Layer masks is the bread and butter in digital image editing. Whether you like it or not, you need to know layer masking if you want to advance your post-processing skills.
There is no escape from it, unfortunately.
The concept of layer masking is kind of intuitive..it does have a bit of a learning curve but it's not super difficult to learn like luminosity masking.
For those of you who are still learning how layer masks work, I hope this post clarifies some of your burning questions.
This FAQ assumes you work in Adobe Photoshop...but fear not!
The principles can be applied in other editing software because the concept of layer masks is pretty much universal.
What are layer masks?
Layer mask or layer masking is a way to change the opacity and either completely or partially conceal a layer non-destructively. This allows flexibility such as selective editing to a specific part of an image or combining parts of different images into a single file.
What does a layer mask do?
It can change the opacity of both pixel and adjustment layer independent of the layer’s opacity adjustment. You can also set the changes to affect the entire layer or part of it.
Where is layer mask in Photoshop?
All layer masks are visible in the Layers panel. To the far left of each layer, there is either a  thumbnail view of the pixel layer or  an icon representing the type of adjustment for an adjustment layer. When a layer has a layer mask attached, you will see this as  a white rectangle (by default) to the right of the thumbnail or adjustment icon with  a chain in between. This means the layer mask is actively linked to that layer.
How to use layer masks?
A layer mask can be used in 3 ways:
- 1To reveal the entire or part of a layer,
- 2To conceal the entire or part of a layer, or
- 3To partially reveal/conceal the entire or part of a layer.
From the left: white, black and partial/grey layer mask
To conceal/reveal a layer, you apply black, white or grey color to the layer mask. This can be done in several ways using the Brush tool, Gradient tool or making a selection then fill it with black/white/grey.
When to use layer masks?
Use layer masks when you want to non-destructively:
- 1Cut out an object from its image,
- 2Combine parts of several images to create a single image (i.e. blending, composite image, etc.), or
- 3Apply an adjustment to only part of an image.
How to add a layer mask?
All adjustment layers have a white layer mask attached by default. There are 2 ways to add a layer mask to pixel layers: Select the pixel layer you want to add a layer mask by clicking on that layer in the Layers panel. Then, click “Add layer mask” icon at the bottom of the Layers panel to add a white layer mask. If you want a black layer mask instead, hold down Opt/Alt before clicking on the icon.
How to add a selection to a layer mask?
Selections in a layer mask refer to the white area within the mask. When you hold down Cmd/Ctrl + click on a layer mask, the selections get activated and you’ll see marching ants outlining it. You can modify a mask by adding or removing selections anytime. To do so, paint black, white or grey directly onto the mask or make a selection with a selection tool then fill in with black/white/grey.
How to copy a layer mask to another layer?
Click to select the layer mask you want to copy. Hold down Opt/Alt + click on the layer mask and drag it to the layer you want to paste it to. If the layer you want to paste to already has a layer mask, a window will pop up to ask for confirmation. Just hit OK and you’re done.
How to copy a layer mask to another file?
First, both files must have the same dimension. Second, you need to have both files opened in Photoshop. In the file you want to copy the layer mask from, click to select the layer mask. In the file you want to paste, click to select the layer you want to paste it to. Then, go to the top menu and choose Select > Load Selection to open up the Load Selection window. From the Document and Channel dropdown menu, make sure it’s the file name you want to copy from and the name of the layer is correct, then hit OK. You should now see marching ants, which means the selection is loaded. Click the “Add layer mask” icon in the Layers panel and you’ll have the exact layer mask from the other file.
How to invert a layer mask?
Select the layer mask you want to invert by first clicking directly on it in the Layers panel. Use keyboard shortcut Cmd/Ctrl + I to invert white to black and vice versa. You can also go to the Properties panel and click the Invert button. If the Properties panel in your Photoshop isn’t in display, go to the top menu, select Window > Properties.
How to make a layer mask black?
By default, layer masks come “unpainted”, i.e. white. This means the entire layer is visible. To make the layer mask black, click on the layer mask itself to select, then fill it with black by (1) going to the top menu, select Edit > Fill and select Black from the dropdown menu before hitting OK, or (2) hold down Shift + Backspace and do the same.
How many layer masks can be added to an image?
According to a thread in Adobe Forum, you can have up to 8000 layers in a single file in Photoshop. That means you can have up to 8000 layer masks.
How to edit a layer mask?
If you want to edit the selection of a layer mask, do so directly on the mask by using a Brush tool or make a selection and fill it with black/white/grey. You can Opt/Alt + click on the layer mask to display the mask in large to make editing the mask easier. If you want to refine the mask, select the layer mask and click Select and Mask in the Properties panel.
How to disable a layer mask?
To temporarily disable a layer mask, hold down shift and click on the layer mask. You should now see a red “X” on it. The mask is now disabled and so does its effect on the layer. Shift + click on the mask again to enable it. If you want to disable a layer mask permanently, you can delete by right-click on it and select Delete Layer Mask.
How to preview a layer mask in large?
Hold down Opt/Alt and click on the layer mask. It will now be displayed on where the image is supposed to be. You can also edit directly on the enlarged version of the mask.
How to remove the background using layer masks?
Removing the background means “cutting out” an object from the rest of an image. Typically, this is done in composite images or digital art. To cut out an object from its image, you need to make a selection of the rest of the image other than the object, then hit the delete key. Sometimes its easier to make a selection of the object first then inverse the selection. You know the object has been cut out when the background becomes checkered.
Learn more about how to remove background in Photoshop.
Can you layer mask in Lightroom?
Adobe Lightroom does not have layer masks.
Where is the layer clipping mask in Photoshop?
Place the cursor between the two layers you want to clip together. Hold down Opt/Alt and you should see a symbol appear. Click it between the two layers and you will see the symbol now appears in the layer above. This means the layer above is now clipped to the layer below it.
Learn more about how to use clipping masks in Photoshop.
Why can't I apply a layer mask?
This typically happens when you have converted the layer into a smart object. You would know this when the smart object icon is present at the lower right-hand corner of the thumbnail. To apply layer mask, the layer must be rasterized. Right-click on the layer and select Rasterize Layer.