Research shaders for compositor ops

assets/blit.slang

@@ -8,7 +8,8 @@
 struct BlitData {
 	float4 rect;   // NDC fit rect: x0, y0, x1, y1
 	uint   selected;
-	uint   pad[3];
+	uint   mode;   // 0 = sample texture, 1 = solid background
+	uint   pad[2];
 };
 
 [[vk::push_constant]]
@@ -33,5 +34,8 @@ VSOutput main(uint vertexIndex : SV_VertexID) {
 
 [shader("fragment")]
 float4 main(VSOutput input) {
+	if (blit.mode == 1) {
+		return float4(0.18, 0.18, 0.18, 1.0);
+	}
 	return textures[NonUniformResourceIndex(blit.selected)].Sample(input.UV);
 }

documents/session-logs/2026-08-09.md

@@ -0,0 +1,59 @@
+# Session Log — 2026-08-09
+
+## Background colour change (blit shader)
+
+- User requested changing the letterbox/pillarbox background from black to neutral grey.
+- Initial attempt: changed the render pass clear color to `(0.5, 0.5, 0.5, 1.0)`. This
+  triggered the NVIDIA validation layer warning
+  `BestPractices-NVIDIA-ClearColor-NotCompressed` — SRGB fast clears only work
+  with 0.0 or 1.0 on NVIDIA tile-based GPUs.
+- Reverted the clear color and implemented the proper solution: draw a fullscreen
+  grey quad in the fragment shader before the texture quad. The render pass clear
+  stays at 0.0 (fast-compressed).
+- Added `mode` field to `BlitData` push constant. Mode 0 samples the texture, mode
+  1 outputs solid grey.
+- User noted that a branch in the shader is free (no warp divergence since `mode`
+  is uniform per draw call). Agreed — no need for a separate clear pipeline.
+- Changed grey from 0.5 to 0.18 (18% grey card, standard in photography/compositing).
+- Fixed a Slang compilation warning by updating the profile from `spirv_1_4` to
+  `spirv_1_6` and explicitly declaring the required capabilities.
+
+## Shader filter node research
+
+- User requested research on: Gaussian blur, CDL, Laplacian, Sobel, sharpen,
+  posterize, pixelize, Kuwahara.
+- Launched a research agent that produced `documents/research/shader-filters.md`
+  covering all filters with formulas, Slang pseudocode, parameter tables, and
+  performance notes.
+
+## Design decisions made during review
+
+1. **Colour space**: all intermediate textures are linear float
+   (`R16G16B16A16_SFLOAT`, `R32G32B32A32_SFLOAT` for Kuwahara tensor). sRGB images
+   are linearized once at load by the Read node. Final blit to sRGB swapchain
+   handles display encoding.
+
+2. **Alpha**: premultiplied everywhere by default. Explicit Unpremult/Premult
+   nodes for operations that need unpremultiplied values (Nuke model).
+
+3. **Edge handling**: per-node parameter, clamp-to-edge default, clamp-to-border
+   option. Affects sampler state, not shader branches.
+
+4. **Premult has no parameters**: removed the empty push constant struct.
+
+## Research document fixes
+
+- Fixed a contradictory sentence about push constant sizes and CDL block size.
+- Added Unpremult (§9) and Premult (§10) sections with full implementations.
+- Added `edge_mode` field to all 5 spatial filter push constant blocks (Gaussian,
+  Laplacian, Sobel, Sharpen, Kuwahara).
+- Restructured the implications section (§12) into open items vs resolved decisions.
+- Expanded all mathematics sections with plain-language explanations suitable for
+  someone without a strong math background.
+
+## Open items for next session
+
+- Begin implementing the actual shader nodes in Prism
+- Node system needs: per-pass resource signatures, scratch texture hooks, per-node
+  sampler choice, compile-time-bounded loop limits
+- Classic Kuwahara is the recommended first implementation (single pass)

justfile

@@ -33,7 +33,15 @@ vendor:
 # Compile shaders from .slang to SPIR-V
 shaders:
 	mkdir -p {{BUILDDIR}}/shaders
-	{{VK_SDK}}/bin/slangc -target spirv -profile spirv_1_4 \
+	{{VK_SDK}}/bin/slangc -target spirv \
+		-profile spirv_1_6+\
+			SPV_GOOGLE_user_type+\
+			spvFragmentFullyCoveredEXT+\
+			spvDerivativeControl+\
+			spvImageQuery+\
+			spvImageGatherExtended+\
+			spvSparseResidency+\
+			spvMinLod \
 		-o {{BUILDDIR}}/shaders/blit.spv assets/blit.slang
 
 # Build all objects, then link

src/main.cpp

@@ -50,7 +50,8 @@ struct TextureResources {
 struct BlitData {
 	f32 rect[4];   // NDC fit rect: x0, y0, x1, y1
 	u32 selected;
-	u32 pad[3];
+	u32 mode;      // 0 = sample texture, 1 = solid background
+	u32 pad[2];
 };
 
 // Typedefs for wapp arrays of our types
@@ -481,12 +482,28 @@ int main() {
 			prRhiCmdBindDescriptorSets(cb, PR_RHI_PIPELINE_BIND_POINT_GRAPHICS,
 			                           app.pipeline_layout, 0, sets);
 
+			// Draw background (fullscreen grey quad)
+			BlitData bg = {};
+			bg.rect[0] = -1.0f;
+			bg.rect[1] = -1.0f;
+			bg.rect[2] =  1.0f;
+			bg.rect[3] =  1.0f;
+			bg.selected = 0;
+			bg.mode     = 1;
+			prRhiCmdPushConstants(cb, app.pipeline_layout,
+			                      (PrRhiShaderStage)(PR_RHI_SHADER_STAGE_VERTEX |
+			                                         PR_RHI_SHADER_STAGE_FRAGMENT),
+			                      0, sizeof(BlitData), &bg);
+			prRhiCmdDraw(cb, 4, 1, 0, 0);
+
+			// Draw texture (contain-fit)
 			BlitData blit = {};
 			blit.rect[0] = fit_rect[0];
 			blit.rect[1] = fit_rect[1];
 			blit.rect[2] = fit_rect[2];
 			blit.rect[3] = fit_rect[3];
 			blit.selected = app.selected;
+			blit.mode     = 0;
 			prRhiCmdPushConstants(cb, app.pipeline_layout,
 			                      (PrRhiShaderStage)(PR_RHI_SHADER_STAGE_VERTEX |
 			                                         PR_RHI_SHADER_STAGE_FRAGMENT),