Psychtoolbox>Screen.{mex*} subfunction

[sourceFactorOld, destinationFactorOld, colorMaskOld]=Screen(‘BlendFunction’, windowIndex, [sourceFactorNew], [destinationFactorNew], [colorMaskNew]);

Return or set the current alpha-blending mode and the color buffer writemask for
window ‘windowIndex’.
Alpha blending is a way to combine color values of pixels already in the window
with new color values from drawing commands. Alpha blending is disabled by
default: If you overdraw some pixel location (x,y) in your window with a new
source color value [Rs Gs Bs As], then the window location gets that color value
assigned: I(x,y) = [Rs Gs Bs As]. However, OpenGL also allows you to combine
such new color values with color values previously stored in that location. The
old values are called destination colors [Rd Gd Bd Ad]. The way old and new
destination and source colors are combined into new destination colors is called
alpha-blending. Formally:
New color at location (x,y) is [Rn Gn Bn An] = blendequation([Rs Gs Bs As], [Rd
Gd Bd Ad]);
where blendequation is a function that describes how the new [Rs Gs Bs As]
color values and previous old values [Rd Gd Bd Ad] should be combined. You can
choose the ‘blendequation’ from a set of defined blend equations via choice of
the ‘sourceFactorNew’ and ‘destinationFactorNew’ arguments. See help
PsychAlphaBlending and the help texts for the functions in that folder for
possible choices of blend factors. The default setting of GL_ONE, GL_ZERO
disables blending.
The most common alpha-blending factors are sourceFactorNew = GL_SRC_ALPHA and
destinationFactorNew = GL_ONE_MINUS_SRC_ALPHA They are needed for proper
anti-aliasing (smoothing) by Screen(‘DrawLines’), Screen(‘DrawDots’) and for
drawing masked stimuli with the Screen(‘DrawTexture’) command. See DotDemo,
LinesDemo, AlphaImageDemo, GazeContingentDemo for a few applications of
alpha-blending.

This function also allows to return and set the color write mask for a window:
You can prevent Psychtoolbox from drawing into and changing the content of one
or more color channels by disabling the color channel for writing. This allows
to perform drawing commands that only affect, e.g., the red chahnnel, but not
the blue, green or alpha channel. You choose the channels by specification of
the ‘colorMaskNew’ vector: The first element selects if writing to the red color
channel are allowed (value greater than zero) or disallowed (value equal zero).
The 2nd element selects the green channels state, the 3rd element selects the
blue channels state and the 4th element selects the alpha channels state. E.g.,
setting colorMaskNew equal to [1 1 0 0] would allow updates of the red and green
channel, but not of the blue and alpha channel. A setting of [0 1 0 1] would
allow updates to the green- and alpha channel, but not the red- and blue channel
etc. The default writemask is ‘all enabled’ ie. [1 1 1 1].

Settings for alpha-blending and color write mask are per window: They can be set
individually for each onscreen window, offscreen window or texture.

###See also: DrawDots, DrawLines, DrawTexture