Understanding Masks

IF YOU'RE AT ALL CONFUSED ABOUT MASKING, THIS QUICK LESSON WILL MAKE IT ALL BECOME CLEAR

Don't let the word "masking" freak you outit's just another one of those holdover terms from the traditional prepress industry that carried over into Photoshop (like the term "unsharp mask" which is a holdover term from conventional darkroom techniques).

Essentially, masking is protecting part of your image while leaving other parts of the image exposed to change. It's kind of like repainting your carif you went to repaint it, you'd have to put masking tape over the parts of the car you didn't want accidentally painted over (like the chrome, the windows, the headlights, etc.). Those areas need to be masked while you paint the rest of the car. That's essentially what masking is all aboutconcealing parts of your image you don't want affected by change.

In this chapter, you're going to create a series of channel masks (that we'll turn into intricate selections), but before we head into that, it helps to get a good understanding of how masks work. We'll start with this simple lesson, so you'll understand what masking is all about. Then, as soon as it clicks with you, we're heading straight into some serious masking, but don't worryyou'll be ready.

Step 1.	WE'RE STARTING WITH A BROWN CARDBOARD LETTER STENCIL Okay, forget Photoshop for now, because this lesson takes place with a regular piece of paper (you don't need to go grab some paper, I've done that for you here). This lesson is based on how a standard letter stencil works (the kind you can still buy to this day at your local art store or office supply store). The one shown here is a cardboard stencil cutout of the words "For Sale," and you see (because the stencil is on top of the right side of the paper) that the stencil letters are cut out of the brown cardboard. So far pretty standard stuff, right? ©SCOTT KELBY
Step 2.	MOVE THE STENCIL SO IT COVERS YOUR PAPER, THEN SPRAY PAINT OVER IT IN BLUE Now, if you were to move this cardboard stencil over to completely cover the yellow paper, then you grabbed a can of blue spray paint and sprayed over the letters (as shown here), you'd get the blue blob you see here, right? But of course you're not worried, because the blue paint will only show through the cutout lettersthat's the whole idea behind stencilsthe brown part protects all the rest of the paper, except for those cutout stencil letters.
Step 3.	WHEN YOU REMOVE THE STENCIL, THE ONLY AREAS THAT ARE BLUE ARE WHERE THE CUTOUT LETTERS WERE When you remove the stencil from on top of the paper, what you have left is the blue stencil letters on the yellow paper. With me so far? I know, "duh!" Okay, now let's take it a step further.
Step 4.	WHAT HAPPENS IF INSTEAD OF YELLOW PAPER, YOU PUT A PHOTO BEHIND YOUR STENCIL? What if, instead of a piece of yellow paper, we used an 8x10" photo print (like the cows shown here, on the left)? If we put our stencil over the cows, the brown areas would cover everything but where those cutout stencil letters are (like the image shown here on the right). Okay, so the only difference is a photo in the background instead of yellow paper. ©ISTOCKPHOTO/MARTINA BERG
Step 5.	THEN SPRAY PAINT IN BLUE OVER THE CUTOUT LETTERS So, if you were to grab your blue spray paint and paint over the entire area where the letters were cut out, the brown areas would protect the rest of the photo, and the only parts of your photo that would change would be those cutout letters. They'd be blue, right?
Step 6.	THE RESULT IS THE SAMETHE CUTOUT AREAS ARE AFFECTED, THE REST IS PROTECTED Pull off the brown cardboard stencil and you'd see that the effect is exactly the samethe blue spray paint only shows through the area of the cutout letters. So, the stencil works the same whether it's a photo or yellow paper. You knew that, but I had to go over it anyway. Now, let's start to relate this to Photoshop.
Step 7.	WHAT IF THE BROWN CARDBOARD WAS BLACK? DOES THAT CHANGE ANYTHING? NOPE If we went back to the store, and bought the same stencil, but this time we bought it in black cardboard (instead of brown), and put this black cardboard stencil over our cow photo, it wouldn't change anything, would it? The cutout letters would still be the part that shows through, and the black areas would still be the part that protects or conceals the rest of the photo.
Step 8.	PHOTOSHOP USES A SIMILAR METAPHOR FOR MASKS, BUT INSTEAD OF A CUTOUT, IT USES A WHITE FILL Now, Photoshop uses this same metaphor for masking, with one small exceptionthe cutout transparent areas are white, instead of see-through (like the stencil shown on the left, where black is protecting the cow image, but the white areas are actually cut out, or transparent). So, if you were to grab a soft brush, and paint over this stencil (called a "mask" in Photoshop) in the same blue, it would look pretty much the same as it did on the brown cardboard stencil, just over black (see the image on the right).
Step 9.	IF YOU PAINTED OVER THIS MASK IN BLUE, THE WHITE AREAS WOULD BE BLUE If you were to remove the mask (the stencil), the effect is exactly the samethe black areas protected the rest of the photo, and the white areas are like a see-through cutout, so anything you do to those cutout areas (like paint in blue) is going to appear on your photo, just like it does here.
Step 10.	BUT WHAT IF INSTEAD OF STENCIL LETTERS, THE CUTOUT WAS IN THE SHAPE OF A COW? Now, what if your stencil isn't made up of letters? What if, instead, you bought another stencil but this one is in the shape of a cow, and as luck would have it, the white cutout just happens to exactly match the size and position of the cow in your photo? (This is no more far-fetched than your average James Bond movie opening sequence.) So, you've got a new black cardboard stencil, and instead of your cutout area being shaped like letters, it's shaped like a cow. You still with me? Good.
Step 11.	YOU COULD STILL PAINT OVER THIS MASK, AND THE BLACK AREAS WOULD BE PROTECTED If you painted in that same blue color over the mask, what would happen (you know by now)? If there was a yellow piece of paper behind that mask, you'd see just the blue shape of a cow on a blank yellow piece of paper. If the cow in the field photo was behind that mask, it would paint blue over the cow and the rest of the photo would be protected from the blue paint.
Step 12.	IF YOU REMOVED THE MASK, YOU'D SEE THAT THE COW WAS AFFECTED, BUT THE BLACK AREAS PROTECTED When you remove the stencil (mask) you'll see the cow is blue, but nothing else is. The black areas of the mask protected the rest of the photo, and the white areas let your changes (the blue paint) affect only those white areas of the photo. This is what masks are all about. That's why you'll hear Photoshop instructors sometimes recite this little rhyme to help students remember how channel masks work in Photoshop: "Black conceals, and white reveals." My wife has a saying I like even better: "Black remains the same, white accepts the change." So, where do we make these masks within Photoshop? In the Channels palette (look at the Alpha 1 channelit's a white cow shape surrounded by black).
Step 13.	IF THE MASK WAS MOSTLY WHITE, LIKE THIS ONE, THE COW WOULD BE PROTECTED INSTEAD Now, let's try the flip side just to wrap things up. Here's the oppositea white mask with a black cow shape in the center. What would happen if you painted in blue over this mask?
Step 14.	IF YOU PAINTED OVER THE WHITE AREAS.... By now, you knowthe white areas would turn blue, the black area is protected, so nothing would happen to whatever is under that black area.
Step 15.	THOSE PARTS OF THE PHOTO WOULD BE AFFECTED, THE BLACK COW IN THE PHOTO WOULD BE PROTECTED So, what's behind that black area? You got it, babya cow. The rest of the photo was affected because those areas accept your change (paint in blue), but the black area remains the same. Okay, that's it, you're ready to bust full force into masking, and that's good because we're going to jump right in with both feet once you turn the page.