Most images you encounter are not so perfect that a magic wand can easily pick it out and select it. Even the quick selection tool often does not do a very good job. And if you do manage to carefully select the right edges, what then? You may want to be able to make that exact selection again later. This is where masking comes in. It's a handy way to define your edge that is utterly open to being altered, copied, you name it whenever you need to in the future. In simple terms, a mask is a map. It defines a hole where everything inside the hole will be affected by whatever you do, and nothing outside will. If you want to paint, no matter how much you scribble outside the lines, what you are actually painting will always stay inside the line you defined as the edge (very handy for painters that don't want to have to carefully figure out where to stop) Masking is used in every area Photoshop is useful for - from Photography to illustration. For this particular example I'm using a drawing - so some aspects will be specific to coloring line art, but in general the concept is clear.
I have this drawing I've done. Normally a magic wand tool would select the white part and I'd be done with it, but in my case, I have the problem that I have really chaotic rough edges. Inevitably some bits will be left out as you can see below when I simply try to fill the shirt with red.
Ok so what now? I want to include those little bits, but it's a serious pain if not impossible to manually go along the edges I want and select everything in one fell swoop. For one thing, the T-shirt is divided into two distinct sections. By using a mask, I can start with my magic wand as I normally would - selecting and then expanding the selection by a pixel or two to make sure I have a nice tight edge (it works the same if you use a higher tolerance, I just like to be doubly sure that the edges fit perfectly by expanding through select - modify - expand selection) Once you have your selection (number 1 below) go to your layers palette and open up a new layer (number 2). With that new layer selected you can hit the button that looks like a rectangle with a circle in it. This is to create a mask.
Once done, you will see a new little image pop up on the layer in black and white. The white represents the areas you WANT to be able to alter, and the black represents area you would like to remain untouched.
When you click on that mask you will note that your color selector has switched to black and white. If you try painting anywhere on your image now, it will add or subtract from the mask, NOT apply paint in any way (white to add areas you would like to affect, black to remove areas).This is where things can get really interesting. There are an infinite number of things you can do to this mask. Make the edges softer, paint in areas using grey to represent opacity, or even add new areas as I will show next.
My mask isn't really complete as it's only half of the shirt. I still have the bottom half to include. To do so, I need to go back to the original layer again and this time select that bottom section. Then go back to the new layer, making sure to click on the mask. This is important! THe mask must be what is highlighted! The second most important thing is to make sure that your foreground color is black, and the background color white. If this is not set up this way, the next step will not work. Go to your menu under edit and hit clear
Your mask will now show two white areas as shown below.
Now is where the handiness of masking really gets noticeable. If I click on the transparency instead of the mask and fill it with red I end up with this:
But, It's obvious that there are little white bits that got left out and need to be included as part of the shirt.
I have on small issue before I can continue. Being that this is an illustration, I need to do a little moving around of layers. I need the line layer to be on top and the fill layers to be beneath it. I always want my lines to be on top. So, I duplicate the background layer, move it to the top and set it to multiply (this causes the whites to become transparent and the blacks to be permanent, effectively transforming my lines layer into nothing but lines.
OK now that I have things in the right orientation, I can click on my mask again (You must have that mask clicked on!). Now, I use a basic paintbrush to paint white into the spots that I want to be included as part of the T-shirt (ie. appear red). It's even easier then it sounds! With a few key strokes I have now defined exactly what part of the image is T-shirt and what is not.
Where to go from here? The logical next step is to mask all the other sections. If you have a huge drawing with multiple areas there are some clever little tricks to make things go faster but let's stick to how we've been doing things so far.
And where to go from here? The possibilities are endless really. You can permanently apply the mask (if you want to do something like layer effects to it), you can open it up as a selection again using select - load selection, or you can copy it to a new blank layer (using alt - drag) and use that new layer to do some painting of highlights or shadows. The great thing about painting on a fresh masked layer, is that in no way do you need to care about edges. No more carefully applying your paintbrush to the exact edge, you can start wherever you like and lazily throw in random quick brush strokes that in about 5 seconds give you something like this:
Have fun!