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

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)