WebGLRenderer: Ensure readback checks work on correct attachment state.#32506
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andredsm
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Mugen87
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Dec 8, 2025
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| // when using MRT, select the correct color buffer for the subsequent read command | ||
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| if ( renderTarget.textures.length > 1 ) _gl.readBuffer( _gl.COLOR_ATTACHMENT0 + textureIndex ); |
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At least according to my understanding the change is correct. gl.getParameter( gl.IMPLEMENTATION_COLOR_READ_FORMAT ) requires a correct framebuffer/attachment setup which is not true without this PR.
Besides, the bounds check is only required when we actually read pixels so gl.readBuffer() can safely be executed at a earlier point in time.
Mugen87
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Dec 27, 2025
Mugen87
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Description
This PR fixes a bug when reading pixels from MRT. When using readRenderTargetPixels or readRenderTargetPixelsAsync, the code checks whether the WebGL context can read a pixel from the requested texture:
https://github.com/mrdoob/three.js/blob/1246386f547c5b4e64d3705debfe5a22f18cc91c/src/renderers/WebGLRenderer.js#L2916C1-L2921C7
This is done in textureFormatReadable function by using
gl.getParameter( gl.IMPLEMENTATION_COLOR_READ_FORMAT ). Nonetheless, at this point, we did not define the readBuffer yet, so the result will always return the format from COLOR_ATTACHMENT0, raising some errors when I attempt to read pixels from other color attachments. The fix was basically moving the readBuffer calling function before this check.An example of this bug: https://codepen.io/andredsm/full/jEqXVme. Check the console to see the error. In the code of this example, if you comment the second read, everything works, since we are only reading from the COLOR_ATTACHMENT0.
Finally, I made this pure WebGL2 example showing that the output from
gl.getParameter( gl.IMPLEMENTATION_COLOR_READ_FORMAT )is affected by the state ofgl.readBuffer: https://codepen.io/andredsm/pen/pvyqEqV.