When a Layer Mask is added to a layer in Photoshop, it essentially places an invisible canvas over the top of it that allows you to control the transparency of the layer in specific areas.
Typically, this is done by using the Brush Tool to paint areas of the mask black, grey, or white. Which color you use determines the opacity of the pixels in that area of the layer. Black equals 0% opacity and white equals 100% opacity.
However, using the Brush Tool isn’t the only way to create a mask. You can also paste onto a Layer Mask’s canvas, which will create a mask using the tones of the image you choose to paste.
This is a great non-destructive technique for adding texture directly to a layer where you want the background to show through.
How it’s done
Select the layer you would like to mask and click the Add Layer Mask icon at the bottom of the Layers panel (Window > Layers).
This will make no immediate difference to the appearance of your document, but you will now notice a blank Layer Mask thumbnail next to your layer.
Once you’ve done that, press Cmd+A to select all, then Cmd+C to copy the image.
Go back to the document containing your Layer Mask, then hold Alt and click the Layer Mask thumbnail. This will make the Layer Mask’s canvas visible.
Press Cmd+V to paste your image onto the canvas, then scale it accordingly.
Following that, click the Layer Mask thumbnail again to hide the canvas and check out the results.
If too much or too little of the layer is visible, or there’s not enough contrast, you can use the Levels adjustment to edit your Layer Mask’s shadows, midtones, and highlights.
To do this, click the Layer Mask thumbnail and press Cmd+L to open the Levels window, then adjust the Input Levels until you’re happy with the result.