Make Layer Masks from Alpha channels
Channels contain the brightness information for the colours that make up an image.
An RGB image, for example, has four channels – one for each colour and a global RGB channel.
Clicking a channel thumbnail in the Channels panel reveals a monochrome image, showing the brightness distribution of that channel.
Alpha channels, however, are created when selections are saved or masks are created.
They also display 8-bit monochrome images, and they can be edited using the painting tools before being turned back into a selection and used to create a mask.
Clipping Layer Masks
A clipping mask can be created easily from a layer with transparent areas via Layer>Create Clipping Mask.
It masks off the image beneath it in the Layers panel, showing only those areas that are overlaid by an object in the clipping mask layer.
If a clipping mask is created from a layer containing text, for example, then the letters will be created using the image beneath.
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