480305 - [c3d] glReadPixels support

Status: RESOLVED WORKSFORME

(Core :: Graphics: CanvasWebGL, enhancement)

Description

[Ilmari Heikkinen]

User-Agent:       Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2a1pre) Gecko/20090226 Minefield/3.2a1pre
Build Identifier: 

There's no gl.readPixels in the canvas3d extension. readPixels is useful for reading from the currently bound framebuffer (not necessarily the visible image), which makes it useful for things like picking by unique color and reading back results of off-screen computation.

Reproducible: Always

Steps to Reproduce:
1. gl.readPixels(0,0,1,1, gl.RGBA, gl.UNSIGNED_BYTE)

Actual Results:  
Error, no method named readPixels.

Expected Results:  
Should return an ImageData object {width: 1, height: 1, data: [r,g,b,a]}.

Comment 1

[Ilmari Heikkinen]

Attachment: gl.readPixels implementation

Comment 2

[Ilmari Heikkinen]

Attachment: Testcase for gl.readPixels

Renders to an FBO, reads from it and uses putImageData to draw the read image on a 2D canvas. Should have a large blank canvas on the left and a small canvas with a clipped downwards-pointing green rectangle on a yellow field.

Comment 3

[Ilmari Heikkinen]

s/rectangle/triangle

Comment 4

[Ilmari Heikkinen]

The above patch copies PRBool IsCallerTrustedForRead() over from nsContentUtils.cpp, as I had no luck with #include "nsContentUtils.h". If someone more familiar with the system manages to make the include work, they shall be praised, and the copied implementation removed.

Another issue is that glReadPixels returns an y-flipped image. I don't know if it should be flipped or left to user.

And I don't know if the data array pixel component order remains r,g,b,a on big-endian, so that should be tested.

Comment 5

[Ilmari Heikkinen]

Attachment: Updated patch, uses CheckSaneSubrectSize from nsCanvasRenderingContext2D.cpp

Comment 6

[Ilmari Heikkinen]

Attachment: Updated test, as glReadPixels now errors on (0,0,0,0) like getImageData

Comment 7

[Arpad Borsos [:Swatinem]]

Closing FIXED as the WebGL support in Gecko does support a readPixels function.