For best results with the Highlight op, the number of colors in the DPS context's gray ramp should be such that
fract(((float) numgrays - 1)* 2. / 3.) == 0
In other words, (numgrays = 4, 7, 6, 8, 16, ....). This ensures that the color 2/3 gray is not halftoned.
Given the limited number of colors usually available in the DPS color cube and gray ramp, images with alpha values that are not completely opaque (1) or completely transparent (0) should be avoided to obtain best image quality.
Compositing operations are only defined for pixels values that are in the gray ramp or color cube specified by the gstate. Compositing pixels with values outside the color cube and gray ramp may not yield expected results.