Masks: Image as Mask (2)

In this basic mask tutorial, we'll make a pop art poster image, using a mask made from this photo of one of my cats:

Start by opening the image that you want to use as the basis of your mask. Duplicate this image by hitting Shift+D. We'll build the pop art version of the cat in the duplicated image and use the original image to create the mask.
In the Materials palette, set the Background color to something suitably pop-ish. (I chose purple). Press Del to delete the current contents of the image. If your original image was a flat file (that is, it has no layers except for Background), then pressing Del fills the image canvas with the Background color. If your image was on a real layer, Del empties the layer, making it completely transparent. If you now have a transparent layer, use the Flood Fill tool to fill the image with the Background color by right-clicking in the image canvas.
Create a new layer in the duplicated image by clicking the New Raster Layer icon on the Layers palette. When the New Raster Layer dialog pops up, just click OK. (Note: You can bypass the New Raster Layer dialog by holding down the Shift key when you click the New Raster Layer icon.)
Next, choose the Flood Fill tool. In the Materials palette, set the Foreground color to some contrasting pop-ish color. (I chose a bright yellow-green.) With the new layer the active layer, click in the image with the Flood Fill tool to fill this layer with the Foreground color. You now have a totally opaque purple layer with a totally opaque green layer above:

Contents of lower layer Contents of upper layer
Now, with the upper layer still the active layer, choose Layers > New Mask Layer > From Image. For 'Source window', choose the image that you want for the basis of your mask (m-cat1.jpg here). Be sure that 'Source luminance' is selected under 'Create mask from'.
When you click OK, a new layer group is created that includes the previous topmost layer with your new mask layer above:

The mask makes some areas of the layer look opaque, some transparent, and some semi-transparent. The result you see in your image will be something like this:

Used in this way, a mask layer can be thought of as "acid" that eats away at the nontransparent areas of a lower layer in the mask's layer group, letting the contents of lower layers outside the layer group show through. (Masks are much more versatile than acid, though, since the layer beneath the mask isn't really changed. The mask just makes some areas of the lower layer appear to be transparent or semi-transparent. So maybe a better way to think of a mask used in this way is as some sort of magical invisibility paint.)
If you like, you can now merge all the layers by right-clicking the Layer button of one of the layers on the Layer palette and choosing Merge > Merge All (flatten). This merges all of the layers together, forming a new Background layer.

You can then apply any other effects you want. I used Effects > Artistic Effects > Posterize to get a pop-art effect:

Note: You can save the file without merging the layers, but if you don't merge the layers, any effects that you apply before saving will affect only the active layer. If you save the file in a format that supports layers (PSP or PSD), then the layers (including the mask) will be preserved and will be available for editing later on. If you save to another format, such as GIF or JPEG, all of the layers will automatically be merged together.


Contents of lower layer	Contents of upper layer