411 REFERENCE · Wikibooks
MATLAB Programming/Psychtoolbox/Screen Commands/FrameOval
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Usage:
Screen(windowPtr,'FrameOval',[color],[rect],[penWidth],[penHeight],[penMode])
Draw the outline of an oval. color is the clut index (scalar or [r g b] triplet) that you want to poke into each pixel; default produces black with the standard CLUT for this window's pixelSize. Default rect is entire window. Default pen size is 1,1.