ntsc: factor common code into snippet; add RGB and YIQ input variants

- Extract shared NTSC logic into render/ntsc_common.wgsl snippet
- sample_ntsc_signal() hook decouples input format from processing
- ntsc_rgb.wgsl: RGB input (converts via rgba_to_luma_chroma_phase)
- ntsc_yiq.wgsl: YIQ passthrough for RotatingCube output
- Add NtscYiq WgslEffect thin wrapper; register both in tests

handoff(Claude): NTSC refactor complete; NtscYiq ready for timeline use with RotatingCube.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

7 files changed, 71 insertions, 32 deletions

diff --git a/src/effects/ntsc_effect.h b/src/effects/ntsc_effect.h
index d8624ea..4737291 100644
--- a/src/effects/ntsc_effect.h
+++ b/src/effects/ntsc_effect.h
@@ -1,12 +1,22 @@
-// NTSC post-process effect with fisheye distortion
+// NTSC post-process effects: RGB and YIQ input variants.
 #pragma once
 #include "effects/shaders.h"
 #include "gpu/wgsl_effect.h"
 
+// RGB input: converts RGB → YIQ internally (standard post-process use).
 struct Ntsc : public WgslEffect {
   Ntsc(const GpuContext& ctx, const std::vector<std::string>& inputs,
        const std::vector<std::string>& outputs, float start_time,
        float end_time)
       : WgslEffect(ctx, inputs, outputs, start_time, end_time,
-                   ntsc_shader_wgsl) {}
+                   ntsc_rgb_shader_wgsl) {}
+};
+
+// YIQ input: input texture already stores luma/chroma/phase (e.g. RotatingCube output).
+struct NtscYiq : public WgslEffect {
+  NtscYiq(const GpuContext& ctx, const std::vector<std::string>& inputs,
+          const std::vector<std::string>& outputs, float start_time,
+          float end_time)
+      : WgslEffect(ctx, inputs, outputs, start_time, end_time,
+                   ntsc_yiq_shader_wgsl) {}
 };
diff --git a/src/effects/ntsc_rgb.wgsl b/src/effects/ntsc_rgb.wgsl
new file mode 100644
index 0000000..09adbf1
--- /dev/null
+++ b/src/effects/ntsc_rgb.wgsl
@@ -0,0 +1,20 @@
+// NTSC post-process effect (RGB input): fisheye distortion, scanlines, color bleeding.
+// Input texture is in RGB color space; converted to YIQ for NTSC processing.
+#include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"
+#include "math/noise"
+#include "math/color"
+#include "math/color_c64"
+#include "debug/debug_print"
+
+@group(0) @binding(0) var input_sampler: sampler;
+@group(0) @binding(1) var input_texture: texture_2d<f32>;
+@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
+
+// RGB input: sample texture and convert to luma/chroma/phase
+fn sample_ntsc_signal(uv: vec2f) -> vec4f {
+  let rgba = textureSample(input_texture, input_sampler, uv);
+  return rgba_to_luma_chroma_phase(rgba, uv.y, YSIZE);
+}
+
+#include "render/ntsc_common"
diff --git a/src/effects/ntsc_yiq.wgsl b/src/effects/ntsc_yiq.wgsl
new file mode 100644
index 0000000..8ab36d3
--- /dev/null
+++ b/src/effects/ntsc_yiq.wgsl
@@ -0,0 +1,19 @@
+// NTSC post-process effect (YIQ input): fisheye distortion, scanlines, color bleeding.
+// Input texture already stores luma/chroma/phase (e.g. from RotatingCube output).
+#include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"
+#include "math/noise"
+#include "math/color"
+#include "math/color_c64"
+#include "debug/debug_print"
+
+@group(0) @binding(0) var input_sampler: sampler;
+@group(0) @binding(1) var input_texture: texture_2d<f32>;
+@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
+
+// YIQ input: texture already stores luma/chroma/phase — pass through directly
+fn sample_ntsc_signal(uv: vec2f) -> vec4f {
+  return textureSample(input_texture, input_sampler, uv);
+}
+
+#include "render/ntsc_common"
diff --git a/src/effects/shaders.cc b/src/effects/shaders.cc
index 44dfa0b..8a81bb0 100644
--- a/src/effects/shaders.cc
+++ b/src/effects/shaders.cc
@@ -55,6 +55,8 @@ void InitShaderComposer() {
                      AssetId::ASSET_SHADER_DEBUG_DEBUG_PRINT);
   register_if_exists("render/scratch_lines",
                      AssetId::ASSET_SHADER_RENDER_SCRATCH_LINES);
+  register_if_exists("render/ntsc_common",
+                     AssetId::ASSET_SHADER_RENDER_NTSC_COMMON);
   register_if_exists("render/fullscreen_vs",
                      AssetId::ASSET_SHADER_RENDER_FULLSCREEN_VS);
   register_if_exists("render/fullscreen_uv_vs",
@@ -103,7 +105,8 @@ const char* flash_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_FLASH);
 const char* scene1_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_SCENE1);
 const char* scene2_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_SCENE2);
 const char* scratch_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_SCRATCH);
-const char* ntsc_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_NTSC);
+const char* ntsc_rgb_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_NTSC_RGB);
+const char* ntsc_yiq_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_NTSC_YIQ);
 
