Import image as a mask or How to use an alpha mask for transparency mask

Question

Hello,
Basically I need to know how to import 1 image into another image as a mask.
What I'm trying to do:
I have a bunch of large images that put together make 1 very large image (ortho land images). I also have corresponding images, lets refer to them as mask images, that would mask out parts of the large ortho image I want to import it too. These mask images basically look like a PS mask. They are feathered black to white images. I need to be able to import them as a mask.
Is there away, if so how?
Thanks!!!!

Answer 1

If the masked out areas of your image are to be black, see if this approach fits your needs and forget about creating a mask. Instead bring your "mask image" into your ortho land image using your favorite technique. Now change the blend mode of this mask image to mutiply. You should now see land wherever your mask is white, black where the mask is black and transition regions where the mask has a gray value.
Alternatively, you can make an alpha channel if you want by turning off all layers except your "mask image", switching to the channel palette, Ctrl clicking on the rgb channel (to create a selection) and then clicking on the Save Selection as Channel icon at the bottom of the palette. You can then create a luminance mask by going back to the layer palette, selecting the layer you want to mask and click the Add LAyer Mask icon at the bottom of the palette.
Paulo