DPI of monitor versus printer

[henmaster]

I have a 15" monitor which is 12 x 9 inches, resolution 1024x768. That means the DPI is 1024/12 = 85 DPI. However, I've read that you must print at 300DPI or higher or you will get a "grainy" picture. Not sure exactly what is meant by grainy, but why does the DPI have to be so much higher for the printer?

 

[RyanM]

The monitor, with 3 colors, can exactly reproduce the necessary color for an image in a single pixel. Also, the light from surrounding pixels blends seamlessly, leading to a good-detail image without high resolution.

A printer, on the other hand, needs 4 colors, and in order to create as good of an image, many hundreds of dots need to be arranged into a precise pattern to reproduce that color. Also, there's no room for "interpolation" on the printer's end, so the image must be high resolution to begin with.

I hope that makes sense. It does in my head, but trying to explain it is difficult.

 

[henmaster]

Thanks, I googled the 4 color part (found out it's called dithering) and I understand it now.