Impostors: rephrase to clarify barycentric interpolation

1 files changed, 23 insertions, 22 deletions

diff --git a/impostor.cginc b/impostor.cginc
index d328223..a60c9ca 100644
--- a/impostor.cginc
+++ b/impostor.cginc
@@ -45,15 +45,20 @@ float4 SampleAtlas(float2 uv, float2 cell) {
     return _ImpostorAtlas.Sample(bilinear_clamp_s, (cell + uv) / _GridResolution);
 }
 
-// Compute blend weights for 3 frames in a grid cell triangle
-// .xyz = weights for frames 1,2,3
-// .w = which triangle (0 = lower-left, 1 = upper-right)
-float4 QuadBlendWeights(float2 frac) {
+// Compute barycentric weights for a point within a grid cell.
+// The cell is a unit square split into two triangles along the (0,0)-(1,1) diagonal:
+//   Triangle 0 (lower-left):  vertices at (0,0), (0,1), (1,1)
+//   Triangle 1 (upper-right): vertices at (0,0), (1,0), (1,1)
+// Returns:
+//   .xyz = barycentric weights for the 3 triangle vertices
+//   .w   = which triangle (0 or 1)
+float4 GridCellBarycentric(float2 p) {
     float4 res;
-    res.x = min(1.0 - frac.x, 1.0 - frac.y);
-    res.y = abs(frac.x - frac.y);
-    res.z = min(frac.x, frac.y);
-    res.w = ceil(frac.x - frac.y);
+    // Branchless barycentric weights (works for both triangles due to symmetry)
+    res.x = min(1.0 - p.x, 1.0 - p.y);  // weight for (0,0) vertex
+    res.y = abs(p.x - p.y);              // weight for (0,1) or (1,0) vertex
+    res.z = min(p.x, p.y);               // weight for (1,1) vertex
+    res.w = ceil(p.x - p.y);             // triangle select: 0 if p.y >= p.x, 1 otherwise
     return res;
 }
 
@@ -98,7 +103,7 @@ struct v2f {
     nointerpolation float2 cell1 : TEXCOORD4;
     nointerpolation float2 cell2 : TEXCOORD5;
     nointerpolation float2 cell3 : TEXCOORD6;
-    float3 blendWeights : TEXCOORD8;
+    nointerpolation float3 blendWeights : TEXCOORD8;
     UNITY_VERTEX_OUTPUT_STEREO
 };
 
@@ -128,19 +133,17 @@ v2f vert(appdata v) {
     float2 gridFloor = floor(grid);
     float2 gridFrac = frac(grid);
 
-    // Compute blend weights and determine which triangle
-    float4 weights = QuadBlendWeights(gridFrac);
-    o.blendWeights = weights.xyz;
+    // Compute barycentric weights and determine which triangle
+    float4 bary = GridCellBarycentric(gridFrac);
+    o.blendWeights = bary.xyz;
 
-    // Frame 1: base corner
+    // Frame 1: (0,0) corner of cell
     o.cell1 = gridFloor;
 
-    // Frame 2: depends on which triangle (lower-left vs upper-right)
-    // weights.w = 0: lower-left triangle -> (0,1) offset
-    // weights.w = 1: upper-right triangle -> (1,0) offset
-    o.cell2 = clamp(gridFloor + lerp(float2(0,1), float2(1,0), weights.w), 0, gridRes - 1);
+    // Frame 2: (0,1) or (1,0) corner depending on which triangle
+    o.cell2 = clamp(gridFloor + lerp(float2(0,1), float2(1,0), bary.w), 0, gridRes - 1);
 
-    // Frame 3: diagonal corner
+    // Frame 3: (1,1) corner of cell
     o.cell3 = clamp(gridFloor + float2(1,1), 0, gridRes - 1);
 
     // Get directions for each frame
@@ -148,7 +151,7 @@ v2f vert(appdata v) {
     float3 dir2 = DirFromCell(o.cell2, gridRes);
     float3 dir3 = DirFromCell(o.cell3, gridRes);
 
-    // Billboard facing the camera direction (not snapped)
+    // Billboard facing the camera direction
     float3 viewWS = normalize(camPos - center);
     float3 right, up;
     BillboardBasis(viewWS, right, up);
@@ -164,12 +167,10 @@ v2f vert(appdata v) {
 #endif
 
     // Compute virtual plane UVs for all 3 frames
-    // pivotToCam and vertexToCam in object space
-    float3 pivotToCamOS = mul((float3x3)unity_WorldToObject, camPos - center);
+    float3 pivotToCamOS = mul((float3x3)unity_WorldToObject, camPos - center) * 100.0;
     float3 vertexPosOS = mul((float3x3)unity_WorldToObject, worldPos - center);
     float3 vertexToCamOS = pivotToCamOS - vertexPosOS;
 
-    // size = 0.5 because the mesh is a unit quad (-0.5 to 0.5)
     o.uv1 = VirtualPlaneUV(dir1, pivotToCamOS, vertexToCamOS, 0.5);
     o.uv2 = VirtualPlaneUV(dir2, pivotToCamOS, vertexToCamOS, 0.5);
     o.uv3 = VirtualPlaneUV(dir3, pivotToCamOS, vertexToCamOS, 0.5);