WebGL auto clear color?

[PrincessGod]

Hi, @greggman .

I found in this sample, when comment these lines

72    //webglUtils.resizeCanvasToDisplaySize(gl.canvas);
73
74    // Tell WebGL how to convert from clip space to pixels
75    //gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);
76
77    // Clear the canvas.
78    //gl.clear(gl.COLOR_BUFFER_BIT);

drawScene() still clear history draw, do you know the reason?

Thank you.

[greggman]

Sorry I guess I never covered that.

Yes, WebGL by default clears the canvas by after the canvas has been compositied.

If you want it not to clear the canvas you need to add preserveDrawingBuffer: true when creating the context as in

const gl = someCanvas.getContext("webgl", {preserveDrawingBuffer: true});

The longer version is:

If you draw to the canvas (gl.drawXXX, gl.clear) then the browser marks that the canvas needs to be recompositied with the page. To do this either either makes a copy of the canvas or swaps the canvas's storage with another storage buffer (double buffered). After the browser has compositied the result it clears the canvas. It does this so that the result is consistent whether or not it copies or it swaps.

[PrincessGod]

Really helpful, thank you very mach!