PSC03: Layer Masks

If you haven't worked with layer masks before, here's a quick explanation on what they do:

Imagine you have two pictures in front of you on your desk, one on top of the other. Naturally you can't see the bottom picture because the top picture is covering it - but imagine taking a pair of scissor or something else and punch holes into the top picture, so that you can see parts of the bottom picture. Layer masks in photoshop work very similar to that. Imagine now that you have those two pictures on two layers in photoshop. By masking part of the top layer, you can have the bottom layer shine through.

Example 1

In Photoshop this gives us endless possibilities. Here's a small example: Imagine you have the same picture on two layers, one being black-and-white, and the other one being in colour. With the black-and-white layer on top, you can now mask parts of this layer away and have the colour image shine through where you want it to.

Let's try that. Fire up Photoshop and open a picture. Open the Layers Palette by pressing F7 or selecing Window -> Layers from the menu.

Open the first picture in Photoshop

Now duplicate your colour layer by dragging it to the new document symbol on the bottom of the layer palette. That is the second icon from the right. Let's make that top layer a black and white image by selecting Image -> Adjustments -> Desaturate. You now have two layers, one background layer being the colour picture and a copy of it being in black-and-white.

Add a layer mask to this top layer by first making sure the top layer is selected and then clicking the little grey box with the white circle in it at the bottom of the layer palette.

Duplicated the layer, make it b&w and add a layer mask

For the moment nothing will change apart from a white box showing up next to the image. This is your mask, and per default it's white which means it is set to reveal everything on that layer. The way to hide parts of that layer is to paint onto the layer mask with a brush - if you paint black, the layer will be conceiled in the areas where you painted, if you paint white, the layer will be revealed.

Let's reset our colour swatches to a black foreground and a white background colour by pressing the the D key.

Select the brush tool by hitting the B key.

Now that we have set black as the foreground colour and selected a brush, make the brush small enough to be able to paint over the area in the layer that you want to have the bottom colour layer shine through, click on the white layer mask in the layer palette in order to select it and then paint away on your image.

Reveal parts of the lower layer by painting black on the layer mask

Example 2

But that's just one of the many different ways in which you can use a layer mask. Let's look at another example. Here I have tried to take a picture in an environment that had quite a high dynamic range, there were dark areas and bright highlights and the camera could either preserve detail in the dark areas but then the highlights were blown out, or the camera was set to correctly expose the hightlights, but then the darker areas were all black. In order to cope with this situation, I set my camera on a beanbag in order to take two identical images with different exposures. Now that I have one image with the darker areas intact, and one with the highlights exposed correctly, using a layer mask to combine the two is just the way to go.

Put both differently exposed images on separate layers and add a black layer mask

First step is to make those two pictures appear on two layers in photoshop. To achieve this, open both pictures, then hit the V key to switch to the move tool. And with the move tool selected, and the shift key pressed, drag the darker image on top of the brighter one. Pressing the shift key makes sure that the two images are perfectly ligned up. And again select the top layer, and add a layer mask by clicking on the layer mask icon at the bottom of the Layer Palette. Now this time let's conceal the darker top layer: In order to create a new layer mask that conceals everything, just hold the Alt or Option key pressed while clicking the mask button. So now you have two layers, the bottom layer being the bright image, and the top layer with the dark image and a mask that conceals the whole layer.

Now if you paint white into that black layer mask, you can make parts of the darker image appear, for example the highlights that are overexposed in the brighter image.

Assuming you still have the foreground and background colours set to black and white, let's switch those around by hitting the x key in order to paint with white.

Another helper in this situation is to reduce the opacity of your brush to about 10% - this way you can gradually make areas of the top layer more transparent by repeatedly brushing over it, instead of punching hard holes into it.

Reveal parts of the top layer by painting white onto the black layer mask

More cool uses for Layer Masks

The really cool thing about layer masks, however, is the fact that they work with every type of layer. Imagine you use a curves adjustment layer in order to increase the contrast of an image in certain areas only. Or increase the colour saturation of a green meadow but leave the saturation of the cows on that meadow untouched.

Layer masks offer you endless possibilities, and once you got the hang of them, and have a better idea about what great results you can achieve with them, I promise you won't look back.