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IMAGE PROGRESSING FUNDAMENTALS

Part 1: Playing with the Pixels

by James Antonakos

Start ý Imaging 101 ý RLE Compressor Tool ý Adjusting the X-Y Resolution ý Adjusting the Z-Axis Resolution ý Simple Image Operations ý Run-Length Compression ý More to Come ý Sources and PDF

ADJUSTING THE Z-AXIS RESOLUTION

The z-axis resolution is another name for the number of intensity levels available for each pixel. Using six bits to represent a pixel allows 64 shades of intensity (0 equals black, 63 equals white). An 8-bit z-axis resolution allows 256 shades.

Just as the x-y resolution affects image quality, so does the z-axis resolution. Photos 3aýe show a progression of processed images, each with a different z-axis resolution. Photo 3a uses a single bit for each pixel intensity, providing just two shades for each pixel. Clearly, there is a great amount of detail missing from the image. Even so, some images, especially those containing only text or simple line drawings, can get by with only two shades. These types of images, called binary images, are easily transmitted over a fax machine.

a)	b)	c)	d)	e)
Photos 3aýeýThese five images show how the z-axis resolution affects the details of an image. The number of bits per pixel are (a) one (two levels), (b) two (four levels), (c) three (eight levels), (d) four (16 levels), and (e) five (32 levels).

Photo 3b looks better with two bits per pixel (four shades) but still lacks detail. Photo 3c, using three bits for each pixel (eight shades), provides the first glimpse of the background in the image. Compare the contour lines in the background of Photos 3cýe to see how additional shades bring out the background details.

Even though Photo 3e uses only 32 shades, its quality is almost as good as the original 64-shade image.

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