97 lines
4.8 KiB
Markdown
97 lines
4.8 KiB
Markdown
# Effects & GPU Renderer
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## How to add a new effect
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1. Create a new file in `apps/web/src/lib/effects/definitions/` (e.g. `brightness.ts`)
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2. Export an `EffectDefinition` — see `blur.ts` as a reference
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3. Register it in `apps/web/src/lib/effects/definitions/index.ts`
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An effect definition has:
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- `type` — unique string identifier
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- `name` — display name
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- `keywords` — for search
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- `params` — user-facing controls (sliders, toggles, etc.)
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- `renderer` — GPU pass templates resolved into shader identifiers + uniforms
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All effects use the shared GPU renderer. TypeScript decides which shader identifiers to run and which uniforms to pass. Rust/wgpu owns device creation, textures, and pass execution.
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## Single-pass vs multi-pass
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The renderer supports a `passes` array. Single-pass effects (e.g. color grading) just have one entry. Multi-pass is needed when an effect has to process its own output — blur (H then V), bloom (extract → blur → composite), glow, etc.
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```typescript
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renderer: {
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passes: [
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{ shader: "my-effect-shader", uniforms: ({ effectParams }) => ({ ... }) },
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],
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}
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```
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### Dynamic pass counts with `buildPasses`
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Some effects need a variable number of passes depending on their parameters (e.g. blur needs more iterations at high intensity to keep quality). For these, add a `buildPasses` function to the renderer:
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```typescript
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renderer: {
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passes: [ /* static fallback — used if buildPasses is absent */ ],
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buildPasses: ({ effectParams, width, height }) => {
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// return EffectPass[] with pre-computed uniforms
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},
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}
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```
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When `buildPasses` is present, all rendering paths use it instead of the static `passes` array. The static array is kept as a structural reference and fallback for effects that don't need dynamic pass counts.
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### Resolving passes — always use `resolveEffectPasses`
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All code that consumes effect passes should go through the helper, never access `definition.renderer.passes` directly:
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```typescript
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import { resolveEffectPasses } from "@/lib/effects";
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const passes = resolveEffectPasses({ definition, effectParams, width, height });
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```
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This handles the `buildPasses` vs static `passes` dispatch automatically.
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### Pipeline
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Linear effect chains go through `gpuRenderer.applyEffect()` in `apps/web/src/services/renderer/gpu-renderer.ts`.
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TypeScript resolves `EffectPass[]` from effect definitions. Each pass contains:
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- `shader` — a stable identifier such as `"gaussian-blur"`
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- `uniforms` — resolved numeric values for that pass
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Rust maps the shader identifier to a precompiled WGSL pipeline in `rust/crates/gpu/src/shader_registry.rs`. Non-linear GPU work such as signed-distance-field generation and mask feathering lives in dedicated Rust pipeline modules, not in TypeScript orchestration.
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## Writing shaders
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Effect-specific WGSL shaders live in `rust/crates/gpu/src/shaders/`. Add the shader file there, then register its identifier in `rust/crates/gpu/src/shader_registry.rs`.
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Available uniforms (automatically injected, no need to pass them manually):
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- `u_texture` — the input texture (sampler2D)
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- `u_resolution` — canvas size in pixels (vec2)
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Any additional uniforms come from the `uniforms()` function in the pass definition.
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**Sampling density and step scaling**
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A fixed kernel (e.g. ±30 samples) can only cover ±30 texels at step=1. When the target sigma grows beyond ~10, the kernel can't cover enough of the Gaussian curve and the result degrades into a box filter.
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The fix is a `u_step` uniform that spaces samples further apart. With step=4 the same 61-sample kernel covers ±120 texels. Bilinear texture filtering smooths the gaps between samples. For very large sigma, combine step scaling with **multi-iteration stacking** (multiple H+V pass pairs via `buildPasses`) — each iteration compounds the blur, and the effective sigma = per-pass sigma × √iterations.
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Keep the step size moderate (≤4) to avoid visible banding. If you need more blur than step=4 allows in a single iteration, add iterations instead of increasing the step further.
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```wgsl
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// u_step scales the distance between samples
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let position = f32(sample_index) * uniforms.step;
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let weight = exp(-(position * position) / (2.0 * uniforms.sigma * uniforms.sigma));
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color += textureSample(input_texture, input_sampler, uv + texel_size * uniforms.direction * position) * weight;
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```
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Do **not** use large step sizes (>6) in a single pass — it creates visible banding regardless of bilinear interpolation. Use multiple iterations instead.
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## Coordinate systems
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Source canvases are imported through `copy_external_image_to_texture()`, which is the boundary where browser canvas data enters the GPU pipeline. If a shader or import path changes, validate orientation explicitly — the renderer assumes a consistent top-left canvas origin by the time results come back to TypeScript.
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