Horizontal overlay causes the left and right halves of the video frame to be
treated as two separate images which are composited, or blended. The resulting
blended image is then stretched to fill the entire display. As an example, if
the graphics board is generating a 2048x768 image, then the final display would
consist principally of the left 1024x768 image, with some parts overlayed by the
right 1024x768 image. The compositing is done within the rectangle defined by
Datapixx(‘SetVideoHorizontalOverlayBounds’, boundsRect). Within the bounding
rectangle, the two images are blended according to an alpha function which is
defined by calling Datapixx(‘SetVideoHorizontalOverlayAlpha’, alphaTable).
Because SetVideoHorizontalOverlayBounds can be called many times per video
frame, horizontal overlays can be used to generate very low latency
gaze-contingent displays, without having to wait for OpenGL to page flip.
See DatapixxGazeContingentDemo.m for an example.