Demo of quantization levels

CS 248 - Introduction to Computer Graphics
Autumn Quarter, 2005
Marc Levoy

The first test image (top row) is synthetic, so it should require the most number of bits to look good. To my eye, the solid glass cone lying on its side (upper-right corner) begins to exhibit quantization artifacts, i.e. contouring, at 5 bits. The second test image is an optically scanned photograph taken by a film camera. To my eye, the linear luminance version begins to show quantization artifacts at 5 bits, but less severely so than the synthetic image, probably because of film or digitization noise. Look in particular at the top surface of the leading edge of the raft. The gamma-corrected version (original^1/2) exhibits fewer artifacts, and for the sake of exaggeration the inverse corrected version (original²) exhibits more. The third image is an optically scanned hand-painted cartoon background. To my eye, it looks fine at 5 bits, but begins to show artifacts at 4 bits. This reduction in the number of bits is at least partially due to the more random nature of the hand painting. Finally, the fourth image is an optically scanned pencil drawing. It does fine down to 3 bits, due mainly to the narrow spatial extent of the "zone of antialiasing", i.e. of midrange gray pixels, over which the quantization occurs. This same property allows text to be antialiased using few bits.

Note: This demo must be viewed on a gamma-corrected display. On the Sweet hall graphics cluster PCs, you can enable gamma correction by typing "xgamma 1.7". If you don't do this, the quantization effects will be hard to compare, and the subtle differences between applying quantization in different spaces will be almost invisible. For more information about gamma correction and viewing web pages, look at our research lab's tutorial about gamma correction.

original quantization space quantized versions
(converted back to linear luminance after quantization if necessary)

linear luminance
6 bits
5 bits
4 bits

perceived brightness (original^1/2)

6 bits
5 bits
4 bits

linear luminance
(original image)
6 bits
5 bits
4 bits

original²
6 bits
5 bits
4 bits

linear luminance
6 bits
5 bits
4 bits

linear luminance
4 bits
3 bits
2 bits

levoy@cs.stanford.edu

original	quantization space	quantized versions (converted back to linear luminance after quantization if necessary)
	linear luminance	6 bits	5 bits	4 bits
	perceived brightness (original^1/2)	6 bits	5 bits	4 bits
	linear luminance (original image)	6 bits	5 bits	4 bits
	original²	6 bits	5 bits	4 bits
	linear luminance	6 bits	5 bits	4 bits
	linear luminance	4 bits	3 bits	2 bits