Question #2
Graded by: Pat Hanrahan's Assistant
Normally images displayed on printers require halftoning or
dithering.
A. (5 points) What is halftoning and why does it work?
Answer:
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.
Answer:
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
150 cells / inch: 2 x 2 cells ==> 2^2 + 1 = 5 levels
- Not enough intensity resolution for typical images
hanrahan@cs.stanford.edu
Copyright © 1996 Pat Hanrahan