Test a blit shader implementation
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# Plan — Fullscreen Texture Blit with Contain-Fit
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## Goal
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Render a texture fullscreen with aspect-ratio preservation. If the texture's
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aspect ratio does not match the window's, draw it to fit within the window —
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never stretched, squashed, or cropped. Tall/narrow textures pillarbox (bars on
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left/right); wide textures letterbox (bars on top/bottom).
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This replaces the Suzanne mesh demo in `main.cpp`, which was for testing the RHI.
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## Fit semantics (contain — never crop)
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```
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scale = min(win_w / tex_w, win_h / tex_h)
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content_w = tex_w * scale
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content_h = tex_h * scale
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rect = centered: left = (win_w - content_w)/2, top = (win_h - content_h)/2
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```
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| Texture vs window aspect | Limiting dim | Result |
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|--------------------------|--------------|-------------------|
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| equal | — | fills exactly |
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| wider (e.g. 2048x512) | width | letterbox (T/B) |
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| taller (e.g. 512x2048) | height | pillarbox (L/R) |
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The fit rect is recomputed per frame from the selected texture's dimensions and
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the current window size, so window resize works without extra handling.
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## Changes
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### 1. RHI — texture size accessor
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Add a value struct and a by-value getter (matches `prRhiGetSurfaceCapabilities`
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pattern):
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```c
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typedef struct PrRhiTextureSize {
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u32 width;
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u32 height;
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} PrRhiTextureSize;
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PrRhiTextureSize prRhiGetTextureSize(PrRhiTexture *texture);
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```
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Files: `pr_rhi_types.h`, `pr_rhi.h`, `vulkan/pr_rhi_vk.h`,
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`vulkan/pr_rhi_vk.c`, `vulkan/pr_rhi_vk_aliases.h`.
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### 2. Offline shader compilation (drop Slang runtime)
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- New `justfile` `shaders` recipe:
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`slangc -target spirv -profile spirv_1_4 -o build/shaders/blit.spv assets/blit.slang`
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(entry points auto-detected from `[shader(...)]` attributes).
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`build` depends on it.
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- `main.cpp` loads `build/shaders/blit.spv` via wapp file I/O
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(`wpFileOpen` / `wpFileGetLength` / `wpFileRead`) into an arena buffer, then
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`prRhiCreateShader`. No `slang.h` includes, no runtime compilation.
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- Drop `-lslang` and the `-I .../slang` include from the build.
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### 3. `assets/blit.slang`
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Push constant block (32 bytes — Slang pads structs to 16-byte alignment, so the
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C++ struct carries explicit `pad[3]` to match):
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```hlsl
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struct BlitData {
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float4 rect; // NDC fit rect: x0, y0, x1, y1
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uint selected;
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uint pad[3];
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};
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```
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- **VS**: generates a 4-vertex triangle-strip quad from `SV_VertexID` (no vertex
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buffer), maps UV 0–1 into the NDC rect.
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- **FS**: `textures[NonUniformResourceIndex(selected)].Sample(uv)` — reuses the
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existing bindless descriptor array.
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### 4. `main.cpp` — clean texture viewer
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Remove everything mesh-related: tinyobj loading, vertex/index buffers,
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`ShaderData` storage buffers / device addresses, the mesh shader + pipeline,
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mouse orbit, and the mesh draw. Drop `-ltinyobjloader -lglm` from the link.
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Remove unused `assets/shader.slang`, `suzanne.obj`, `suzanne.mtl`.
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New flow: RHI init → window/instance/pdev/surface/device/swapchain → load 7
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textures → bindless descriptor set (variable count 7) → load `blit.spv` →
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blit pipeline layout (`VERTEX|FRAGMENT` 32-byte push range) → blit pipeline
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(no vertex input, `TRIANGLE_STRIP`, swapchain color format, no depth,
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`cull NONE`, dynamic viewport/scissor).
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Render loop per frame:
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- `compute_fit_rect()` from the selected texture's dims + window size
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- bind blit pipeline + descriptor set, push `BlitData{ rect, selected }`,
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`prRhiCmdDraw(cb, 4, 1, 0, 0)`
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- `+/-` cycles the selected texture; resize recomputes fit automatically
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### 5. Test textures (PIL + `build/bin/toktx`)
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Four generated KTX files in `assets/`, loaded alongside the 3 Suzanne textures
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(`texture_count = 7`, explicit path array):
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| File | Size | Shows |
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|-----------------|-----------|--------------------------|
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| `test_square.ktx` | 1024x1024 | bars on both axes |
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| `test_fill.ktx` | 1920x1080 | fills the 16:9 window |
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| `test_wide.ktx` | 2048x512 | letterbox (T/B) |
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| `test_tall.ktx` | 512x2048 | pillarbox (L/R) |
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Each with distinct gradients + a border grid so any stretch/squash is visible.
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Generated with `--genmipmap` for mip-aware sampling.
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## Verification
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`just build && just run` — each texture fits without crop/stretch, `+/-` cycles,
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window resize keeps fit, no Slang runtime in the binary.
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## Verification results (2026-08-08)
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Fit behavior confirmed by capturing the window (X11 driver) and sampling pixels:
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| Texture | Window | Result verified |
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|---------------------|---------------|------------------------------------------------|
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| square 1024x1024 | 16:9 wide | pillarbox — pure-black L/R bars, full height |
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| test_wide 2048x512 | 16:9 wide | letterbox — pure-black T/B bars, full width |
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| test_tall 512x2048 | 16:9 wide | pillarbox — pure-black L/R bars, full height |
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| test_fill 1920x1080 | 16:9 wide | fills exactly — no bars anywhere |
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| square (resized) | portrait | fit recomputed per frame — flips to letterbox |
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Texture cycling (`+/-`, `SDLK_PLUS/KP_PLUS/EQUALS`, `SDLK_MINUS/KP_MINUS`) is a
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straightforward `selected` bump in the key handler; it was reviewed but not
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exercise-tested in the headless verification env (KWin/XWayland drops XTEST
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synthesised keys). The other fit cases were verified by temporarily launching
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each texture as the initial selection.
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### Bugs found and fixed during verification
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1. **`VK_SUBOPTIMAL_KHR` aborted the app.** `prRhiAcquireNextImageVk` /
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`prRhiPresentVk` routed `SUBOPTIMAL` into `_checkVk` → `__builtin_trap()`.
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Both now treat it like `OUT_OF_DATE` (return `PR_RHI_SWAPCHAIN_OUT_OF_DATE`).
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Triggered immediately under X11/XWayland.
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2. **Swapchain recreate ignored surface extent.** `prRhiRecreateSwapchainVk`
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hard-coded the passed width/height; on surfaces whose `currentExtent` is
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meaningful (X11) that mismatched the drawable and re-looped on SUBOPTIMAL.
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Now queries `vkGetPhysicalDeviceSurfaceCapabilitiesKHR` and falls back to the
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passed size only when `currentExtent == 0xFFFFFFFF`.
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3. **App used logical window size for the swapchain.** Under HiDPI the drawable
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differs from `SDL_GetWindowSize` (1920x1080 logical → 2400x1350 drawable at
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1.25x scale), which is the root cause of #1 under XWayland. main.cpp now uses
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`SDL_GetWindowSizeInPixels` for swapchain width/height, fit math, viewport
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and scissor.
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