 // Compute shaders
 const char* gen_noise_compute_wgsl =
diff --git a/src/effects/shaders.h b/src/effects/shaders.h
index 0686b7e..527a8a3 100644
--- a/src/effects/shaders.h
+++ b/src/effects/shaders.h
@@ -17,7 +17,8 @@ extern const char* flash_shader_wgsl;
 extern const char* scene1_shader_wgsl;
 extern const char* scene2_shader_wgsl;
 extern const char* scratch_shader_wgsl;
-extern const char* ntsc_shader_wgsl;
+extern const char* ntsc_rgb_shader_wgsl;
+extern const char* ntsc_yiq_shader_wgsl;
 
 // Compute shaders
 extern const char* gen_noise_compute_wgsl;
diff --git a/src/effects/ntsc.wgsl b/src/shaders/render/ntsc_common.wgsl
index 3ee02bc..61ea254 100644
--- a/src/effects/ntsc.wgsl
+++ b/src/shaders/render/ntsc_common.wgsl
@@ -1,11 +1,5 @@
-// NTSC post-process effect: fisheye distortion, scanlines, and color bleeding.
-// Produces a retro CRT/NTSC look using YIQ color space and C64-style dithering.
-#include "sequence_uniforms"
-#include "render/fullscreen_uv_vs"
-#include "math/noise"
-#include "math/color"
-#include "math/color_c64"
-#include "debug/debug_print"
+// Shared NTSC post-process constants and functions.
+// Requires sample_ntsc_signal(uv: vec2f) -> vec4f to be defined by the includer.
 
 const PI = 3.14159265;
 const TAU = 6.28318530718;
@@ -26,11 +20,6 @@ const CROSS_TALK = 0.1;
 const CROSS_Y_INTERFERENCE = 30.;
 const CHROMA_MOD_FREQ = (0.4 * PI);
 
-
-@group(0) @binding(0) var input_sampler: sampler;
-@group(0) @binding(1) var input_texture: texture_2d<f32>;
-@group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
-
 // Barrel (fisheye) distortion: strength > 0 = barrel, < 0 = pincushion
 fn fisheye(uv: vec2f, scale: f32) -> vec2f {
   const strength = vec2f(0.1, 0.24);
@@ -45,20 +34,6 @@ fn vignette(uv: vec2f) -> f32 {
   return smoothstep(0., vignetteSmoothness, amount);
 }
 
-// returns Luma, chroma subcarrier level, phase, transparency
-fn get_luma_chroma_phase_a(uv: vec2f) -> vec4f {
-  let rgba = textureSample(input_texture, input_sampler, uv);
-  return rgba_to_luma_chroma_phase(rgba, uv.y, YSIZE);
-}
-
-fn get_value(uv: vec2f, off: f32, yscale: f32) -> vec4f {
-  return get_luma_chroma_phase_a(uv + off * vec2f(1., yscale));
-}
-
-fn peak(y: f32, ypos: f32, scale: f32) -> f32 {
-  return clamp((y - 1.) * scale * log(abs(y - ypos)), 0.0, 1.0);
-}
-
 // 6-taps Luma horizontal filtering
 //   fs = 3.84 MHz (Nyquist 1.92 MHz)
 //   Passband: 0–2.8 MHz
@@ -80,6 +55,14 @@ const chroma_filter = array<f32, 2 * 6 + 1>(
    0.0000,  0.3456,  0.2984,  0.1678,  0.0234, -0.0892
 );
 
+fn get_value(uv: vec2f, off: f32, yscale: f32) -> vec4f {
+  return sample_ntsc_signal(uv + off * vec2f(1., yscale));
+}
+
+fn peak(y: f32, ypos: f32, scale: f32) -> f32 {
+  return clamp((y - 1.) * scale * log(abs(y - ypos)), 0.0, 1.0);
+}
+
 fn get_signal(uv: vec2f, d: f32) -> vec4f {
   var signal = vec4f(0.0);
   for (var i = 0; i <= 12; i += 1) {
@@ -88,7 +71,7 @@ fn get_signal(uv: vec2f, d: f32) -> vec4f {
     let sumc = chroma_filter[i] * get_value(uv, offset * d * CHROMA_SIZE, 0.67);
     signal += vec4f(suml.x, sumc.y, sumc.z, suml.a);
   }
-  let base = get_luma_chroma_phase_a(uv);
+  let base = sample_ntsc_signal(uv);
   return mix(signal, base, vec4f(LUMA_BLUR, CHROMA_BLUR, CHROMA_BLUR, 1.));
 }
 
diff --git a/src/tests/gpu/test_demo_effects.cc b/src/tests/gpu/test_demo_effects.cc
index b9d1463..9d8cb7d 100644
--- a/src/tests/gpu/test_demo_effects.cc
+++ b/src/tests/gpu/test_demo_effects.cc
@@ -76,6 +76,9 @@ static void test_effects() {
       {"Ntsc", std::make_shared<Ntsc>(
                    fixture.ctx(), std::vector<std::string>{"source"},
                    std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
+      {"NtscYiq", std::make_shared<NtscYiq>(
+                      fixture.ctx(), std::vector<std::string>{"source"},
+                      std::vector<std::string>{"sink"}, 0.0f, 1000.0f)},
   };
 
   int passed = 0;