2d scale rot

jaesung-cs · jaesung-cs · commit aeae877ac985 · 2025-02-11T21:35:15.000+09:00
diff --git a/src/shader/projection.comp b/src/shader/projection.comp
@@ -48,7 +48,7 @@ layout(std430, set = 2, binding = 0) writeonly buffer DrawIndirect {
 };
 
 layout(std430, set = 2, binding = 1) writeonly buffer Instances {
-  vec4 instances[];  // (N, 12). 3 for ndc position, 1 padding, 4 for scale rot, 4 for color.
+  vec4 instances[];  // (N, 12). 3 for ndc position, 1 padding, 4 for rot scale, 4 for color.
 };
 
 layout(std430, set = 2, binding = 2) buffer VisiblePointCount {
@@ -130,10 +130,14 @@ void main() {
   float D = sqrt((a - b) * (a - b) + 4.f * c * c);
   float s0 = sqrt(0.5f * (a + b + D));
   float s1 = sqrt(0.5f * (a + b - D));
-  // decompose to R^T S^2 R
+  // decompose to R S^2 R^T
   float sin2t = 2.f * c / D;
   float cos2t = (a - b) / D;
   float theta = atan(sin2t, cos2t) / 2.f;
+  float cos_theta = cos(theta);
+  float sin_theta = sin(theta);
+  // R*S
+  mat2 rot_scale = mat2(s0 * cos_theta, s0 * sin_theta, -s1 * sin_theta, s1 * cos_theta);
 
   pos = projection * pos;
   pos = pos / pos.w;
@@ -184,6 +188,6 @@ void main() {
   float opacity = gaussian_opacity[id];
 
   instances[inverse_id * 3 + 0].xyz = pos.xyz;
-  instances[inverse_id * 3 + 1] = vec4(s0, s1, cos(theta), sin(theta));
+  instances[inverse_id * 3 + 1] = vec4(rot_scale[0], rot_scale[1]);
   instances[inverse_id * 3 + 2] = vec4(color, opacity);
 }
diff --git a/src/shader/splat.vert b/src/shader/splat.vert
@@ -1,7 +1,7 @@
 #version 460
 
 layout (std430, set = 1, binding = 1) readonly buffer Instances {
-  vec4 instances[];  // (N, 12). 3 for ndc position, 1 padding, 4 for scale rot, 4 for color.
+  vec4 instances[];  // (N, 12). 3 for ndc position, 1 padding, 4 for rot scale, 4 for color.
 };
 
 layout (location = 0) out vec4 out_color;
@@ -11,19 +11,16 @@ void main() {
   // index [0,1,2,2,1,3], 4 vertices for a splat.
   int index = gl_VertexIndex / 4;
   vec3 ndc_position = instances[index * 3 + 0].xyz;
-  vec2 scale = instances[index * 3 + 1].xy;
-  vec2 rot = instances[index * 3 + 1].zw;
+  mat2 rot_scale = mat2(instances[index * 3 + 1].xy, instances[index * 3 + 1].zw);
   vec4 color = instances[index * 3 + 2];
 
   // quad positions (-1, -1), (-1, 1), (1, -1), (1, 1), ccw in screen space.
   int vert_index = gl_VertexIndex % 4;
   vec2 position = vec2(vert_index / 2, vert_index % 2) * 2.f - 1.f;
 
-  mat2 rot2d = mat2(rot.x, rot.y, -rot.y, rot.x);
-
   float confidence_radius = 3.f;
 
-  gl_Position = vec4(ndc_position + vec3(rot2d * (scale * position) * confidence_radius, 0.f), 1.f);
+  gl_Position = vec4(ndc_position + vec3(rot_scale * position * confidence_radius, 0.f), 1.f);
   out_color = color;
   out_position = position * confidence_radius;
 }
