Quick Mask Mode. Guys it's pretty awesome. Does anyone ever use this? If you're sitting there like "IDK, what's a Quick Mask," the answer is, "No, it's not when you use a layer mask as fast as you can." Quick Mask is a tool in Photoshop that lets you more precisely edit your selections. You can activate it by pressing the "Q" key or clicking the little rectangle-with-a-dotted-line-circle-in-it at the bottom of your tools palette. Some of you may already know what it is but hardly ever think to use it for anything. Well I'm here to let you know that it can be incredibly handy if you use it correctly.
Once you've selected something in your image and activated the Quick Mask Mode, it will apply a somewhat see-through red color to everything that you do not have selected. From there on out it works very much like a regular layer mask, except you can see the mask sitting on top of your picture. Painting with white will add things to your selection (take the red away), while painting with black will mask things away from your selection (naturally that would mean it adds red). Then when you hit "Q" again to get out of Quick Mask Mode you'll see the little "marching ants" of your selection around everything that you changed to be not red.
I don't use Quick Mask super often, but there is one technique for which I do employ it quite frequently. While editing images, especially those that were shot on location, I like to blur the background (and a tiny amount of the foreground) so my subject looks even sharper. (You can read more in depth about some ways to make your subject "pop" in this post.) Basically, I'm fudging a slightly shallower depth of field. In order to blur just the background, you of course need a good selection of your model first. I ALWAYS take the time to intricately pen tool my subject.
For this forced/faked shallower depth of field effect, you want to be sure the ground around your subject is just as sharp as they are or it's not going to be believable. After all, they are on the same focal plane. I'll use the rectangular marquee tool (because if nothing is selected then Quick Mask Mode is not very helpful) and select a wide rectangle going across my image where my model's feet meet the ground (see above). However, the focus on the ground would not have a hard edge between what's in focus and what is not, and we want a smooth transition. Once you're in Quick Mask Mode you can actually "feather" your selection by applying a blur filter right to your mask.
You could just feather your selection by a lot without ever using Quick Mask, but then you can't really tell quite how feathered/soft your selection is. Using Quick Mask you can see a visual representation of exactly how gradual of a transition you're creating. I used the Gaussian blur filter at about 200%. (Above is my Quick Mask selection before the blur and below is what it looked like after.) Also, while in Quick Mask Mode you can actually use the transform tool to stretch and/or squish your selection as needed (pretty handy). Once you're happy with the selection, hit "Q" again to exit Quick Mask Mode.
Now, since you've already taken the time to pen tool your subject, you can hold down SHIFT + COMMAND (control for PC) and click on the layer of your silhouetted model and it will add them to your selection. Now that you have a smooth transition on the ground and nice sharp edges on your subject selected, you can press SHIFT + COMMAND + i to inverse your selection (aka selecting everything except your subject and the ground right around them). Once you've flopped your selection you can blur the background. I like to use the Lens Blur Filter. Also, I always duplicate my layer first so I still have a version of my image that is un-blurred just in case I need it later on in my edit.
"But why can't you just blur the layer and then use a layer mask to bring back your subject and the ground?" Good question. The reason I go about doing the blurring in this fashion is because if you just blur the whole image with the subject and everything as one, you'll end up with a sort of fuzzy-halo around the edges of your subject. Their pixels will be slightly blurred into the surrounding background, so even if you mask away the subject from the blurred layer you won't have a nice sharp edge, because the background has obtained some of their edges. Make sense?
This is just one of what I'm sure are many crafty ways to use the Quick Mask Mode to your advantage. You can apply any filter to a Quick Mask, so who knows what other interesting things you could achieve ... and for what reason. Do any of you guys use it for anything? Well hopefully after reading this post you'll have at least one use for it!