Graphics Programs Reference
In-Depth Information
At first, it might seem that the Mix node is messy, but this difference is due to the way
both the nodes assume whether the alpha input is premultiplied or straight. When
using the transparent alpha mode, Blender renders the image with a premultiplied
alpha. Since the Alpha Over node assumes that the input is premultiplied by default,
it performs calculations correctly. However, the Mix node assumes that the input has
a straight alpha.
To solve the issue, an Alpha Convert node should be connected to the lower
Image
socket of the
Mix
node. By selecting the
Premul to Straight
mode in the Alpha Con-
vert node, the straight alpha mode is created from premultiplied alpha input. When
this is fed to the Mix node, this fringe issue is resolved.
The following screenshot represents the use of the Alpha Convert node to solve the
fringe issue for a Mix node, with the transparent alpha mode being used:
