I need a link to a disussion about rendering RGB to halftone or another related monochromatic rendering algorithm that does this:

Convert colors (RGB) to different size black dots (not Pixels) on a white background for monochrome grayscale. For example, 300dpi graphic input may come out to a 60dpi rendering, very much like what you see in B&W newspaper pictures.

What I'm doing is converting RGB raster to a vector mode plotter, using HPGL.