Possible Duplicate:
Why are some pages in my PDF coming out bold?

I have a PDF document that I made in LaTeX. Some pages contain figures I created in METAPOST. Everything is super-smooth in my PDF viewer, i.e. it's clearly nicely vectorized as it should be when I zoom way in.

However, upon printing the document (on a HP laser printer at the school), only the text-only pages appear smoothly in a good resolution as they should be. The pages with the figures get printed with a rougher lower resolution—even the text on those pages!

I tried two different laser printers at the school, one connected to Mac OS X and the other to Linux. Similar weird results for both.

Any ideas what could be causing this?

I suspect that pages containing the figures are being treated as images, and printed as images by the printer. But I have no idea whether this is the case, and if this is the case, I have no idea why this should be happening.

UPDATE: Could it have something to do with the fact that the figure contains a gray color (somewhere between white and black)? Does that cause the printer to go into some kind of a “color mode” for that page? Could that explain this?