Unavoidable gridlines in pcolor pdf output

I started reading some of Adobe's documentation to find out if there's
something I've overlooked, and noticed that when I read the PDF
Reference in Preview.app, some example images have zoom-dependent
gridlines that look a lot like the output from pcolor. If even Adobe
creates PDF files that have this kind of artifacts, maybe it's a
rendering bug in the other viewers, or possibly the imaging model is


Jouni K. Sepp�nen