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Inconsistent rendering of alpha channel of an external image with Use Alpha disabled
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Mac OS X 10.9.5 MacBook Pro Retina, Mid 2012
Broken on Blender 2.74 and 2.75 both CPU and GPU (Nvidia GT 650M) rendering

Open the file and keep pressing Shift-Z to switch between rendered and solid viewport. For me, image is sometimes rendered as horizontal strips and sometimes as a grid.

It looks like alpha channel of images that have Use Alpha unchecked, although always rendered white when connected directly, somehow influences the mix node in a strange way.

Event Timeline

Andrej Ivanis (aivanis) raised the priority of this task from to 90.
Andrej Ivanis (aivanis) updated the task description. (Show Details)
Andrej Ivanis (aivanis) edited a custom field.
Sergey Sharybin (sergey) lowered the priority of this task from 90 to 50.Jun 20 2015, 10:33 PM
Sergey Sharybin (sergey) changed the task status from Unknown Status to Unknown Status.Jun 22 2015, 4:10 PM

Looked into the file, and this is actually a limitation caused by OIIO -- you opened the same image as two different datablocks (which blender usually forbids to do actually) and used different settings for them. And because OIIO is actually using filename as a main handle, this confuses it.

While it's possible to solve the things for SVM backend it'll be still really problematic to solve for OSL. I would love to look into a more generic approach here and for now will consider it a TODO:

So thanks for the report, but closing as TODO for now. Meanwhile you can work this around by packing the image.