Fix perspective texture mapping

1 files changed, 75 insertions, 49 deletions

diff --git a/src/swgfx.c b/src/swgfx.c
index 612f82d..3bec663 100644
--- a/src/swgfx.c
+++ b/src/swgfx.c
@@ -297,15 +297,12 @@ static inline sgVec2i Clip(const swgfx* gfx, const sgVec2i p) {
   return max2i(lower, min2i(upper, p));
 }
 
-static inline R BarycentricInterp1(sgVec3 bar, R a, R b, R c) {
+static inline R BarycentricInterp(sgVec3 bar, R a, R b, R c) {
   return bar.x*a + bar.y*b + bar.z*c;
 }
 static inline sgVec2 BarycentricInterp2(sgVec3 bar, sgVec2 a, sgVec2 b, sgVec2 c) {
   return add2(add2(scale2(a, bar.x), scale2(b, bar.y)), scale2(c, bar.z));
 }
-static inline sgVec2 PerspectiveInterp2(sgVec3 bar, sgVec3 depths, R z, sgVec2 a, sgVec2 b, sgVec2 c) {
-  return scale2(BarycentricInterp2(div3(bar, depths), a, b, c), z);
-}
 
 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;
@@ -327,9 +324,7 @@ static inline sgVec3 Barycentric(sgVec2 p0, sgVec2 p1, sgVec2 p2, sgVec2 p) {
   return (sgVec3){a,b,c};
 }
 
-// Input triangle in screen space. Retains Z for depth testing and vertex
-// attribute interpolation.
-static void DrawTriangle2(swgfx* gfx, const sgTri3* const tri) {
+static void DrawTriangle2(swgfx* gfx, const sgTri2* const tri) {
   assert(gfx);
   assert(tri);
   const sgVec2 p0 = (sgVec2){tri->p0.pos.x, tri->p0.pos.y};
@@ -355,68 +350,104 @@ static void DrawTriangle2(swgfx* gfx, const sgTri3* const tri) {
       //   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 R z = BarycentricInterp1(bar, tri->p0.pos.z, tri->p1.pos.z, tri->p2.pos.z);
-        R* depth = Depth(gfx, x, y);
-        if ((0.f <= z) && (z <= 1.f) && (z <= *depth)) {
-          *depth = z;
-          const sgVec3  depths = (sgVec3){tri->p0.pos.z, tri->p1.pos.z, tri->p2.pos.z};
-          const sgVec2  uv     = PerspectiveInterp2(bar, depths, z, tri->p0.uv, tri->p1.uv, tri->p2.uv);
-          const sgPixel colour = Sample(gfx->texture, uv);
-          //const sgPixel colour = (sgPixel){255, 0, 255, 255};
-          // TODO: When doing lighting, need to tone-map here.
-          /*const int r = (int)(uv.x * 255.f);
-          const int g = (int)(uv.y * 255.f);
-          const sgPixel colour = (sgPixel){r, g, 255, 255};*/
-          const sgVec2i pix = (sgVec2i){x,y};
-          SetPixel(gfx, pix, colour);
-        }
+        assert((bar.x + bar.y + bar.z - 1e7) <= 1.f);
+        const sgVec2 uv = BarycentricInterp2(bar, tri->p0.uv, tri->p1.uv, tri->p2.uv);
+        const sgPixel colour = Sample(gfx->texture, uv);
+        SetPixel(gfx, (sgVec2i){x,y}, colour);
       }
     }
   }
 }
 
-static inline sgVec3 PerspDivide(sgVec4 v) {
-  return (sgVec3){v.x / v.w, v.y / v.w, v.z / v.w};
+static inline sgVec4 PerspDivide(sgVec4 v) {
+  return (sgVec4){v.x / v.w, v.y / v.w, v.z / v.w, v.w};
 }
 
 // TODO: Compute a viewport matrix in sgViewport() instead.
-static inline sgVec3 ViewportTransform(sgViewport_t vp, sgVec3 ndc) {
-  return (sgVec3){
+static inline sgVec4 ViewportTransform(sgViewport_t vp, sgVec4 ndc) {
+  return (sgVec4){
     .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,
-    .z = ndc.z};
+    .z = ndc.z*0.5f + 0.5f,
+    .w = ndc.w};
 }
 
-static inline sgVec3 ViewportToWindow(sgViewport_t vp, sgVec3 p) {
-  return (sgVec3){p.x, (R)vp.height - p.y, p.z};
+static inline sgVec4 ViewportToWindow(sgViewport_t vp, sgVec4 p) {
+  return (sgVec4){p.x, (R)vp.height - p.y, p.z, p.w};
 }
 
