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problem with alpha channel
Closed, InvalidPublic


System Information
Win 7 PRO 64 bit and quadro FX 1700

Blender Version
Broken: Blender 2.63-2.69
Worked: not working

Short description of error
I am trying to save a raster image file from composite in blender with alpha channel. So I have two images to do it. The first one is a color image and the second one is a B&W image that will be an alpha channel for color image. I made a node setup as it is in attached file. I thought that the final image alpha will be the same as the original file, but it is not. I tried also doing the alpha channel from the color image by using the node RGB to BW but it also fails. I do not know if I am doing something wrong or it is a bug
Exact steps for others to reproduce the error
blend file is enough



Event Timeline

daniel (midan) added a project: BF Blender.
daniel (midan) set Type to Bug.
daniel (midan) added a subscriber: daniel (midan).
daniel (midan) created this task.
daniel (midan) claimed this task.
daniel (midan) raised the priority of this task from to Needs Triage by Developer.
Brecht Van Lommel (brecht) closed this task as Invalid.

The problem here is that the Blender compositor works with a premultiplied alpha standard, while this setup gives it straight alpha. Some information on that can be found here:

To create an image with premultiplied channels, you can use a Set Alpha node to put the alpha alongside the RGB channels, and then use an Alpha Convert node to convert straight to premutiplied alpha.

I did what you have adviced, brecht ( if I understood you well). But the problem still occurs. The final alpha channel of the image after connecting file F.png and F_a.png is not the same as the original file F_a.png. If you could set the right node setup to achieve the expected result I would be grateful.

Ah, you also need to set the color space on the alpha input image to non-color or linear I think, otherwise it is assumed to be sRGB but alpha is not actually color data.

THANK YOU BRECHT :) I changed the color space of alpha input and it works as expected. Now I can go further...