangleAabb2(const sgTri2 tri) {
   return (sgAABB2){.pmin = min2(min2(tri.p0, tri.p1), tri.p2),
                    .pmax = max2(max2(tri.p0, tri.p1), tri.p2)};
 }
 
+static inline sgVec2i Clip(const swgfx* gfx, const sgVec2i p) {
+  assert(gfx);
+  constexpr sgVec2i lower = (sgVec2i){0,0};
+  const     sgVec2i upper = (sgVec2i){gfx->viewport.width  - 1,
+                                      gfx->viewport.height - 1};
+  return max2i(lower, min2i(upper, p));
+}
+
 static inline R f(sgVec2 a, sgVec2 b, sgVec2 p) {
   return (a.y - b.y)*p.x + (b.x - a.x)*p.y + a.x*b.y - b.x*a.y;
 }
@@ -222,10 +289,77 @@ static inline sgVec3 Barycentric(const sgTri2 tri, sgVec2 p) {
     f(tri.p0, tri.p1, p) / f(tri.p0, tri.p1, tri.p2)};*/
   const R b = f(tri.p0, tri.p2, p) / f(tri.p0, tri.p2, tri.p1);
   const R c = f(tri.p0, tri.p1, p) / f(tri.p0, tri.p1, tri.p2);
-  const R a = /*f(tri.p1, tri.p2, p) / f(tri.p1, tri.p2, tri.p0);*/1 - b - c - 1e-7;
+  const R a = /*f(tri.p1, tri.p2, p) / f(tri.p1, tri.p2, tri.p0);*/1.f - b - c - (R)1e-7;
   return (sgVec3){a,b,c};
 }
 
+static void DrawTriangle2(swgfx* gfx, const sgTri2* tri) {
+  assert(gfx);
+  assert(tri);
+  const sgAABB2 bbox = TriangleAabb2(*tri);
+  // We consider (x,y) to be the pixel center.
+  // Draw all pixels touched by the bounding box. TODO: Multi-sampling.
+  sgVec2i pmin = (sgVec2i){(int)bbox.pmin.x, (int)bbox.pmin.y};
+  sgVec2i pmax = (sgVec2i){(int)(bbox.pmax.x + 0.5f), (int)(bbox.pmax.y + 0.5f)};
+  // Clip to screen space.
+  pmin = Clip(gfx, pmin);
+  pmax = Clip(gfx, pmax);
+  // Draw.
+  for   (int y = pmin.y; y <= pmax.y; ++y) {
+    for (int x = pmin.x; x <= pmax.x; ++x) {
+      const sgVec2 p = (sgVec2){(R)x, (R)y};
+      // TODO: there is an incremental optimization to computing barycentric coordinates;
+      //  read more about it.
+      const sgVec3 bar = Barycentric(*tri, p);
+      // We need to check the third coordinate.
+      //   a + b + c = 1
+      //   So, e.g., if a >= 0 and b >= 0, then we have c <= 1, but we could also have c <= 0.
+      //   In the case c <= 0, then point is outside the triangle.
+      if ((bar.x >= 0) && (bar.y >= 0) && (bar.z >= 0)) {
+        const sgVec2i pi = (sgVec2i){(int)x, (int)y};
+        sgPixels(gfx, 1, &pi, (sgPixel){255, 255, 255, 255});
+      }
+    }
+  }
+}
+
+static inline sgVec3 PerspDivide(sgVec4 v) {
+  return (sgVec3){v.x / v.w, v.y / v.w, v.z / v.w};
+}
+
+// TODO: Compute a viewport matrix in sgViewport() instead.
+static inline sgVec2 ViewportTransform(sgViewport_t vp, sgVec3 ndc) {
+  return (sgVec2){
+    .x = (ndc.x+1.f) * ((R)vp.width/2.f)  + (R)vp.x0,
+    .y = (ndc.y+1.f) * ((R)vp.height/2.f) + (R)vp.y0};
+}
+
+static inline sgVec2 ViewportToWindow(sgViewport_t vp, sgVec2 p) {
+  return (sgVec2){p.x, (R)vp.height - p.y};
+}
+
+static inline sgVec2 TransformPosition(const swgfx* gfx, sgVec3 p) {
+  assert(gfx);
+  // Model to clip space.
+  const sgVec4 p_clip = Mat4MulVec4(gfx->mvp, Vec4FromVec3(p, 1));
+  // TODO: Backface culling.
+  // Perspective divide.
+  const sgVec3 p_ndc = PerspDivide(p_clip);
+  // TODO: Clip.
+  const sgVec2 p_vp = ViewportTransform(gfx->viewport, p_ndc);
+  return ViewportToWindow(gfx->viewport, p_vp);
+}
+
+static void DrawTriangle3(swgfx* gfx, const sgTri3* tri) {
+  assert(gfx);
+  assert(tri);
+  const sgVec2 p0 = TransformPosition(gfx, tri->p0);
+  const sgVec2 p1 = TransformPosition(gfx, tri->p1);
+  const sgVec2 p2 = TransformPosition(gfx, tri->p2);
+  const sgTri2 tri2 = (sgTri2){p0, p1, p2};
+  DrawTriangle2(gfx, &tri2);
+}
+
 #define is_pow2_or_0(X) ((X & (X - 1)) == 0)
 
