How do I apply a channel as a layer mask?

[J Isner]

In my document I have a layer L and a channel C.  How can I apply C as the layer mask for L?  I want direct way to do it that does not go through an intermediate selection.  Thanks!

[Trevor.Dennis]

Ctrl click the alpha channel before applying the layer mask, or creating a new Adjustment Layer.

[J Isner]

Thanks, but that goes through an intermediate selection.  Selections drop the bit depth to 8-bit.  Masks are 16-bit.  I want everything to be done in 16 bit.

[rayek.elfin · Accepted solution]

1. create a blank, white mask on your layer.
2. make sure the mask is selected.
3. go to Image > Apply Image.
4. select the channel you want from the channel dropdown.
5. click ok.

[D Fosse]

An easier alternative is to copy the entire image, alt + click mask to target, and paste.

The problem with Apply Image is that you need two extra clicks in the dialog. If you need a luminosity mask on an adjustment layer, it's not even actionable, because you have to select the specific adjustment layer.

This is when laziness kicks in and you just load a luminosity selection instead (either ctrl + RGB channel, or easier, ctrl + alt + 2).

I must confess I often do this, and the bit depth be damned. With a modest adjustment it shouldn't make much of a difference, but more severe adjustments should really be with a 16 bit mask.

[J Isner]

That's it!  Thanks.  The Apply Image dialog is a bit confusing but I think I get it.  Setting Blend mode = Normal will replace whatever the mask currently contains, whether it's black, white, or something else.  That's the behavior I wanted.  With a blend mode other than Normal, the selected channel will combine with whatever's currently in the mask according to the blend mode.  For example, Multiply will darken whatever's currently in the mask and Screen will lighten.