Bright/Contrast Doesn't respect Pre-multiplied Alpha
Closed, ResolvedPublic

Description

System Information
Ubuntu 16.04 LTS
GeForce GTX 560 Ti

Blender Version
Broken: 2.78c
Worked: ?

Short description of error
Changing a render with a Bright/Contrast node before an Alpha Over node also changes the image it's being laid over.

Exact steps for others to reproduce the error

  • Render a scene in Cycles (should be the same in Internal) with alpha Transparency turned on.
  • Add a Bright/Contrast node to the render 'Image' output.
  • Output of Bright/Contrast goes into the bottom 'Image' input of an Alpha Over node. Set top input to a non-white/black colour (so we can see the changes with the Bright/Contrast node).
  • Alpha Over goes to composite and/or viewer node.
  • Changing values on the Bright/Contrast node changes the background colour, even though it's before the Alpha Over node in the chain.

This can be fixed by checking the 'Convert Premul' checkbox on the Alpha Over node. This adds a dark border around the rendered object. This can then be fixed by adding a 'Alpha Convert' node in front of the 'Bright/Contrast' node.

I realise that this is down to the differences in Straight and Premultiplied Alpha, so it's not necessarily a "bug", but it would be logical, in my mind, for all nodes to assume the image inputs are using the same alpha mode as the default for the rendered image. At the minute, to do a simple Alpha Over with some colour manipulation requires a couple extra steps which might throw new users.