 static size_t align(size_t size) {
@@ -295,14 +429,44 @@ void sgPresent(swgfx* gfx, sgVec2i dimensions, sgPixel* screen) {
   }
 }
 
-void sgCam(swgfx* gfx, sgVec3 position, sgVec3 forward) {
+static void sgUpdateViewProjection(swgfx* gfx) {
+  assert(gfx);
+  gfx->viewProj = Mat4Mul(gfx->proj, gfx->view);
+}
+
+static void sgUpdateMvp(swgfx* gfx) {
+  assert(gfx);
+  gfx->mvp = Mat4Mul(gfx->viewProj, gfx->model);
+}
+
+void sgModelId(swgfx* gfx) {
   assert(gfx);
-  gfx->view = Mat4Look(position, forward, Up3);
+  sgModel(gfx,
+    (sgVec3){0,0,0},
+    (sgVec3){1, 0, 0},
+    (sgVec3){0, 1, 0},
+    (sgVec3){0, 0, 1});
+}
+
+void sgModel(swgfx* gfx, sgVec3 position, sgVec3 right, sgVec3 up, sgVec3 forward) {
+  assert(gfx);
+  gfx->model = Mat4FromVec3(right, up, forward, position);
+  sgUpdateMvp(gfx);
+}
+
+void sgView(swgfx* gfx, sgVec3 position, sgVec3 forward) {
+  assert(gfx);
+  const sgMat4 camera = Mat4Look(position, forward, Up3);
+  gfx->view = Mat4InverseTransform(camera);
+  sgUpdateViewProjection(gfx);
+  sgUpdateMvp(gfx);
 }
 
 void sgPerspective(swgfx* gfx, R fovy, R aspect, R near, R far) {
   assert(gfx);
   gfx->proj = Mat4Perspective(fovy, aspect, near, far);
+  sgUpdateViewProjection(gfx);
+  sgUpdateMvp(gfx);
 }
 
 void sgViewport(swgfx* gfx, int x0, int y0, int width, int height) {
@@ -317,34 +481,13 @@ void sgClear(swgfx* gfx) {
 
 void sgPixels(swgfx* gfx, size_t count, const sgVec2i* positions, sgPixel colour) {
   assert(gfx);
+#define XY(X,Y) Pixel(gfx->colour, gfx->dims.x, gfx->dims.y, X, Y)
   for (size_t i = 0; i < count; ++i) {
     const sgVec2i p = positions[i];
     *XY(p.x, p.y) = colour;
   }
 }
 
-static void DrawTriangle2(swgfx* gfx, const sgTri2* tri) {
-  assert(gfx);
-  assert(tri);
-  const sgAABB2 bbox = TriangleAabb2(*tri);
-  for   (int y = bbox.pmin.y; y <= bbox.pmax.y; ++y) {
-    for (int x = bbox.pmin.x; x <= bbox.pmax.x; ++x) {
-      const sgVec2 p = (sgVec2){x, y};
-      // TODO: there is an incremental optimization to computing barycentric coordinates;
-      // read more about it.
-      const sgVec3 bar = Barycentric(*tri, p);
-      // We need to check the third coordinate.
-      //   a + b + c = 1
-      //   So, e.g., if a > 0 and b > 0, then we have c < 1, but we could also have c < 0.
-      //   In the case c < 0, then point is outside the triangle.
-      if ((bar.x > 0) && (bar.y > 0) && (bar.z > 0)) {
-        const sgVec2i pi = (sgVec2i){(int)x, (int)y};
-        sgPixels(gfx, 1, &pi, (sgPixel){255, 255, 255, 255});
-      }
-    }
-  }
-}
-
 // TODO: DrawTriangle3 with clipping. Leave DrawTriangle2 to not clip for
 //       performance; assume that 2D triangles are within bounds.
 // TODO: If the triangle is out of bounds, skip entirely.
@@ -365,26 +508,51 @@ void sgTriangles2(swgfx* gfx, size_t count, const sgTri2* tris) {
 
