Images Compression

1
Images Compression

• Compression eliminates data to reduce file size
• Two variables
• Reduction in color depth
• Reduction in image quality

2
Images Image for Comparison
Traquair at Sunset Windows BMP (85K)
3
Images BMP vs. GIF
Displayed at 100
Original BMP (85K)
GIF Adaptive Palette (19K)
GIF Web Palette (6K)
GIF Web Palette Dithered (12K)
4
Images BMP vs. GIF
Magnified at 300
Original BMP (85K)
GIF Adaptive Palette (19K)
GIF Web Palette (6K)
GIF Web Palette Dithered (12K)
5
Images BMP vs. JPEG
Displayed at 100
Original BMP (85K)
JPEG Quality9 (22K)
JPEG Quality5 (11K)
JPEG Quality1 (9K)
6
Images BMP vs. JPEG
Magnified at 300
Original BMP (85K)
JPEG Quality9 (22K)
JPEG Quality5 (11K)
JPEG Quality1 (9K)
7
Images BMP vs. PNG
Displayed at 100
Original BMP (85K)
PNG 24-bit (42K)
PNG 8-bit - Adaptive (10K)
PNG 8-bit Web Palette 100 Dither (10K)
8
Images BMP vs. PNG
Magnified at 300
Original BMP (85K)
PNG 24-bit (42K)
PNG 8-bit - Adaptive (10K)
PNG 8-bit Web Palette 100 Dither (10K)
9
Images Comparisons
10
Images Comparisons
Original BMP (85K)
JPEG Quality5 (11K)
GIF Web Palette Dithered (12K)
PNG 8-bit - Adaptive (10K)
11
Images Comparisons
4-Up view in Fireworks 3
12
TTYP Which type why?

• GIFs do a better job with images that have blocks
  of solid colors and hard edges.
• JPEGs do a better job with photographic images.
• PNGs can do well with both, but have limited
  browser support.

13
Images Comparisons

• For pages of comparisons, see this graphics
  bible
• Weinman, L. (2002).
  (4th ed.). Indianapolis New Riders Publishing.

14
TTYP Identical Images?

• You have a Web page that contains two copies of
  the same image displayed at the same size. Yet
  one takes longer to display than does the other.
  Why?

15
Images - Resolutions

• To go from analog to digital requires sampling
• Continuous tone to sampled pixels
• In printing dots per inch (dpi)
• On the screen pixels per inch (ppi)

16
Images - Resolutions

• Most monitors display at 72 to 96 pixels per
  inch.
• If you scan an image at 300 dpi, when it displays
  on your screen, it will display about four times
  as big as its original size.