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Layer masks - Photoshop Guide - Barry Beckham and David Rowley explain how layer masks work.
Words & Pictures Barry Beckham/David Rowley
Layer masks can be a little tricky to grasp straight away as the work you do within the mask is not directly seen. However, the effects the mask has on your image can be seen. We have created a red and blue layer simply as an example and the icon to create a layer mask is found at the bottom of the layers palette as shown below.
To add a mask, click on your layer and then click the mask icon and the mask will appear alongside your thumbnail.
When a layer mask is selected your colour picker defaults to black and white and they are the only two colours that can be used on the mask. If you do try selecting another colour, the colour will default to the equivalent shade of grey. The colour picker is shown on the toolbar below.
Create this simple two layer example yourself and add your own layer mask.
When a layer mask is selected the icon to the right of the little eye shown above changes to the mask icon and this tells you that you are about to edit the mask and not the image.
You could create exactly the same effect with the eraser tool, but once you remove those pixels from the blue layer they would be gone forever. You cannot easily replace those pixels if you need to make adjustments later. Using your own example switch the colour picker to white by clicking the two headed arrow in the colour picker box.
Now spray white into the picture area and the real magic of the layer mask is revealed. You can undo any masking that you did with the black spray by spraying white. No pixels have ever been removed from your little example, just masked. Hence the name layer mask.
The layer mask can be temporarily switched off by holding down the shift key and clicking within the mask thumbnail in the layers palette. A red cross will appear on the mask and the effect will be turned off. To turn the mask on again simply click within the mask thumbnail.
You can view just the mask by holding down the alt key and clicking the mask thumbnail from within the layers palette. Your image on screen will change to show just the mask. To return to your image click on the layer eye within the layers palette.
You can use any of the Photoshop tools within a layer mask, try a gradation tool set foreground to transparent.