Changing the Alpha Mode of an Image in Cycles doesn't have expected effect.
Open, ConfirmedPublic

Description

Images with non-color data such as normal maps that also have an alpha channel containing other data such as a bump map have the color data affected by the alpha channel, even when the Alpha Mode is set to Straight.

When the alpha channel on the image is disabled or when the image file has the alpha channel removed the texture behaves as expected, but this prevents using the data in the alpha channel.

Using Blender 2.79 Cycles renderer.

Details

Type
Bug
Philipp Oeser (lichtwerk) triaged this task as Normal priority.

I can confirm this behaviour and would also expect 'straight' alpha mode to not affect color data.
Maybe @Sergey Sharybin (sergey) can have a look?

Brecht Van Lommel (brecht) raised the priority of this task from Normal to Confirmed.Jan 3 2018, 12:36 AM
Brecht Van Lommel (brecht) claimed this task.

I'll fix it so we don't premultiply alpha for non-color data. For the OSL / OIIO texture cache case this might be complicated.

Until this is fixed, there is a workaround:

  • create two image texture nodes, referencing both the same image (note: this is not the same as duplicating the texture node).
  • check 'Use Alpha' for the first, uncheck it for the second
  • use the first node to connect RGB, the second to connect alpha.

Didn't check, though, whether this will double memory usage.