# 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)