How to create a gradually transparent mask in photoshop from a black->grays->white image

Question

There have been some posts regarding what I describe in the title, but none seem to understand what the aim is. So I'll try to described it further here:

I want to take an image, which has in it black, various increasingly light grays, all the way to white, in a complex arrangement (essentially a grayscale image). With that image, I want to re-define it so that the black is transparent and white is opaque, with the grays between representing partially transparent, with the ones closest to white the least transparent. I think you have the idea by now.

I then want to use that to be a mask that sits on top of another layer. So, for example, that mask on top of a solid colour of red would then look like the same original image, but instead of the black, with grays and white, it would look red where black was, then increasingly lighter reds until where white was in the original remains white here, with very light pinks near it, representing the very light grays.

Sounds like it should be really straightforward, and the sort of thing Photoshop is designed for, but no explanation of it so far has laid out in clear, simple steps, how to do this.

Please help?

Answer 1

What you need is a layer mask. They already work like you described - black being transparent, white being opaque and everything in between of gradual levels of opacity.

If your image is already a grayscale (as you said), all you need to do is to copy it and apply as a layer mask to any other layer.

One of the easiest way of doing it is to simply click ctr+a then ctr+c with only your grayscale image visible. That should copy it to the clipboard.

Then add a layer mask to the layer you want to mask: Layer ->Layer Mask -> from transparency.

You should see the mask in your layers window now as an additional thumbnail to the right. Simply alt+click on that thumbnail. Now you are in the Mask edit mode, paste (ctrl+v) your mask from clipboard and make any adjustments if needed. Than click on the layer main thumbnail to see affects.