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I would like to erase by hand with a brush or eraser tool, but I only want darker colors to be erased. For example, if a pixel under the brush is 100% black (luminosity=0), it would get 100% erased (turned to opacity/alpha=0%). If a pixel is a dark blue (say luminosity=20), it would get 80% erased (alpha=20%). If a pixel is solid white, it would be untouched (alpha=100%).

This question is about photoshop, but an answer that works in a different but free graphics program will be just as good or better. Thank you.

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For example, you can use Photoshop Knockout layers in conjunction with BlendIf.

Here I have a Radial Gradient that punches a hole in the Group 1 group. It's set to Shallow Knockout to act only inside that group, fill is set to 0% to ignore the layer contents (black color):

enter image description here

Next if I change the Blend If Underlying Layer slider, this gradient starts to lose transparency if the layer below is darker or brighter that specific value:

enter image description here

An example where a letter A punches through pixel values that are stronger that 80% (Underlying Layers is set to 204: 255*0.8) enter image description here

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