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Photo in many aspects surpasses well-known programs.
But working with channels is doubly disappointing. 
Channels _must_ be editable, especially an alpha channel.
Painting directly, copying, pasting etc. should be implemented.

Composite alpha is a convenient feature as some kind of a spare channel, 
but it is not a replacement for true arbitrary alpha channel.

Thank you.

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If you click on Composite Alpha it does allow you to edit the alpha channel of the currently selected pixel layer, but there are a number of limitations and undesirable behaviors related to channel editing that really should be addressed:

  • You can't view a single layer's alpha to isolate it while editing (you can edit just the selected layer, but it is displayed in composite with the other visible layers).
  • The images seem to be worked internally as pre-multiplied, which means that if you zero the alpha of some pixel it will in effect erase and not just hide the content of the other channels for that pixel.
  • When you paint on a particular channel, it still takes its color from that channel of the selected color, so choosing white or black will still paint "opaque" onto the alpha channel.  This would be OK for the color channels, but it is a problem for the alpha channel as it is additive: painting 0% opacity over the alpha channel has no effect, so to clear it you need to use the eraser tool.
  • When you copy, you are copying all of the channels, including those that are hidden.  This would be ok in and of itself, except that pasting creates a new layer instead of modifying the existing one, and when you merge (Layer -> Merge Selected), all of the channels are included (again, not just those marked as editable).

 

Note: in case you haven't seen it, there is a tutorial for most of what is currently possible: https://affinity.serif.com/en-us/tutorials/photo/desktop/video/334256548/

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