Graphics
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Graphics |
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A bit-map image is stored in the same manner that the computer monitor contents are stored in video memory. A monitor screen image is composed of pixels, similar to the dots in a photograph in a newspaper. The number of pixels that compose a monitor image determine the quality of the image (resolution). The more pixels (e.g., 640 X 480; listed as horizontal X vertical numbers), the higher the resolution. The pixels of a screen image are represented differently depending upon the type of image/monitor display.
In
a monochrome (black/white), image, (like the example at the
left <--), each pixel is stored as a single 0 or 1 value
(bit). A 640 X 480 monochrome image would require
38.4KB of storage.
A grayscale image, (like the example at the right-- >),
usually requires that each pixel be stored as a value between 0 - 255
(byte). Where the value represents the shade of gray of the
pixel. A 640 X 480 grayscale image would require 307.2KB of
storage.
Bit-Map Image Artifacts
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One of the chief problems of a bit-map image representation is that the image cannot easily be scaled without introducing image artifacts. The most noticable artifact is a 'cookie-cutter' or 'stair-stepping' effect around the edges of the objects in the images. This side-effect introduced when scaling bit-maps is termed aliasing or jaggies. Many better image processing programs attempt to reduce this jagged affect through anti-aliasing. The process of anti-aliasing requires the image application to identify and alter the edges of the image by smoothing the pixel's graduations, (shades/colors). The smoothing process reduces the sharp grayscale or color transitions adjacent to the edges. This process takes advantage of human vision system tendency to merge adjacent gradual transitions in chrominance. A process sometimes referred to as dithering. |
In order to see the application of these effects, (jaggies, anti-aliasing and dithering), take place, right click on the grayscale image above and select "Save image as..." from the pop-up menu to save the image to your hard drive.
[Download
and install the shareware program Paint
Shop Pro if you do not alreday have it.] Locate the Paint
Shop Pro image processing application from the 
,
Program menu. Open the saved grayscale image file in Paint
Shop Pro. From the Zoom menu, (located below the
File menu), select a 3:1 zoom factor. Examine the
edges to see the affects of the jaggies, anti-aliasing
and dithering), that has taken place due to the scaling.
(If you cannot locate Paint Shop Pro,
click
here to view a 250% scaled view of the
image in a new browser window.)
In a color image each pixel is represented as three values, one for each of the primary colors (red, green and blue - RGB). Actually these are the additive primary colors; while other color representation systems do exist this discussion will only concern itself with the RGB system.} The size of a color image depends upon the number of shades of each primary color that is stored.
8-Bit Bit-Map Color Images
One common semi-standard that can store 256 different colors is termed 8-bit color. Monitors are capable of displaying millions of colors. This requires that 8-bit color images have color look-up tables, (CLUTs), stored with them to represent which 256 colors, out of the millions possible, are to be used in the image. A 640 X 480 8-bit color image would require 307.2KB of storage, (the same as 8-bit grayscale). This yields acceptable color quality, but does not compare very well to 35mm photographic quality.
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8-bit color Bit-Map image example |
24-Bit Bit-Map Color Images
Another semi-standard that does yield photographic color quality is 24-bit color. Each pixel value is represented as three bytes, (one for each primary RGB color). Thus 256 different shades of red, green and blue is possible for each pixel; yielding 256 X 256 X 256 possible combined colors, (16,777,216). A 640 X 480 24-bit color image would require 921.6KB of storage. (Actually most 24-bit images are 32-bit images. The extra byte of data for each pixel is used to store an alpha channel value representing special effect information.)
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24-bit color Bit-Map image example |
Compare the two color images above. Notice the anti-alias dithering that has been performed on the 8-bit image to compensate for the limited colors. (It is most noticable in the green leaves.) While 24-bit color compares well with 35mm photographs in terms of color, most monitors today lack the approximately 4000 X 4000 resolution of 35mm images.
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© CS Dept Va Tech, 1997-1998. |
All rights reserved. |
