Reference: setPostProcessing

setPostProcessing(S) sets the set of post-processing operations to S in the current 2D visualization-window. S is a set of string identifiers of pre-defined post-processing operations, including

invert (inverts each pixel's R, G, and B value),
invertValue (inverts each pixel's V value),
invertLightness (inverts each pixel's L value),
greyscale (sets each pixel's S value to zero),
swapBW (swaps white and black pixels),
flipV (flips vertically, i.e. reflects in the line y = h/2, where h is the height of the pixmap), and
flipH (flips horizontally, i.e. reflects in the line x = w/2, where w is the width of the pixmap).