 void sgTriangles(swgfx* gfx, size_t count, const sgTri3* tris, const sgNormal*) {
   assert(gfx);
+  assert(tris);
   for (size_t i = 0; i < count; ++i) {
-    // Ignore projection matrix for now. Rasterize 2D triangles.
-    const sgTri3* tri3 = &tris[i];
-    const sgTri2 tri2 = (sgTri2) {
-      .p0 = (sgVec2){tri3->p0.x, tri3->p0.y},
-      .p1 = (sgVec2){tri3->p1.x, tri3->p1.y},
-      .p2 = (sgVec2){tri3->p2.x, tri3->p2.y},
-    };
-    DrawTriangle2(gfx, &tri2);
+    const sgTri3* tri = &tris[i];
+    DrawTriangle3(gfx, tri);
+  }
+}
+
+void sgTrianglesIndexed(swgfx* gfx, size_t numIndices, const sgIdx* indices, const sgVec3* positions) {
+  assert(gfx);
+  assert(indices);
+  assert(positions);
+  for (size_t i = 0; i < numIndices; i+=3) {
+    const sgIdx i0 = indices[i];
+    const sgIdx i1 = indices[i+1];
+    const sgIdx i2 = indices[i+2];
+    const sgVec3 p0 = positions[i0];
+    const sgVec3 p1 = positions[i1];
+    const sgVec3 p2 = positions[i2];
+    const sgTri3 tri = (sgTri3){p0, p1, p2};
+    DrawTriangle3(gfx, &tri);
+  }
+}
+
+void sgTrianglesIndexedNonUniform(swgfx* gfx, size_t numTris, const sgTriIdx* tris, const sgVec3* positions) {
+  assert(gfx);
+  assert(tris);
+  assert(positions);
+  for (size_t t = 0; t < numTris; ++t) {
+    const sgTriIdx* triIdx = &tris[t];
+    const sgTri3 tri = (sgTri3){
+      positions[triIdx->v0.position],
+      positions[triIdx->v1.position],
+      positions[triIdx->v2.position]};
+    DrawTriangle3(gfx, &tri);
   }
 }
 
-static inline void AssertViewportWithinBuffer(swgfx* gfx) {
+static bool ViewportWithinBuffer(swgfx* gfx) {
   assert(gfx);
   const sgViewport_t vp = gfx->viewport;
-  assert((vp.x0 + vp.width)  <= gfx->dims.x);
-  assert((vp.y0 + vp.height) <= gfx->dims.y);
+  return ((vp.x0 + vp.width)  <= gfx->dims.x) &&
+         ((vp.y0 + vp.height) <= gfx->dims.y);
 }
 
 void sgCheck(swgfx* gfx) {
   assert(gfx);
-  AssertViewportWithinBuffer(gfx);
+  assert(ViewportWithinBuffer(gfx));
 } 
diff --git a/test/test.c b/test/test.c
index 668dec8..be3be93 100644
--- a/test/test.c
+++ b/test/test.c
@@ -1,3 +1,5 @@
+#include "test.h"
+
 #include <swgfx.h>
 
 #include <assert.h>
@@ -24,7 +26,7 @@ static bool WritePPM(int width, int height, const RGB* image, const char* path)
   return true;
 }
 
-void ToRGB(int width, int height, const sgPixel* rgba, RGB* rgb) {
+static void ToRGB(int width, int height, const sgPixel* rgba, RGB* rgb) {
   assert(rgba);
   assert(rgb);
   for (int y = 0; y < height; ++y) {
@@ -71,18 +73,21 @@ static void TestTriangle(swgfx* gfx) {
   
   ToRGB(BufferWidth, BufferHeight, colour, rgb);
   WritePPM(BufferWidth, BufferHeight, rgb, "triangle.ppm");
+  // TODO: Assert the contents. Turn this file into a unit test executable.
+  //  Write a helper function that first writes the image to a file and then
+  //  asserts its contents.
 }
 
-int main() {
-  swgfx* gfx = 0;
-  if (!(gfx = sgNew())) {
-    fprintf(stderr, "Failed to create swgfx\n");
-    return 1;
+#define GFX_TEST(NAME, FUNC) \
+  TEST_CASE(NAME) {\
+    swgfx* gfx = nullptr;\
+    if (!(gfx = sgNew())) {\
+      SET_FAILURE("Failed to create swgfx\n", false);\
+    }\
+    FUNC(gfx);\
+    sgDel(&gfx);\
   }
-  
-  TestTriangle(gfx);
-  
-  sgDel(&gfx);
-  return 0;
-}
 
