Message-Id: <mailto:199507141552.KAA22927@library.wustl.edu> Date: Fri, 14 Jul 1995 11:51:07 -0400 From: Cynthia Frazier <mailto:cf12@CORNELL.EDU> Subject: Bit depth vs dpi To: Multiple recipients of list IMAGELIB
When I teach the concepts of dots per inch (dpi) and bit depth, I stress that each value affects the quality of the final output.Dots per inch affects the graininess of the image: Is it funny paper quality (low dpi) or National Geographic quality (high dpi).
Bits per pixel (bpp or bit depth) affects the number of colors in the image: Again is it funny paper (just a few colors), TV quality (usually thousands of colors or 16 bpi) or it is millions of subtle shades like a high-end photograph.
I use an image of a cube to represent a pixel (picture element) to get the point across...and I describe the flat area on the face of the cube as the dots per inch, and the depth of the cube as the bits per pixel.
The most important determinant of the necessary dpi and bpp is the nature of the output. If your intent is to include an image in a Hypercard stack that will be distributed in student labs-- your final outcome will have to be only 72 dpi and 8-bit pixel depth. These same parameters apply if you plan to use the image as a GIF on the World Wide Web. (There are some finer points about downsizing to get rid of moire patterns and using JPEG in WWW, but that's another lesson.)
If, on the other hand, you plan to print the image to another medium, your requirements may be much greater. High-end printing can require information as high as 1200 to 2400 dpi and 32 bpp; however, if printing to a laser printer, your needs may be much lower. Most people think that printing to a laser writer requires the equivalent in dpi to the resolution of the lase rprinter. In fact, (and my experience here is limited to Apple printers) images are printed in cells and therefore a 300 dpi printer requires only about 150 dpi in the image.
This goes on and on...but there are some good resources on the web at the following urls:
http://info.med.yale.edu/caim/M_III_6.HTML http://info.med.yale.edu/caim/M_III_5.HTML http://sunsite.unc.edu/otis/notes/otis-compression.html#dimensions