Question #2

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

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

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

• Not enough intensity resolution for typical images