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+# Effects & WebGL 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` — always `webgl`
+
+All effects use WebGL. Even simple single-value effects like brightness or contrast are trivial shaders — there's no reason to leave the GPU pipeline for them.
+
+## Single-pass vs multi-pass
+
+The `webgl` 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.
+
+```typescript
+renderer: {
+  type: "webgl",
+  passes: [
+    { fragmentShader: myShader, uniforms: ({ effectParams }) => ({ ... }) },
+  ],
+}
+```
+
+All WebGL rendering — both the main renderer and the effect preview — goes through `applyMultiPassEffect` in `apps/web/src/services/renderer/webgl-utils.ts`. Don't add a new rendering path somewhere else; update that function if needed.
+
+## Writing fragment shaders
+
+Shaders live in `apps/web/src/lib/effects/definitions/`. The shared vertex shader (`effect.vert.glsl`) maps clip space to UV coordinates — don't replace it unless you have a specific reason.
+
+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 — the most common mistake**
+
+Always use a step of 1 texel when sampling neighbors. Do not scale the step size with the blur radius or intensity — it creates visible discrete artifacts (ghosting/glow look) because there are large gaps between samples that the GPU fills with linear interpolation instead of your intended curve.
+
+```glsl
+// correct — step is always 1 texel, loop count controls radius
+for (int i = -30; i <= 30; i++) {
+  color += texture2D(u_texture, v_texCoord + texelSize * u_direction * float(i)) * weight;
+}
+
+// wrong — stepping 6 texels at a time looks ghosty at high intensity
+vec2 offset = texelSize * u_direction * u_radius;
+color += texture2D(u_texture, v_texCoord + offset * 2.0) * someWeight;
+```
+
+If you need a large radius with a fixed kernel size, increase the number of samples rather than the step.
+
+## Y-flip and coordinate systems
+
+Source textures (uploaded from canvas) are Y-flipped via `UNPACK_FLIP_Y_WEBGL`. Intermediate FBO textures (rendered by WebGL between passes) are not. In practice this cancels out correctly as long as you use the shared vertex shader — it maps clip space Y consistently so both texture types sample correctly.
+
+If you write a custom vertex shader or do manual coordinate math, be aware that canvas and WebGL have opposite Y origins (canvas: top-left, WebGL: bottom-left). Getting this wrong produces an upside-down result with no obvious error.