Effects & GPU Renderer

How to add a new effect

1. Create a new file in apps/web/src/lib/effects/definitions/ (e.g. brightness.ts)
2. Export an EffectDefinition — see blur.ts as a reference
3. Register it in apps/web/src/lib/effects/definitions/index.ts

An effect definition has:

• type — unique string identifier
• name — display name
• keywords — for search
• params — user-facing controls (sliders, toggles, etc.)
• renderer — GPU pass templates resolved into shader identifiers + uniforms

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.

Single-pass vs multi-pass

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.

renderer: {
  passes: [
    { shader: "my-effect-shader", uniforms: ({ effectParams }) => ({ ... }) },
  ],
}

Dynamic pass counts with buildPasses

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:

renderer: {
  passes: [ /* static fallback — used if buildPasses is absent */ ],
  buildPasses: ({ effectParams, width, height }) => {
    // return EffectPass[] with pre-computed uniforms
  },
}

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.

Resolving passes — always use resolveEffectPasses

All code that consumes effect passes should go through the helper, never access definition.renderer.passes directly:

import { resolveEffectPasses } from "@/lib/effects";

const passes = resolveEffectPasses({ definition, effectParams, width, height });

This handles the buildPasses vs static passes dispatch automatically.

Pipeline

Linear effect chains go through gpuRenderer.applyEffect() in apps/web/src/services/renderer/gpu-renderer.ts.

TypeScript resolves EffectPass[] from effect definitions. Each pass contains:

• shader — a stable identifier such as "gaussian-blur"
• uniforms — resolved numeric values for that pass

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.

Writing shaders

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.

Available uniforms (automatically injected, no need to pass them manually):

• u_texture — the input texture (sampler2D)
• u_resolution — canvas size in pixels (vec2)

Any additional uniforms come from the uniforms() function in the pass definition.

Sampling density and step scaling

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.

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.

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.

// u_step scales the distance between samples
let position = f32(sample_index) * uniforms.step;
let weight = exp(-(position * position) / (2.0 * uniforms.sigma * uniforms.sigma));
color += textureSample(input_texture, input_sampler, uv + texel_size * uniforms.direction * position) * weight;

Do not use large step sizes (>6) in a single pass — it creates visible banding regardless of bilinear interpolation. Use multiple iterations instead.

Coordinate systems

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.