White to alpha?

[Ozzieing]

I have a greyscale image of a shadow which is a mixture of colours from white to dark grey. What I want is for the "whiteness" of a pixel to denote how alpha it is. i.e. a perfectly white pixel would be completely transparent whereas a perfectly black pixel would be completely opaque. I've had a search on the internet but everything I can see to do with this kind of thing is about simply removing the white background of an image - not introducing this kind of alpha levelling.

Any help would be much appreciated!

 

[Rich54]

I am not one of the gurus, but I just tried doing what you describe and got it done very easily using "Blend If" sliders. This YouTube video explains Blend If and the example is very similar to what you are trying to do, i.e., make the whites transparent. https://www.youtube.com/watch?v=ePnHZPmhjsM

 

[Tom Mann]

What you are looking for (...I think...) is called a "Luminosity Mask".

Here's the basics:
http://www.photoshopessentials.com/photo-editing/luminosity-mask/

And here is a longer article that concentrates less on the mechanics of generating a luminosity mask, and more on the uses and results:
https://iso.500px.com/luminosity-masks-in-digital-blending/

HTH,

Tom M