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|This manual is a rewrite of the 1.6 manual. It is not complete.|
The Transparency mask is another of the non-destructive editing tools available to you.
It lets you selectively show and/or hide parts of a layer. By using a mask to just hide some pixels you are able to avoid deleting (permanently!) parts of an image that you just might want in the future.
In addition, it allows you to do things like remove a portion of a layer in the layer stack so you can see what's behind it. A very common example would be if you wanted to do some sky replacement. Suppose you had a picture of a barn but it was taken on a kind of sunless, boring, drab, cloudy day and you wanted to replace the sky with one of the many available Creative Commons sky textures from the internet. All you would have to do is add your two images, making sure that the original of the barn is on top. Now add a Transparency mask to that image and click on the mask in the Layer docker. You'll see that the Advanced Color Selector turns to grayscale only. This is because the you can only paint on the Transparency mask in shades of gray, from pure black (fully transparent) to full white (fully opaque). Since you want to replace your sky with the one in the image underneath you would take a brush, set your color to black and then start to paint over your sky. With the first brushstroke you'll see that the image below starts to be revealed. The more you paint the more you reveal.
Here, the original sky on the right is being replaced by the sky on the left.
Of course, if you go too far and paint out part of roof or a tree or whatever, all you have to do is change the color to white and paint it back in.
This is really what we really mean when we say non-destructive. This example is just one of the myriad of ways to leverage the amazing power and flexibility of Transparency Masks.
So to review...Major advantages of using a Transparency mask, is that it is non-destructive. You can always fine-tune and edit what you want visible and any layer. If you discover you've hidden part of your paint layer accidentally, you can always show it again just by painting white on your transparency mask.
This makes for a workflow that is extremely flexible and tolerant of mistakes and encourages rough drafts that can be improved upon.