Incorrect RGB display in Image Viewer when not viewing alpha channel #74709
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Reference: blender/blender#74709
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Version: 2.82a (Linux 64)
RGB_bug.blend
How to reproduce:
The above image looks as though the rim of the shape is solid grey, but in fact the explicit RGB values do fade from grey to black, due to the blur node.
When viewing an image in Color without alpha, or in Red, Green, or Blue Display mode, the image viewer seems to "pre-divide", or perhaps "un-premultiply" the RGB values, which causes incorrect values to display. This makes it very difficult for the user to work with alpha channels!
Adding an Alpha Convert node makes matters even worse:
Now, the explicit RGB values really are a solid grey, but in the image view the rim is displayed as getting brighter! Not a good workflow at all!
Added subscriber: @rocketman
Added subscriber: @mano-wii
The color that is read in the
ibuf
comes with alpha premultiplied.I don't quite understand why it happens, but I imagine it is important for the final image.
(This is an area that I'm not familiar with).
Thankyou, @mano-wii . It doesn't seem to affect the final image. If, for instance, you save the image to a file, the saved image will have the correct colors (the values explicitly stated when you mouse over the pixels), whether or not you enable alpha. It will not look like the result displayed in the viewer. The viewer is showing RGB values which don't seem to relate with how the final image looks.
Added subscriber: @ankitm
#74586 Switching between passes in the image editor can make the channel selector invalid.
Changed status from 'Needs Triage' to: 'Confirmed'
It deserves an investigation.
I'm confirming as a bug.
Added subscriber: @JacquesLucke
This is from the manual page: https://docs.blender.org/manual/en/latest/compositing/types/converter/alpha_convert.html
While I'm not entirely sure, I don't think there is a real bug here... Some time ago I heard that this is more a general design issue of how alpha is handled in the compositor.
@JacquesLucke As far as I'm aware, the compositor handles alpha perfectly fine. This is an issue specifically with how the Blender image viewer presents an image buffer when only the RGB channels are displayed. Here is a very simple example:
Here I have taken a simple color grid image, reduced the alpha, and displayed the result in the backdrop, showing only the RGB and not Alpha. The "Set Alpha" node does not (nor should it) have any effect on the RGB values. Thus, when viewing only the RGB channels, I would expect the displayed image to be unchanged (and when I save the image as RGB, it is). And yet, reducing the Alpha causes it to be shown brighter in Blender! Again, I must stress that this result does not appear in the saved image, and no other image viewer to my knowledge displays images like this.
It is as if a "Premul to Straight" operation has been performed prior to displaying the RGB image buffer in Blender. This is not, under any circumstances, useful to me as a user! When I want to see the RGB buffer without alpha, that means I want to see the RGB buffer without alpha, not what the RGB buffer would look like if I did a premul-to-straight conversion beforehand.
Added subscriber: @Jeroen-Bakker
Changed status from 'Confirmed' to: 'Resolved'
This issue has been resolved by D8234. Please report any new issues as new reports