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>

10 files changed, 76 insertions, 35 deletions

diff --git a/PROJECT_CONTEXT.md b/PROJECT_CONTEXT.md
index 93e67ca..6b4468f 100644
--- a/PROJECT_CONTEXT.md
+++ b/PROJECT_CONTEXT.md
@@ -34,7 +34,7 @@
 - **Timing System:** **Beat-based timelines** for musical synchronization. Sequences defined in beats, converted to seconds at runtime. Effects receive both physical time (constant) and beat time (musical). Variable tempo affects audio only. See `doc/BEAT_TIMING.md`.
 - **Workspace system:** Multi-workspace support. Easy switching with `-DDEMO_WORKSPACE=<name>`. Organized structure: `music/`, `weights/`, `obj/`, `shaders/`. Shared common shaders in `src/shaders/`. See `doc/WORKSPACE_SYSTEM.md`.
 - **Audio:** Sample-accurate sync. Zero heap allocations per frame. Variable tempo. OLA-IDCT synthesis (v2 .spec): Hann analysis window, rectangular synthesis, 50% overlap, click-free. V1 (raw DCT-512) preserved for generated notes. .spec files regenerated as v2.
-- **Shaders:** Parameterized effects (UniformHelper, .seq syntax). Beat-synchronized animation support (`beat_time`, `beat_phase`). Modular WGSL composition with ShaderComposer. 27 shared common shaders (math, render, compute). Reusable snippets: `render/scratch_lines`, `math/color` (YIQ/NTSC), `math/color_c64` (C64 palette, Bayer dither, border animation).
+- **Shaders:** Parameterized effects (UniformHelper, .seq syntax). Beat-synchronized animation support (`beat_time`, `beat_phase`). Modular WGSL composition with ShaderComposer. 27 shared common shaders (math, render, compute). Reusable snippets: `render/scratch_lines`, `render/ntsc_common` (NTSC signal processing, RGB and YIQ input variants via `sample_ntsc_signal` hook), `math/color` (YIQ/NTSC), `math/color_c64` (C64 palette, Bayer dither, border animation).
 - **3D:** Hybrid SDF/rasterization with BVH. Binary scene loader. Blender pipeline.
 - **Effects:** CNN post-processing: CNNEffect (v1) and CNNv2Effect operational. CNN v2: sigmoid activation, storage buffer weights (~3.2 KB), 7D static features, dynamic layers. Training stable, convergence validated.
 - **Tools:** CNN test tool operational. Texture readback utility functional. Timeline editor (web-based, beat-aligned, audio playback).
diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index 2e229cb..9299a93 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -31,7 +31,7 @@ Completed task archive. See `doc/archive/` for detailed historical documents.
 
 ## March 2026
 
-- [x] **NTSC post-process effect** — Fisheye distortion + NTSC scan-line simulation as `WgslEffect` thin wrapper. Files: `src/effects/ntsc.wgsl`, `ntsc_effect.h`. Tests: 35/35.
+- [x] **NTSC post-process effect** — Fisheye distortion + NTSC scan-line simulation as `WgslEffect` thin wrappers. Common logic in `render/ntsc_common` snippet (`sample_ntsc_signal` hook). Two variants: `ntsc_rgb.wgsl` (RGB→YIQ internally, `Ntsc`) and `ntsc_yiq.wgsl` (YIQ passthrough, `NtscYiq`, for RotatingCube output). Files: `src/effects/ntsc_rgb.wgsl`, `ntsc_yiq.wgsl`, `src/shaders/render/ntsc_common.wgsl`, `ntsc_effect.h`. Tests: 36/36.
 
 - [x] **Scratch post-process effect** — Film-scratch overlay effect. Scratch logic extracted into reusable snippet `src/shaders/render/scratch_lines.wgsl` (`#include "render/scratch_lines"`). Applied to first two timeline sequences. Tests: 35/35.
 
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;
diff --git a/workspaces/main/assets.txt b/workspaces/main/assets.txt
index a3b1165..b50f2fb 100644
--- a/workspaces/main/assets.txt
+++ b/workspaces/main/assets.txt
@@ -94,7 +94,9 @@ SHADER_SCENE1, WGSL, ../../src/effects/scene1.wgsl, "Scene1 effect shader"
 SHADER_SCENE2, WGSL, ../../src/effects/scene2.wgsl, "Scene2 effect shader"
 SHADER_RENDER_SCRATCH_LINES, WGSL, ../../src/shaders/render/scratch_lines.wgsl, "Film scratch lines snippet"
 SHADER_SCRATCH, WGSL, ../../src/effects/scratch.wgsl, "Scratch effect shader"
-SHADER_NTSC, WGSL, ../../src/effects/ntsc.wgsl, "NTSC effect shader"
+SHADER_NTSC_RGB, WGSL, ../../src/effects/ntsc_rgb.wgsl, "NTSC effect shader (RGB input)"
+SHADER_NTSC_YIQ, WGSL, ../../src/effects/ntsc_yiq.wgsl, "NTSC effect shader (YIQ input)"
+SHADER_RENDER_NTSC_COMMON, WGSL, ../../src/shaders/render/ntsc_common.wgsl, "NTSC shared constants and functions snippet"
 SHADER_DEBUG_DEBUG_PRINT, WGSL, ../../src/shaders/debug/debug_print.wgsl, "Debug print snippet"
 
 # --- Sequence Shaders ---