Learn the fundamentals of layer masks in Photoshop, a powerful and non-destructive way to hide or reveal parts of a layer without permanently erasing any pixels.
In the Layers panel, click on the layer you want to add a mask to. This will make it the active layer.
Click the Add Layer Mask icon (a rectangle with a circle inside) at the bottom of the Layers panel. A white mask thumbnail will appear next to the layer's thumbnail.
Choose the Brush Tool from the toolbar (or press B). Ensure your foreground and background colors are set to black and white. You can press D to reset them.
Make sure the mask thumbnail is selected (it will have a white border around it). Paint with black on your image where you want to hide parts of the layer. The black paint on the mask makes the corresponding area of the layer transparent.
Press X to swap your foreground color to white. Paint with white on the areas you want to bring back or reveal. Painting with white makes the layer opaque again.
Pro Tips for Working with Layer Masks
Painting on the mask with shades of gray will create semi-transparent effects. The darker the gray, the more transparent the layer becomes.
Press D to reset colors to default black/white. Press X to swap your foreground and background colors.
Hold Alt (or Option on Mac) and click on the mask thumbnail to view the black and white mask directly on your canvas.
Hold Shift and click on the mask thumbnail to temporarily disable it and see your layer without the mask's effect. A red 'X' will appear over it.
With the mask thumbnail selected, press Ctrl + I (or Cmd + I on Mac) to invert the mask, swapping all black and white areas.
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