I posted a question on the Affinity Forum and here's the question and reply from one of the members. I did not know what an alpha channel was and googled it. Thought that you may find this of interest. (Maybe I'm the only one who didn't know)
In Photoshop, I can copy an image or just a channel, say the Red channel in RGB by Ctrl+A and then Ctrl+C or create a luminosity mask by copying the L-channel in Lab. I then can open a mask by Alt+click on the mask and paste the copied image into the mask by Crtl+V The image will now appear in the mask and I can use levels or curve adjustments on the mask. In Affinity Photo, I'm not able to do that, a new pixel layer appears on top and I cannot get this into a mask. Is there a special way to do it? I'm sure one must be able to create such a mask. Thank you
Ganna -- I am by no means expert in this, but here is my understanding. You cannot copy a pixel layer, and then paste it into a mask layer. I imagine that, despite its on-screen representation, the mask layer has only Alpha components, while the pixel layer has RGBA components. The two don't play together well in AP.
What you CAN do is (i) right click on the channel you want to deal with (e.g., in a photo named "Flower" right click on the "Flower Red" entry in the Channels panel) and choose "Create Grayscale Layer" in the pop up menu. Once you have the grayscale version of the Red channel, you can do anything you want to it. Then, with the grayscale layer selected, (ii) choose "Rasterize to Mask" from the Layers menu and drag the new mask layer into the proper position.
The other thing is that you CAN apply certain adjustments to Masks - specifically, any adjustment that lets you specify that it applies to the Alpha component. That includes Levels, Curves, and Channel Mixer adjustments. Simply create the adjustment layer, choose Alpha from the drop down menu, and drag the adjustment onto the Mask thumbnail so that the VERTICAL blue line appears just to its right.
Attached is the pdf file with the googled explanation
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