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I've got two TIFF files with RGB and multiple alpha channels. The RGB channels are pixel by pixel identical. The alpha channels have values from 0 to 255. How can I compare an alpha channel in each file pixel by pixel? I need some kind of way of clearly visualizing the differences. The images are 300 x 384 pixels.

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Say one file is called a.tif and the other b.tif. The channels of interest are both called Alpha 1. Say we're working on Windows.

In a.tif

  • Select Alpha 1 by clicking on channel in Channels window
  • Select All (Ctrl-a)
  • Copy (Ctrl-c)
  • New File (Ctrl-n). Call this file c.tif
  • In new file Paste (Ctrl-p) Call this layer A

In b.tif

  • Select Alpha 1 by clicking on channel in Channels window
  • Select All (Ctrl-a)
  • Copy (Ctrl-c)
  • In c.tif Paste (Ctrl-p) Call this layer B

In c.tif

  • Right click on the top layer
  • Blending options... | Difference

c.tiff will be completely black (value 0) for all pixels if layer A and B are identical. You can check this either by scanning across with your mouse and using the Info window. Or by applying Image|Auto contrast to each layer one at a time.

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