[quote=“Wile E. Coyote”]For PDF output, see [url]Colored 2D histograms in pdf output
For [E]PS output, see the “WARNING” section in the end of the “ROOT - How to use the PostScript interface ?” page.[/quote]
It seems the problem I had is not related to ROOT but to the settings in Acrobat Reader. I am not a Mac user so I do not know where it is in Mac, but in Acrobat Reader in Windows, under edit->prefrences->page display, there is something called “Smooth Text” under rendering. The thin white lines disappeared when I turned off “Smoth line art” and “Enhance thin lines”. May be this helps others with similar problems.