Basic Layer Masking
Tutorial by Doug Nelson
Layer masking provides a non-destructive way to hide or show selected areas of a layer.
1. Let's start with an image. In this case I've chosen a violet on a contrasting background.
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2. I add a layer mask by going to the Layers Pallette and clicking on the Add Layer Mask icon. This will add the layer mask and show a plain, white thumbnail next to your image thumbnail. (Note: Layer masks cannot be added to locked layers or to the "Background" layer. Notice in this example I've renamed my Background layer to unlock it. Rename a layer by double-clicking on the name.)
3. Alt-clicking or Option-clicking on the layer mask thumbnail will display it in your editing window. As you can see it is 100% white. Anything white on a layer mask will show the layer. In this case it's all white, so we can see the entire layer. Anything black on a layer mask will hide, or "mask", that part of the layer. Gray tones will partially mask the layer.
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4. We could paint on the layer mask, either while it's visible, or while viewing the original image. Remember: painting black will hide, painting white will reveal. Since I want to mask just the flower, I need a black flower on a white background. I'll inspect the color channels to see if there's one that can give us a headstart. We're in luck, the green channel has the high contrast between the flower petals and the background that I'm looking for. I hit Ctrl-A or Command-A to select all, then Ctrl-C or Command-C to copy the green color channel. On low-contrast images I usually just paint directly on the mask, but any technique you know to isolate your masking subject will work for this.
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5. Going back to my main image, I Alt-click or Option-click on the layer mask thumbnail to show it in the editing window. Then I press Ctrl-V or Command-V to paste my green channel information into the layer mask.
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6. This has way too much information, plus I want good blacks and whites, not a lot of grays, so I use the Threshold command (Image>Adjustments>Threshold) to isolate just the flower.
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7. I clean up the area I don't want masked by painting with white. I also go over the flower with black to give a good, solid mask. To provide a less severe mask around the edges, I also apply a small amount of gaussian blur.
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8. Clicking on our main image thumbnail reveals the result of our efforts.
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9. You can disable a layer mask at any time by shift-clicking on the layer mask thumbnail. Repeat to enable.
10. Here I've clicked on the layer mask icon and inverted (Ctrl-I or Command-I, or Image>Adjustments>Invert). This exchanges the black and white values, therefore reversing the layer mask.
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11. Layer masking provides an excellent opportunity to experiment with your images, all the while remaining completely non-destructive and reversible. Here I've copied the mask we just made and applied it to a Hue/Saturation adjustment layer.
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