Recently I [attempted] to offer a clear explanation of what a mask is. Now I'm going to give you a quick demonstration of how it works. Pt 3 will be some actual use cases you might enjoy trying.

I'm going to take a brightly colored image and create a very basic B&W layer on top of it. I'm then going to show you how some different masks (applied to the B&W layer) might change the image.

First off, here's our main image. This is a shot I took of pearl harbor during Christmas, from the memorial area. If you've never been to pearl harbor it's quite a place, no history book can quite compare to being there. But I digress:

Also, next to the image itself I'm going to show you the layers associated with it (including the mask).

Ok, so in the upper right hand corner we have our basic background layer (with no mask). This is our starting point.

I'm now going to add just a single new layer above the background which is going to turn the image black and white:

Ok, cool...so take a look at the layer menu now. We can see there is a channel mixer layer sitting above the background and that it has a mask on it! Notice the mask is all white? Remember, all white = apply this to the whole image and all black = don't apply this to any of the image.

Remember when I said we could do anything to a mask that we can do to an image? What do you think would happen if I tried painting on the mask? Lets see!

I'm going to pull out my handy paintbrush and choose black as a color (remember black on a mask means it's going to block the effect of a given layer).

Any guesses as to what's going to happen to my image when I paint some of the mask black?

Ok, ok...you guys are a smart class. Obviously I couldn't fool you. Yes...part of the image is now color. As we can see if we look at the mask, a portion is now black (where I 'painted' it black) and that in that same area you can now 'see down through' to the original color background layer.

Hmm, so painting pure black on a mask blocks things out...but what if i only wanted to...you know, sort partially block it out. What would happen if we were to paint light gray, medium or dark gray on the mask?

Starting from a fresh mask...lets see:

Hey, look at that. using a simple rectangular selection tool I've painted the mask in blocks of gray. All black (on the mask) is full color to almost white (on the mask) which is nearly full black and white.

Now some of you are probably saying to yourself; 'That's all fine and good but when am I ever going to mask big blocks of an image?  And don't tell me I have to paint by hand?'.

This is where the power of combining photoshops tools come in...you can create a gradiated selection and use it as a mask. This is one of the cornerstones of professional image editing. Behold:

Notice there is no hard edge anymore? Instead we have a graceful transition between B&W and color. Think about that for a second, If you have the power to make seamless changes in localized areas of the photo...you can do just about anything.

Whew, that's enough for today! :)

Cheers, -B