>Psychtoolbox>PsychAlphaBlending

matProduct=AlphaTimes(mat1, alpha)

Calculate the product of an image matrix and alpha as would Screen Alpha
blending: Pointwise multiply and round. Argument ‘alpha’ may
be either a scaler or the alpha plane. If an alpha plane, it must have
the same x y dimensions of argument ‘mat’.

see also: AlphaSum, PsychAlphaBlending