will-read-frequently.md

willReadFrequently

Status: explainer.

Adds a willReadFrequently options to CanvasRenderingContext2DSettings to allow signaling that this a Canvas that gets directly read a lot.

This allows UA to optimize Canvas2D for direct reading access through getImageData.

Rationale

There's an important use case for apps/games that are very reliant on getImageData. This operation is particularly slow when Canvas2D is being backed by accelerated graphics (GPU), that speed up considerably most other use cases. Historically, browsers have tried heuristics to decide if a canvas is being read frequently or not, to switch code paths. This is unreliable, complex and brittle. To prevent this, we propose a hint option that will allow developers to tell the browser that this is a canvas that rely on getImageData performance.

Proposal

dictionary CanvasRenderingContext2DSettings {
  // ... current values
  boolean willReadFrequently = false;
};

When the user sets willReadFrequently to true, the UA can optimize for read access, usually by not using the GPU for rendering.

Open issues and questions

None.

Example usage

// Javascript example
const canvas = document.createElement('canvas');
const ctx = canvas.getContext('2d', {willReadFrequently: true});

Alternatives considered

References

• https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext