How to use masks to make a composite
The first step in creating a composite image from two or more shots is to create a multi-layered file by dragging the layer from the Layers panel in one image to the other image, or by simply copying and pasting it in.
Next, click the Add layer mask icon at the bottom of the Layers panel. You’ll see the mask thumbnail adjacent to the selected layer in the Layers panel.
Choose the Brush tool from the Tools panel, press D to ensure the brush is set to black, then start to paint over the areas you want to mask in the attached layer.
Choose a soft-edged brush to create a mask with a soft edge, or a hard one to give a clean edge.
As you paint, notice how the mask thumbnail changes in the Layers panel. Press X to switch to a white brush, then paint out the mask if necessary.
You can also apply a gradient to a mask. Just pick the Gradient tool from the Tools panel and drag across the image.
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