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New Photoshop user trying it for the first time. I'm trying to get an inverted alpha "channel" but I can't. There are things I'm missing.
Steps I took:

  1. Create sRGB 512 * 512 pixel document.
  2. In an empty layer say, layer 1, use one click in the center with brush tool with settings:
    color black, hardness 50, size 256.
  3. Create new layer say, layer 2 and fill it with black.
  4. Select transparency of layer 1 (Ctrl + click its thumbnail).
  5. Create mask on layer 2.
  6. With layer 2's mask selected, invert colors (ctrl + i).
  7. Make sure both layers are visible and only them.

I expected to see a total black as the layers perfectly complement each other, with any pixel having alpha 1 as: layer 1 alpha + layer 2 alpha (which is 1 - layer 1 alpha).
Then I said it's maybe because of the color space and I tried linear RGB. Here are the results: results of alpha inverted and blended
What am I missing and how to get a perfect "complement" of the alpha of a layer?

  • How did you work linearly in Photoshop? – Wolff Nov 1 at 16:17
  • It may not be the most proper way: Create a sRGB (default) document. Then Edit > Convert to profile. In basic mode, Destination Space > Profile = Custom RGB. I set Gamma to 1 and white point to D65 (as I deemed it most "neutral" reading on Wikipedia). – mireazma Nov 1 at 17:32
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You have a misconception. If you have two layers of 50% opaque black with blending mode Normal you will not get the same effect as a single layer 100% opaque black. The result you got is exactly as expected.

If there's a background which has brightness A, the bottom black layer passes it as 0,5A. The top black layer passes 50% of it, the result is 0,25A, not zero.

The total opaqueness can be achieved only by having in every pixel 100% opacity at least in one of the layers. That's because the opacity combination is done by multiplying, opacities are not summed.

If you have a 2 layer image (blending=Normal), let the bottom layer be fully opaque. Let the top layer have more or less transparency where something of the bottom layer must be seen.

You may miss a way to force the alpha work in a different way. Unfortunately there's no tools in the user interface to control how the opacities of the layered stuff are taken into the account.

Photoshop has many places to keep data which is used in opacity calculations. Decreased opacity can be in the layer content as your soft brush shows. Layers have opacity; less than 100% decreases the total opacity. If you double click the layer icon you get a dialog for layer blending interaction dialog (you can also define there layer styles). Then you can have a layer mask and possibly inserted auxiliary channels.

You can see the layer mask in the channels panel and by closing RGB channels you can edit the layer mask on the screen to make transformations. You can create a layer mask from the layer content opacity. Details of this all are beyond the scope of this answer. See Photoshop User's Guide.

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  • This is by the way the same reason vector graphics show trough on edges. Same assumption error. – joojaa Nov 1 at 16:28
  • You're right. As I see it the src_color * src_alpha + dest_col * (1 - src_alpha) thing still holds but it's done once between checker background and layer 1 and once between the result and layer 2. – mireazma Nov 1 at 20:29

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