Fix #14954 - Add willReadFrequently: true to CanvasLayerRenderer context generation settings

[Menecats]

Fixes #14954 by adding willReadFrequently in context generation settings for CanvasLayerRenderer.

[mike-000]

This will disable hardware acceleration and reduce rendering performance. What are you doing to produce the warning?

[Menecats]

Hi @mike-000, I'm getting this message when reading the canvas image data directly inside a postrender event handler (see following partial code).

I've tought of adding a configuration option that would allow the user to specify the getContext options directly but I was unsure of where to place it. (my general idea was: new ImageLayer({ source: [...], contextSettings: {willReadFrequently: true}))

My code that produces the warning:

const layer = new ImageLayer({
  source: [...]
});

layer.on('prerender', e => {
  // Draw mask that will block part of the view

  e.context.globalCompositeOperation = 'source-in';
});

layer.on('postrender', e => {
  e.context.globalCompositeOperation = 'source-atop';

  // Tint part of the view

  const imageData = e.context.getImageData(0, 0, canvasWidth, canvasHeight);

  // Send the image data via presentation API to a chromecast or other low power casting device
});

[ahocevar]

In this case the warning issued by the Developer Console is not appropriate. Because you create content on the canvas, read it, and then create new content and read it again. I don't think the willReadFrequently: true flag would improve the overall performance here. It would be interesting to benchmark this use case.