Not to be confused with a quick mask, a clipping mask is in fact a series of layers; the dynamic between the base layer and the layers above it define the image that will be revealed. Various combinations of opacity and blending can result in extraordinary composite images.
The advantage of this method is simple: edit the base layer and you edit the shape of the composite image.
The Basics
1. Create or choose your base layer. By default, any new layer is called "untitled". I have renamed mine "base layer" for the sake of clarity.
Keep in mind that the visible content of the base layer becomes the visible shape of the content above it. Any areas in this layer that are transparent will remain transparent in the composite image as well.
2. Create a new layer above the base layer or or drag a layer from another image onto the base layer. as seen here, i have dragged a layer from another file onto my example file.
3. Create the "clipping mask" .
select your new layer then Layer > Create Clipping Mask (alternately, you can Option + Apple+G)
Add additional layers as needed by repeating this step. Drag and drop the overlying layers in any order that best suits your design.
As an additional refinement, note that any of the layers involved can have its opacity adjusted and the blending mode as a number of choices.
(I do miss the layers menu that Photoshop offers with the layer palette but that is more habit than anything else..)
Familiarity with Photoshop is helpful in learning Pixelmator. Once you get over the look of the UI, it is very easy to adjust to Pixelmator. See the quicktime movie of My First Attempt to Create a Clipping Mask with Pixelmator. After a couple of false starts, i was able to find all the relevant tools quickly. It was very easy, making Pixelmator an cheap and reliable substitute for Adobe Photoshop.
My first real attempts at using Pixelmator were to create composite images for a client (Toy-TMA.com) without reading the manual.
Pixelmator: Creating a Clipping Mask | DigitalAppleJuice.com…
tutorial pixelmator- creating a clipping mask…