refactor: extract YIQ and C64 dither to common WGSL shaders

- math/color.wgsl: add rgba_to_yiqa, yiqa_to_rgba, rgba_to_luma_chroma_phase
- math/color_c64.wgsl: new file with C64 palette, Bayer 8x8, Dither()
- ntsc.wgsl: include both, remove local duplicates; Dither() now takes xsize/ysize

handoff(Claude): YIQ/dither helpers now reusable by other effects

1 files changed, 36 insertions, 0 deletions

diff --git a/src/shaders/math/color.wgsl b/src/shaders/math/color.wgsl
index 9352053..0639bf7 100644
--- a/src/shaders/math/color.wgsl
+++ b/src/shaders/math/color.wgsl
@@ -1,4 +1,40 @@
 // Common color space and tone mapping functions.
+// Includes: sRGB, ACES, HSV, YIQ (NTSC) conversions.
+// Use rgba_to_luma_chroma_phase() to encode YIQ into a temp buffer for NTSC-style effects.
+
+const TAU_COLOR = 6.28318530718;
+
+// RGB <-> YIQ (NTSC) color space
+fn rgba_to_yiqa(rgba: vec4f) -> vec4f {
+  const m = mat4x4f(
+    0.299,  0.596,  0.211, 0.,
+    0.587, -0.274, -0.523, 0.,
+    0.114, -0.322,  0.312, 0.,
+       0.,     0.,     0., 1.,
+  );
+  return m * rgba;
+}
+
+fn yiqa_to_rgba(yiq: vec4f) -> vec4f {
+  const m = mat4x4f(
+    1.000,  1.000,   1.000, 0.,
+    0.956, -0.272,  -1.106, 0.,
+    0.621, -0.647,   1.703, 0.,
+       0.,     0.,      0., 1.,
+  );
+  return m * yiq;
+}
+
+// Convert RGBA to packed luma/chroma/phase signal for NTSC processing.
+// Returns vec4f(luma, chroma_level, phase/TAU, alpha).
+// ysize: virtual scanline count (e.g. 33.*8.); drives 22.5-degree inter-line phase shift.
+fn rgba_to_luma_chroma_phase(rgba: vec4f, uv_y: f32, ysize: f32) -> vec4f {
+  let yiq = rgba_to_yiqa(rgba);
+  let chroma_level = length(yiq.yz);
+  let mscanl = (uv_y * ysize) % 2.0;
+  let phase = atan2(yiq.y, yiq.z) - 0.3926991 + mscanl * 0.19634954;
+  return vec4f(yiq.x, chroma_level, phase / TAU_COLOR, yiq.a);
+}
 
 // sRGB to Linear approximation
 // Note: Assumes input is in sRGB color space.