What is a Layer Mask?

The principle of layer masks explained

The perhaps most powerful technique within Photoshop is the use of layer masks. But maybe you wonder, what is a layer mask? Most people to whom I explain it find it difficult. It is actually quite confusing, but try to understand the explanation below, because the key to good and enjoyable working with Photoshop is understanding how it works.
Don't panic if you don't understand it the first time you read it. Read it a second time, and try to play with it in Photoshop.

If you are not sure what layers are, read my What are layers post first.

Why use Layer Masks?

With layer masks it's much easier to blend parts of layers. What I mean by this is best explained with the following example.
Look at the image below. Here I explain to myself how a camera works :-).

What is a Layer Mask? Putting me and me together is easy with a layer mask
Putting me and me together is easy with a layer mask

To make this image, I didn't have to make a difficult accurate selection. I did the following:

  • I placed the two images on top of each other and aligned them (to be explained later).
  • The top layer is the image where I'm sitting, so this is all I see at that moment.
  • In the upper layer I created a so-called Layer Mask, nothing happens yet.
  • I made this mask black (Edit> Fill...). The complete layer becomes transparent so I see the bottom image of me standing.
  • And then I painted with white and a soft brush on where I sit, until I'm completely visible.
    By using that mask in combination with a soft brush you can see that the top layer (a part of it) blends seamlessly with the bottom layer.

Do not worry if you do not immediately understand how I did it, for now it's important that you understand what the mask is for.

Take a look at the following images to help you understand the principle.

What is a Layer Mask? An example of using a layer mask
Part of the layer 0 you can see through the transparent areas of layer 1, thanks to the layer mask of layer 1
What is a Layer Mask? So black hides the content of a layer to make it transparent
So black hides the content of a layer to make it transparent, so now you can see what is a layer mask.

So what is a Layer Mask?

  • A layer mask is a grayscale image that indicates where the layer is transparent
  • With a layer mask you can hide parts of a layer
  • With a layer mask, the entire content of a layer is retained, but you only see a part of it
  • Where it is black in the mask, the layer content is transparent (invisible, you may see the contents of the underlying layer if there is one)
  • Where it is white in the mask, you see the layer content
  • Where it is gray in the mask, the layer content is 'semi-transparent'

With the knowledge you now have, try to predict what you see on your screen in the example below, where you see a part of the layer panel and at the right of the top layer's thumbnail image you see the layer mask with a black, a white and a gray part.
What do you think is the effect?

What is a Layer Mask? Two layers, the top layer with a layer mask.
Two layers, the top layer with a layer mask. What do you see on the screen? (Click to find out if you were right)

I hope your question what is a Layer Mask? is clearly answered in this post.
If you have questions or suggestions, please leave a message.

In a future post I will elaborate on different ways to use layer masks.

A final tip: pay attention to whether you are working in the layer itself or in the mask. You can see this on the white edge around the layer miniature or around the mask. See the images below.

What is a Layer Mask? Layer active.
You see the white stroke around the layer thumbnail image? Everything you do effects the content of the layer.
What is a Layer Mask? Mask active
The white stroke around the Layer Masks indicates that everything you do effects the mask

A brief summary:

What is a Layer Mask? A brief summary
What is a Layer Mask? A brief summary

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