Techno Page: Image Compression

Preliminary Expectoration: these are highly preliminary, subjective and informal notes on a complex topic. Discussions of DCT, et cetera, are best left to the experts.

From a practical point of view, it's worth finding out which of the two image formats supported by the distributed Mosaic viewers -- JPEG or GIF -- gives the best quality for the lowest storage space and transmission time.

The images were all prepared in Adobe Photoshop. The quality levels on the JPEG images refer to the picture quality settings of a slider bar for picture quality versus compression; the two are, non-mathematically speaking, inversely proportional. From Photoshop, only 24-bit RGB color images are saveable as JPEGs, and only 8-bit color images are saveable as GIFs. The black and white image was an 8- bit greyscale. The viewer is JPEGview, running on a Macintosh Centris 650 with 16 Mb RAM and standard 8-bit color graphics.

The images are linked below. You'll need a decent amount of screen real-estate to do a subjective comparison yourself, but you can still do 1-on-1 comparisons with a 14" screen

Data

Black and White
UC Berkeley Towers (detail). Photo: Ansel Adams

GIF: 33,476 bytes

JPEG-fair: 11,425 bytes: 34% of GIF's size
JPEG-good: 27,307 bytes: 81% of GIF's size
JPEG-better: 37,303 bytes: 114% of GIF's size
JPEG-excellent: 56,022 bytes: 160% of GIF's size

Color
Kinaree statue at the Grand Palace, Bangkok. Photo: Michael Buckley

PICT (uncompressed 24-bit color scan): 296,820 bytes

GIF (dithered 8-bit color): 104,098 bytes

JPEG-fair: 15% of GIF's size
JPEG-good: 42,624 bytes: 41% of GIF's size
JPEG-better: 86,064 bytes: 82% of GIF's size
JPEG-excellent: 188,428 bytes: 181% of GIF's size; 63% of PICT's size.

Interpretation

In all cases, the JPEG-fair option is to be shunned unless compression is at an absolute premium. The black and white image was blurry, while enormous square artifacts marred the color image.

Black and White

The JPEG-good option produced the best tradeoff between clarity and size; some geometric distortion (bowing) is noticeable in the rightmost roof, and the leftmost chimney is slightly darker than in the GIF original. This option produces a file 80% the size of the GIF. Better quality JPEG modes did provide some additional clarity; in particular, the trees surrounding the central complex had more detail to them. Nonetheless, these files actually took up more space than the GIF, probably because irrelevant color information was recorded.

Color

The JPEG-better option provided the best tradeoff between clarity and size among the JPEG options, but, subjectively, it fell slightly short of the GIF (and, of course, its lesser cousins fell shorter). The failings of JPEG-better most noticeable to me were:
  1. the chromatic aberrations around the cheek and breast of the statue; the reddish tint was magnified in all but the highest quality option.
  2. geometric distortion of the statue's arms and face; the face was broadened slightly.
  3. impurity in the blue-green hexagon to the right of the statue.

The disk space JPEG-better saves over GIF is about 20%. This is not inconsiderable with a large archive; if you have an 80 Mb hard drive, 80 Mb of JPEG-better images beats the heck out of 100 Mb of the equivalent GIF images. Nonetheless, JPEG-better images do not take 20% less time to load than the equivalent GIFs. Although 20% less data comes across the network, the viewer takes a comparatively longer time -- a few extra seconds -- to reduce the 24-bit color JPEG image to 8-bit color than it does to display a GIF image that's 8-bit color to begin with. In other words, the viewer has to dynamically reduce the colors each time that the JPEG image is loaded, whereas the reduction is done once for GIFs, when the image is first prepared. Naturally, this delay will vary with the speed of the workstation and the efficiency of the viewer software; you can confirm this by loading the images yourself. Moreover, if you happen to have 24-bit color, all bets are off. Nonetheless, for the average Mosaic viewing situation -- and I consider mine fairly average -- the time delay for a JPEG-better image to display is greater than that for a GIF.

It's worth noting that JPEG-excellent takes up only 63% of the size of the uncompressed 24-bit PICT file; its quality is markedly superior to the GIF. If there's a probability that plenty of people will view it with 24-bit color workstations, JPEG-excellent provides a good space-quality tradeoff.

Conclusion

Best overall solutions:

J.K. Cohen/jkcohen@uci.edu/February 26, 1994

Jump to:

J.K. Cohen's Author Page