# Question #2

Normally images displayed on printers require halftoning or dithering.

A. (5 points) What is halftoning and why does it work?

What?
Halftoning approximates a continuous grey / color image with a clustered dot dither matrix. It is particularly good for printers because the matrix creates blobs of ink that gradually increase in intensity.
Why?
There is a trade-off between spatial resolution for intensity resolution because the eye spatially averages the intensities.

B. (10 points) Suppose a grey-scale image is printed on a 300 dots per inch laser printer. The operator decides to print the image using 3 different size halftone cells (each halftone cell is replicated across the page).

10 halftone cells per inch
60 halftone cells per inch
150 halftone cells per inch
Which printout will look best? Describe why the other two printouts will not look as good.

60 halftone cells per inch will look the best because it offers a good compromise between spatial resolution and intensity level resolution.

10 cells / inch: 30 x 30 cells ==> 30^2 + 1 = 901 levels
• Excessive levels and low spatial resolution

60 cells / inch: 5 x 5 cells ==> 5^2 + 1 = 26 levels
• Good

150 cells / inch: 2 x 2 cells ==> 2^2 + 1 = 5 levels
• Not enough intensity resolution for typical images

hanrahan@cs.stanford.edu