-static inline sgVec3 TransformPosition(const swgfx* gfx, sgVec3 p) {
+static inline sgVec4 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);
+  const sgVec4 p_ndc = PerspDivide(p_clip);
   // TODO: Clip.
-  const sgVec3 p_vp = ViewportTransform(gfx->viewport, p_ndc);
+  const sgVec4 p_vp = ViewportTransform(gfx->viewport, p_ndc);
   return ViewportToWindow(gfx->viewport, p_vp);
 }
 
 static void DrawTriangle3(swgfx* gfx, const sgTri3* const tri) {
   assert(gfx);
   assert(tri);
-  const sgVec3 p0  = TransformPosition(gfx, tri->p0.pos);
-  const sgVec3 p1  = TransformPosition(gfx, tri->p1.pos);
-  const sgVec3 p2  = TransformPosition(gfx, tri->p2.pos);
-  const sgVec2 uv0 = tri->p0.uv;
-  const sgVec2 uv1 = tri->p1.uv;
-  const sgVec2 uv2 = tri->p2.uv;
-  const sgTri3 tri_screen = (sgTri3){
-    (sgVert3){p0, uv0},
-    (sgVert3){p1, uv1},
-    (sgVert3){p2, uv2}};
-  DrawTriangle2(gfx, &tri_screen);
+  // TODO: Inline the transform here and interleave its operations to perform
+  //  backface culling and clipping as early as possible.
+  const sgVec4 p0    = TransformPosition(gfx, tri->p0.pos);
+  const sgVec4 p1    = TransformPosition(gfx, tri->p1.pos);
+  const sgVec4 p2    = TransformPosition(gfx, tri->p2.pos);
+  const sgVec2 p0_2d = (sgVec2){p0.x, p0.y};
+  const sgVec2 p1_2d = (sgVec2){p1.x, p1.y};
+  const sgVec2 p2_2d = (sgVec2){p2.x, p2.y};
+  const sgAABB2 bbox = TriangleAabb2(p0_2d, p1_2d, p2_2d);
+  // 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);
+  // Setup for perspective texture mapping.
+  // 'w' is view-space z.
+  const sgVec3 depths      = (sgVec3){p0.z, p1.z, p2.z};
+  const sgVec3 one_over_zs = (sgVec3){1.f / p0.w, 1.f / p1.w, 1.f/ p2.w};
+  const sgVec3 u_over_zs   = (sgVec3){tri->p0.uv.x / p0.w, tri->p1.uv.x / p1.w, tri->p2.uv.x / p2.w};
+  const sgVec3 v_over_zs   = (sgVec3){tri->p0.uv.y / p0.w, tri->p1.uv.y / p1.w, tri->p2.uv.y / p2.w};
+  // 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(p0_2d, p1_2d, p2_2d, 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)) {
+        assert((bar.x + bar.y + bar.z - 1e7) <= 1.f);
+        const R p_one_over_z = dot3(bar, one_over_zs);
+        const R p_u_over_z   = dot3(bar, u_over_zs);
+        const R p_v_over_z   = dot3(bar, v_over_zs);
+        const R p_depth      = dot3(bar, depths);
+        const R z            = 1.f / p_one_over_z;
+        const sgVec2 uv      = (sgVec2){p_u_over_z * z, p_v_over_z * z};
+        R* depth = Depth(gfx, x, y);
+        if ((0.f <= p_depth) && (p_depth <= 1.f) && (p_depth <= *depth)) {
+          *depth = p_depth;
+          const sgPixel colour = Sample(gfx->texture, uv);
+          // TODO: When doing lighting, need to tone-map here.
+          /*const int d = (int)(z*255.f);
+          const sgPixel colour = (sgPixel){d,d,d,255};*/
+          //const sgPixel colour = (sgPixel){255, 0, 255, 255};
+          /*const int r = (int)(uv.x * 255.f);
+          const int g = (int)(uv.y * 255.f);
+          const sgPixel colour = (sgPixel){r, g, 255, 255};*/
+          SetPixel(gfx, (sgVec2i){x,y}, colour);
+        }
+      }
+    }
+  }
 }
 
 #define is_pow2_or_0(X) ((X & (X - 1)) == 0)
@@ -585,12 +616,7 @@ void sgPixels(swgfx* gfx, size_t count, const sgVec2i* positions, sgPixel colour
 void sgTriangles2(swgfx* gfx, size_t count, const sgTri2* tris) {
   assert(gfx);
   for (size_t i = 0; i < count; ++i) {
-    const sgTri3 tri3 = (sgTri3) {
-      .p0 = (sgVert3){.pos = (sgVec3){tris[i].p0.pos.x, tris[i].p0.pos.y, 0}, .uv = tris[i].p0.uv},
-      .p1 = (sgVert3){.pos = (sgVec3){tris[i].p1.pos.x, tris[i].p1.pos.y, 0}, .uv = tris[i].p1.uv},
-      .p2 = (sgVert3){.pos = (sgVec3){tris[i].p2.pos.x, tris[i].p2.pos.y, 0}, .uv = tris[i].p2.uv},
-    };
-    DrawTriangle2(gfx, &tri3);
+    DrawTriangle2(gfx, &tris[i]);
   }
 }