# `react-doctor/webgl-no-sync-readback-in-animation-loop`

Synchronous GPU readback inside animation loop

- **Category:** Performance
- **Severity:** warn
- **Source:** oxlint-plugin-react-doctor
- **Framework:** global
- **Enabled when:** always

## Validate the diagnostic

Confirm the reported code matches this rule before you edit it.

Confirm the reported code matches `react-doctor/webgl-no-sync-readback-in-animation-loop`. Compare it with the Before example and verify the same API, framework, and execution context. Treat it as a false positive only when one of those conditions differs.

## Compare the code

The corrected pattern shows a focused way to address the diagnostic.

### Reported pattern

```ts
const gl = canvas.getContext("webgl2");
const pixel = new Uint8Array(4);
const animate = () => {
  gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
  requestAnimationFrame(animate);
};
```

### Corrected pattern

```ts
const gl = canvas.getContext("webgl2");
const pixel = new Uint8Array(4);
const animate = () => {
  requestAnimationFrame(animate);
};
const samplePixel = () => {
  gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
};
canvas.addEventListener("click", samplePixel);
```

## How to fix

Use the corrected pattern as a reference. Preserve behavior that the rule does not require you to change.

### Copyable fix prompt

Copy this self-contained prompt into your coding agent after you confirm the diagnostic.

````text
Fix every confirmed `react-doctor/webgl-no-sync-readback-in-animation-loop` diagnostic in the current repository.

Required change:
- Move GPU readback to a discrete or asynchronous path and reuse the latest completed result during frames.

Reference transformation:

Before:
```ts
const gl = canvas.getContext("webgl2");
const pixel = new Uint8Array(4);
const animate = () => {
  gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
  requestAnimationFrame(animate);
};
```

After:
```ts
const gl = canvas.getContext("webgl2");
const pixel = new Uint8Array(4);
const animate = () => {
  requestAnimationFrame(animate);
};
const samplePixel = () => {
  gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
};
canvas.addEventListener("click", samplePixel);
```

Constraints:
- Confirm the reported code matches the Before pattern.
- Make the smallest change that fixes the root cause.
- Preserve behavior and interfaces unrelated to this diagnostic.
- Reuse existing project components, utilities, and conventions.
- Do not introduce render-phase side effects, render-phase state updates, or Hooks rule violations.
- Preserve resource ownership and cleanup. Do not move work into a hotter render or frame path.
- Adapt identifiers and framework details instead of copying blindly.
- Do not disable the rule or suppress matching code.

Verification:
- Run focused tests for the changed behavior.
- Run React Doctor and confirm this diagnostic no longer appears.
- Report the files changed and any checks you could not run.
````