I have this image:
And I would like to extract the dark parts (shadows) to make a transparency consisting of black pixels at different alpha channel values. Here is a crude example image of what I am trying to extract:
This way, if I put solid colors behind the image, that texture will show up over the new color and make it look nice and purdy.
I suppose another way I can ask this question is how I can turn a layer working with an "overlay" or "linear burn" blend option selected into a png consisting of black pixels at different alpha channel values.
Tough question to put in words, my apologies for any confusion.
Best bet is probably Select > Color Range. Then click a white portion of the canopy and play with the "Fuzziness" slider to get you where you want.
Once you do that click okay and your'e left with a selection. Make the selection a Mask (probably have to invert the mask afterwards). Then simply lower opacity for the layer and Save for Web.
You can save specific steps of various transparency....
Here's my take on it, it's a bit complicated but very effective, and can especially be used when you scan a drawing and want to take the pencil tracing from the white background in order to change the color. (Switch the Ctrl for Cmd if you are on Mac.)
This should give you a super clean result!