I have some white pillows on a 100% white background. While the pillows are "white", they are certainly darker than the background. So, after making some erasures of whatnot, I'm using DARKEN blending mode to comp them onto a SOLID WHITE layer. This works fine.
I'd like to put a drop shadow layer style behind the pillows, but I don't want/desire to mask them.
Is there a quick way get a Selection from the bits that become VISIBLE from DARKEN blending mode, even if there's a lot of white pixels in the layer? Basically I want everything BUT the 100% white to get Selected, without laborious masking.
"Putting the HARM in 'harmonica' since 2005."
I'd do a linked Levels Adjustment Layer to the pillows layer (ALT/OPT-click between the layers so the Adjustment is only affecting the pillow layer) and darken them as far as possible. Super-dark. You'll be removing the Adjustment Layer later so the appearance doesn't matter, just want them as dark as you can get them while minimally affecting their white background. Then pop into Channels and find the channel with the greatest contrast. CTL/CMD-click that channel to make a selection of the white areas (if the pillows aren't dark enough to not be partially selected, you can also Magic Wand the white area of the channel). Invert your selection (SHIFT-CTL/CMD-I).
Create a new layer, call it "Pillowshadows" or whatever you like. Fill the selection with white, add the Drop Shadow Layer Effect, and set the layer to Multiply. Link Pillowshadow to Pillows so you can easily move them together. Delete the earlier Adjustment Layer. That should give you an editable shadow for your pillows without any masking.
It is easier to destroy than to create.
More fun, too.