You have just seen how you can use quick masks to “paint on” selection regions and apply effects and adjustments to very specific image areas. It is now time to learn about another important mask type: the layer mask.

With a layer mask, you can basically paint a layer on or off an underlying image. This is a great way to make composite photos with elements from multiple images.

You can also apply a black and white gradient to a layer mask, with the excellent effect of having the image layer with the mask appear as a gradient over the background. This works because the black pixels of the gradient will not display the layer associated with the layer mask, but the white gradient pixels will. The result is a gradual fading out of the layer according to the black and white gradient you apply. If this seems a little confusing right now don’t worry. The following examples should clear things up somewhat.

To create a layer mask, you can click the new layer mask icon at the bottom of the layers palette. (The icon looks like a grey square with a circle in it.). When you click the new layer mask icon, a layer mask will be added to whatever layer is currently active.

You can tell if there is a layer mask associated with a layer by the white square and chain links that appear in the layer.

You will also see a grey square with a white circle next to the eye icon. This means that any edits to this layer will be applied to the mask. If you click the square with the checker pattern, the mask icon (a grey square with a circle) will turn into a paint brush icon indicating that the layer can be edited. Click the white square to the right of the chain links to place the layer back in mask mode.

If the layer is in mask mode, you can paint on it with black to reveal the image layer/layers beneath. If you paint on it with white, you will cover up the image beneath.

This image shows a picture of a sand dune, with a greyscale picture of a man overlaid on top. The layers palette for this composite image is shown below.

You will notice that there are two layers in the palette. The bottom layer (background) contains the image of the sand dune. The top layer contains the greyscale image.

You can see that there has been a layer mask added to the top layer. This was done by clicking the new layer mask icon.

The grey square with the white circle that is located next to the eye icon indicates that any editing performed on this layer will be applied to the layer mask. That is to say, the layer mask is currently active.

If we select a paintbrush from the toolbox, and make sure that the foreground colour is black, we can paint on to the layer mask to reveal the background layer underneath.

In this example, you can see the brushstrokes where black was painted onto the layer mask. To prevent the underlying layer from coming through, you can change the foreground colour to white and then paint over the layer mask.

You can zoom in and use a fine tipped brush to paint detailed areas, specifying precisely what regions of the background will be visible. Here is our image zoomed in:

Here is the result after the layer was repositioned over the background with the Move tool, and retouched with the Healing Brush in edit mode.

You now have a means of specifying exactly what parts of an image layer will be viewed on top of the image layers beneath. You could potentially have several layers, each with an associated layer mask, contributing to the final image.

It is a simple matter to make a selection from one image, use Edit->Copy to copy the selection to the clipboard, and then paste the selection to a new layer created for another image. You can then apply a layer mask to the new layer, and fine tune the selection with the painting tools.

As mentioned previously, you can combine a gradient with a layer mask for a neat effect.

Here we have an image of a palm tree, and a plain black canvas. If we make the palm tree image the active document, and then choose Select->All, we can then copy the selection and paste it to a new layer over the black canvas background.

Here is the layers palette after the palm tree image has been pasted to a new layer over the black canvas background.

Since both layers are set to visible, only the top layer will be seen, as it is 100% opaque.

If you click the new layer mask icon, a layer mask will be added to the active layer.

Here is the layers palette after adding a layer mask to the active layer.

By default, the layer mask is active when you create it. If you wanted to edit the image on the layer (palm tree) you would click the small square containing the palm tree thumbnail. The grey square with the white circle that is next to the eye icon would be replaced with a brush. You could then go ahead and edit the layer without editing the mask.

If the grey square with the white circle is next to the layer (instead of the brush icon) then you will be editing the mask and not the layer.

Where you apply black to the mask, the underlying layer will be visible. Where you apply white to the mask, the top layer (the one with the mask) will be visible. If you apply a black and white gradient to the mask, the top layer will be visible as a gradient over the underlying layer.

Here is the same layers palette after adding a simple black and white gradient to the layer mask. As usual, you can adjust the transparency of the layer, or add other effects if you wish. Just remember to click the proper thumbnail in the layer, so you can edit the layer and not the mask. 

Here is the resulting image, with all layers visible. The black part of the gradient allows the underlying layer to come through. The white part of the gradient displays the top layer.