763190 - WebGL readPixels shouldn't call EnsureBackbufferClearedAsNeeded

Status: RESOLVED FIXED

(Core :: Graphics: CanvasWebGL, defect)

Description

[Benoit Jacob [:bjacob] (mostly away)]

Attachment: readPixels should not clear the backbuffer

See this discussion:

  https://www.khronos.org/webgl/public-mailing-list/archives/1206/msg00006.html

Section 2.2 in the spec has been updated to clarify that with preserveDrawingBuffer=false, the clearing only occurs if the canvas has been invalidated by a draw operation. In this respect, readPixels is not a draw operation hence it shouldn't request a clear.

Gregg's testcase,

  http://jsfiddle.net/greggman/hDcxp/1/

clears the canvas in green and does a readPixels in a setTimeout callback. It should not result in a clearing of the buffer. Currently in Firefox it does, because our readPixels implementation calls EnsureBackbufferClearedAsNeeded. This seems unneeded, and removing it fixes it. Notice that there is no security issue here because this only applies to the default framebuffer (non-default FBOs are still initialized if they had uninitialized renderbuffers) and on context creation the default framebuffer does get cleared already.

Comment 1

[Benoit Jacob [:bjacob] (mostly away)]

https://tbpl.mozilla.org/?tree=Try&rev=8cd80ba5c8f3

Comment 2

[Benoit Jacob [:bjacob] (mostly away)]

http://hg.mozilla.org/integration/mozilla-inbound/rev/d4820f5ac825

Comment 3

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/d4820f5ac825