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