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Basic situation:

I have an image with a complex alpha channel (ie. feathered, complex non-linear edges).

I have an image I want to overlay on top of the first image using a blend option like multiply or maybe screen.

...but I must preserve the EXACT alpha channel of first image.

Can I do this in photoshop?

If I paste the second image it creates a new layer, and the multiply, screen etc options overwrite the alpha values on the first image (layer 1).

Seems like there should be an easy way to do this?

I saw this question: How do I change RGB value without changing the Alpha channel?

...but messing around with the lock-alpha options didn't seem to do anything.

NB. The reason I mention the complex edges in the image is that the feature tool doesn't select the alpha channel correctly; simply select-using-magic on layer 1, goto layer 2, delete doesn't even remotely work.

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I think what you're after will be possible and easy, using a clipping mask. Do you have some example images to work with? – Marc Edwards Jul 11 '12 at 10:20
up vote 3 down vote accepted

Simply use a clipping mask within Photoshop.

Option (mac) or Alt (Win) click between two layers. The Top layer will then be restricted to only displaying in areas where the bottom layer exists.

Clipping Mask

share|improve this answer
+1 for the spiffy animated demo! – Lauren Ipsum Jul 11 '12 at 12:22

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