Convert opacity value to grayscale luminosity

Question

I would like to use Gimp to prepare files for riso printing and for this I need to convert each individual color layer from opacity (x%) to greyscale ((100-x)% luminosity).

Precisely, this is the conversion I need:

• [color] at 100% opacity -> black
• [color] at 75% opacity -> grey (25% luminosity)
• [color] at 50% opacity -> grey (50% luminosity)
• [color] at 31 % opacity -> grey (69% luminosity) etc.

And [color] can be replaced by any existing color. I have one layer per color.

Would you know how to do this? In Photoshop, there is a shortcut to do this at seen here: https://youtu.be/UrFq5KliP04?t=263 This is exactly what I would like to reproduce.

Thanks in advance for your help.

Answer 1

This is very similar to xenoid's Lock Alpha method, but using some shortcuts, and a change to the workflow so you can keep the original colour image unaltered.

Before beginning, make sure you have saved your original colour XCF document.

1. Hit Ctrl+D to duplicate the document, so that any changes you make won't affect your original colour document. The duplicate document will open in a new image tab

2. Hit D to set the default foreground and background colours, i.e. black FG, white BG

3. Engage the Lock Alpha Channel* button for the layer

4. Do Edit > Fill with FG colour, or use the shortcut Ctrl+, <---- that's the comma key

Repeat steps 3 and 4 for each layer you want to convert

When you have finished, save your separations XCF file with a new file name so you don't overwrite the original colour document.

*Note: The Lock Alpha Channel function doesn't have a default keyboard shortcut, however it would be possible to set one up in Edit > Keyboard Shortcuts, perhaps something like Ctrl+Alt+A, so it would be possible to do this using shortcuts alone.

Answer 2

Simple solution

• Add a white layer as background
• Alpha-lock the layer

• Bucket fill the color layer with black

So, from this:

To this:

On the top image, alpha in the middle of the gradient is 50%, and in the result image, the color in the middle is (189,189,189) which is indeed the 50% luminosity gray.

A bit more complicated, but better to work with, solution

If you want to keep the color layers intact, there is a slightly more protracted way using layer groups. You keep the color layer with a black layer over it in a group (one such group per color layer), and set the black layer to Composite mode: Clip to backdrop. This has the same result as bucket-filling the alpha-locked color layers.

Clip to backdrop can be set in the layer attributes dialog, or directly in the right-click menu for the layer:

This is slightly more complicated to set up, but you can edit your color layers and see the result in real-time. And if you want to see the color, you just make the black layer invisible.