I have a love/hate relationship with masking and compositing in Photoshop. On the one hand, I love how layer masks can be used to make selective adjustments to an image. On the other hand, I really hate compositing. Thanks to the ridiculous representation of image manipulation in movies and TV shows, most people think that completely changing a face or an environment is a click or two away. The nightmarish result of that attitude happens when I do a group portrait of a large family (or worse, a massive family reunion) and the client wants me to composite Suzie's head from photo 1 with Joe's from photo 3, Jim's from photo 4, Mary's from photo 8—oh, and while you're at it, could you swap out the eyes on Bob, Jane, and Dave? And the baby would probably look better if you turned its head 30 degrees to the left. I cringe just thinking about it.
I hate compositing, but knowing how to do it comes in handy. Look at my banner, for example. I wasn't about to actually take a photo of a bunch of photos being dropped into a take-out box that just happened to have my photo logo on the side. So I used masking and compositing to get the photo I wanted.
I mask way more frequently than I composite. When I whiten teeth, for example, I do an overall image adjustment, add a black mask, and then use a white brush to paint in the effect on the teeth. That kind of fairly casual brush-in masking is the least tedious. Other cases require complex selections. Here's a quick example of a not-so-complex selection that will make the principles clear to readers who are not in the habit of using masks:
In the above photo (part of an ABC Paris project I have been expanding lately), I want to bring out the sky a little more. That's easy enough to do with a curves layer:
I pull the curve down and get a darker sky, but here's the problem: the architectural portion of the photo is already pretty dark. Any darker and it will lose all of the shadow details, which is pretty much a cardinal sin for most photographers. What to do? I don't want the shadow detail police knocking on my door, but I do want a more dramatic sky. Here's where the mask comes in.
When you add a curves adjustment layer, Photoshop has already assumed that you want a mask to go with it. The default mask is white, which means that the adjustment is going to show everywhere by default. Click on the mask and paint in some black, however, and you can mask out the adjustment. In fact, it's not about black and white, you can mask with shades of gray (i.e. at a lower opacity) for even more control. But in this example, I just want a clean mask: black out the building and leave the curves adjustment for the sky only.
Since this isn't meant to be a full-fledged tutorial, let me just give a quick rundown of the process: With the image layer active, I used the magic wand tool (which is way more fun than, say, the pen tool, because hey! it's magic) to create a selection (look for a post on selections soon).
Once the dotted lines commonly known as "dancing ants" indicate that I have the building selected, I make the mask part of the curves adjustment layer active. Even though I made the selection on the background layer, I can use the selection on any layer as long as those ants are still dancing. Next, I simply use the paint bucket tool to dump black into the selection.
And that's it. My sky is now more blue. I went from this sky:
to this sky:
without changing the building in any way.
That's just a simple example of how masking can help. I could spend a whole year doing masking a compositing tutorials, but then I'd have to spend the next 5 in an asylum. So, in order to keep what's left of my sanity, I will do a few this month but will also do some completely unrelated posts. For those of you brave few who want to share by linking back to this post, do some masking and/or compositing and show us your work. (the FAQ explains how)
NOTE: I changed this month to masking only. (4/5/10)