+GFX_TEST(triangle, TestTriangle)
+
+int main() { return 0; }
diff --git a/test/test.h b/test/test.h
new file mode 100644
index 0000000..cdd2f05
--- /dev/null
+++ b/test/test.h
@@ -0,0 +1,248 @@
+// SPDX-License-Identifier: MIT
+#pragma once
+
+#ifdef UNIT_TEST
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#if defined(__DragonFly__) || defined(__FreeBSD__) || defined(__FreeBSD_kernel__) ||     \
+    defined(__NetBSD__) || defined(__OpenBSD__)
+#define USE_SYSCTL_FOR_ARGS 1
+// clang-format off
+#include <sys/types.h>
+#include <sys/sysctl.h>
+// clang-format on
+#include <unistd.h>        // getpid
+#endif
+
+struct test_file_metadata;
+
+struct test_failure {
+        bool present;
+        const char *message;
+        const char *file;
+        int line;
+        bool owned;
+};
+
+struct test_case_metadata {
+        void (*fn)(struct test_case_metadata *, struct test_file_metadata *);
+        struct test_failure failure;
+        const char *name;
+        struct test_case_metadata *next;
+};
+
+struct test_file_metadata {
+        bool registered;
+        const char *name;
+        struct test_file_metadata *next;
+        struct test_case_metadata *tests;
+};
+
+struct test_file_metadata __attribute__((weak)) * test_file_head;
+
+#define SET_FAILURE(_message, _owned)                                                    \
+        metadata->failure = (struct test_failure) {                                      \
+                .present = true,                                                         \
+                .message = _message,                                                     \
+                .file    = __FILE__,                                                     \
+                .line    = __LINE__,                                                     \
+                .owned   = _owned,                                                       \
+        }
+
+#define TEST_EQUAL(a, b)                                                                 \
+        do {                                                                             \
+                if ((a) != (b)) {                                                        \
+                        SET_FAILURE(#a " != " #b, false);                                \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_NOTEQUAL(a, b)                                                              \
+        do {                                                                             \
+                if ((a) == (b)) {                                                        \
+                        SET_FAILURE(#a " == " #b, false);                                \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_TRUE(a)                                                                     \
+        do {                                                                             \
+                if (!(a)) {                                                              \
+                        SET_FAILURE(#a " is not true", false);                           \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_STREQUAL(a, b)                                                              \
+        do {                                                                             \
+                if (strcmp(a, b) != 0) {                                                 \
+                        const char *test_strequal__part2 = " != " #b;                    \
+                        size_t test_strequal__len =                                      \
+                            strlen(a) + strlen(test_strequal__part2) + 3;                \
+                        char *test_strequal__buf = malloc(test_strequal__len);           \
+                        snprintf(test_strequal__buf, test_strequal__len, "\"%s\"%s", a,  \
+                                 test_strequal__part2);                                  \
+                        SET_FAILURE(test_strequal__buf, true);                           \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_STRNEQUAL(a, b, len)                                                        \
+        do {                                                                             \
+                if (strncmp(a, b, len) != 0) {                                           \
+                        const char *test_strnequal__part2 = " != " #b;                   \
+                        size_t test_strnequal__len2 =                                    \
+                            len + strlen(test_strnequal__part2) + 3;                     \
+                        char *test_strnequal__buf = malloc(test_strnequal__len2);        \
+                        snprintf(test_strnequal__buf, test_strnequal__len2,              \
+                                 "\"%.*s\"%s", (int)len, a, test_strnequal__part2);      \
+                        SET_FAILURE(test_strnequal__buf, true);                          \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_STREQUAL3(str, expected, len)                                               \
+        do {                                                                             \
+                if (len != strlen(expected) || strncmp(str, expected, len) != 0) {       \
+                        const char *test_strequal3__part2 = " != " #expected;            \
+                        size_t test_strequal3__len2 =                                    \
+                            len + strlen(test_strequal3__part2) + 3;                     \
+                        char *test_strequal3__buf = malloc(test_strequal3__len2);        \
+                        snprintf(test_strequal3__buf, test_strequal3__len2,              \
+                                 "\"%.*s\"%s", (int)len, str, test_strequal3__part2);    \
+                        SET_FAILURE(test_strequal3__buf, true);                          \
+                        return;                                                          \
+                }                                                                        \
+        } while (0)
+
+#define TEST_CASE(_name)                                                                  \
