Each pixel in a color rendered image consists of three values or channels. Each of the three channels represents the amount of red, green, or blue in the image. Maya image files also contain an alpha channel (or mask channel) which represents the presence and opaqueness of objects, and a luminance channel which represents the intensity or brightness of the image.
You can display each of these channels individually as planes using the options on the Display menu. If you are in the scene painting view, the values that display are for the snapshot of the view, not just the rendered strokes.
You can also use the Display RGB Channels icon on the toolbar.
You can also use the Display Alpha Channel icon on the toolbar.
Alpha channels for the scene painting view are completely white. To see the alpha of the scene painting view, render the scene (Render > Render Current Frame) and in the Render View, select the Display > Alpha Channel.
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