>Psychtoolbox>PsychGLImageProcessing

rval = kPsychNeedTripleWidthWindow

Return a flag that you can pass to the ‘imagingmode’ parameter of
Screen(‘OpenWindow’) in order to request use of triple-width windows.
These onscreen windows are treated by all drawing functions (and geometry
query functions like Screen(‘Rect’, window); or Screen(‘WindowSize’,
window); as if they have three times the width of the underlying framebuffer in
pixels.

This is used for special packed-pixel display modes, where the content of
three visual stimulus pixels is packed into one single output framebuffer
pixel for video scanout. E.g., certain special medical displays for use
in Radiology (cfe. “Eizo RadiForce GS-521”) are able to display
calibrated pure luminance images with a luminance precision of 8 bits
per grayscale pixel at > 15 Megapixels resolution. These displays are driven
over a DVI-D interface with a special pixel data transmission format:
Three horizontally adjacent 8-bit luminance pixels are encoded into
one 24-bit RGB output pixel. The monitor in “Subpixel drive” mode will
output these three “color values” to the three luminance subpixels of
a regular display pixel, thereby achieving three times the horizontal
resolution of the video mode: 2560x2048 –> 7680x2048 ~ 15.6 Megapixels.
To allow natural stimulus drawing our window needs to be three times the
width of the output framebuffer, and the imaging pipeline will pack three
stimulus pixels into the color channels of one output pixel.

Usually user scripts won’t pass this flag, but only special setup
routines that are part of the Psychtoolbox itself.