+        static void __test_h_##_name(struct test_case_metadata *,                         \
+                                     struct test_file_metadata *);                        \
+        static struct test_file_metadata __test_h_file##_name;                            \
+        static struct test_case_metadata __test_h_meta_##_name = {                        \
+            .fn = __test_h_##_name,                                                       \
+            .failure = {},                                                                \
+            .name = #_name,                                                               \
+            .next = 0,                                                                    \
+        };                                                                                \
+        static void __attribute__((constructor(101))) __test_h_##_name##_register(void) { \
+                __test_h_meta_##_name.next = __test_h_file##_name.tests;                  \
+                __test_h_file##_name.tests = &__test_h_meta_##_name;                      \
+                if (!__test_h_file##_name.registered) {                                   \
+                        __test_h_file##_name.name = __FILE__;                             \
+                        __test_h_file##_name.next = test_file_head;                       \
+                        test_file_head = &__test_h_file##_name;                           \
+                        __test_h_file##_name.registered = true;                           \
+                }                                                                         \
+        }                                                                                 \
+        static void __test_h_##_name(                                                     \
+            struct test_case_metadata *metadata __attribute__((unused)),                  \
+            struct test_file_metadata *file_metadata __attribute__((unused)))
+
+extern void __attribute__((weak)) (*test_h_unittest_setup)(void);
+/// Run defined tests, return true if all tests succeeds
+/// @param[out] tests_run if not NULL, set to whether tests were run
+static inline void __attribute__((constructor(102))) run_tests(void) {
+        bool should_run = false;
+#ifdef USE_SYSCTL_FOR_ARGS
+        int mib[] = {
+                CTL_KERN,
+#if defined(__NetBSD__) || defined(__OpenBSD__)
+                KERN_PROC_ARGS,
+                getpid(),
+                KERN_PROC_ARGV,
+#else
+                KERN_PROC,
+                KERN_PROC_ARGS,
+                getpid(),
+#endif
+        };
+        char *arg = NULL;
+        size_t arglen;
+        sysctl(mib, sizeof(mib) / sizeof(mib[0]), NULL, &arglen, NULL, 0);
+        arg = malloc(arglen);
+        sysctl(mib, sizeof(mib) / sizeof(mib[0]), arg, &arglen, NULL, 0);
+#else
+        FILE *cmdlinef = fopen("/proc/self/cmdline", "r");
+        char *arg = NULL;
+        int arglen;
+        fscanf(cmdlinef, "%ms%n", &arg, &arglen);
+        fclose(cmdlinef);
+#endif
+        for (char *pos = arg; pos < arg + arglen; pos += strlen(pos) + 1) {
+                if (strcmp(pos, "--unittest") == 0) {
+                        should_run = true;
+                        break;
+                }
+        }
+        free(arg);
+
+        if (!should_run) {
+                return;
+        }
+
+        if (&test_h_unittest_setup) {
+                test_h_unittest_setup();
+        }
+
+        struct test_file_metadata *i = test_file_head;
+        int failed = 0, success = 0;
+        while (i) {
+                fprintf(stderr, "Running tests from %s:\n", i->name);
+                struct test_case_metadata *j = i->tests;
+                while (j) {
+                        fprintf(stderr, "\t%s ... ", j->name);
+                        j->failure.present = false;
+                        j->fn(j, i);
+                        if (j->failure.present) {
+                                fprintf(stderr, "failed (%s at %s:%d)\n", j->failure.message,
+                                        j->failure.file, j->failure.line);
+                                if (j->failure.owned) {
+                                        free((char *)j->failure.message);
+                                        j->failure.message = NULL;
+                                }
+                                failed++;
+                        } else {
+                                fprintf(stderr, "passed\n");
+                                success++;
+                        }
+                        j = j->next;
+                }
+                fprintf(stderr, "\n");
+                i = i->next;
+        }
+        int total = failed + success;
+        fprintf(stderr, "Test results: passed %d/%d, failed %d/%d\n", success, total,
+                failed, total);
+        exit(failed == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
+}
+
+#else
+
+#include <stdbool.h>
+
+#define TEST_CASE(name) static void __attribute__((unused)) __test_h_##name(void)
+
+#define TEST_EQUAL(a, b)                                                                 \
+        (void)(a);                                                                       \
+        (void)(b)
+#define TEST_NOTEQUAL(a, b)                                                                 \
+        (void)(a);                                                                       \
+        (void)(b)
+#define TEST_TRUE(a) (void)(a)
+#define TEST_STREQUAL(a, b)                                                              \
+        (void)(a);                                                                       \
+        (void)(b)
+#define TEST_STRNEQUAL(a, b, len)                                                        \
+        (void)(a);                                                                       \
+        (void)(b);                                                                       \
+        (void)(len)
+#define TEST_STREQUAL3(str, expected, len)                                               \
+        (void)(str);                                                                     \
+        (void)(expected);                                                                \
+        (void)(len)
+#endif