indexed color

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indexed color

A method of compressing an image that enables 8 bits per pixel to look almost as good as 24 bits per pixel. The technique determines the 256 most frequently used colors in the image and creates a color lookup table, also called a "color map" or "color palette," that is stored with the image. Rather than each pixel in the image having all three RGB colors (one 8-bit red, one 8-bit green and one 8-bit blue), each pixel contains one 8-bit number that indexes into the 256-color lookup table, which contains the RGB values (see illustration below).

It Used to Be a Major Problem
When early computer screens were commonly limited to 256 colors, indexed color methods were essential. Even then, two indexed photos on screen at the same time with vastly different color schemes would overload the hardware's color capacity and cause images to display improperly. Today, computer hardware easily renders full 24-bit color, but 8-bit indexed images are still widely used to save bandwidth and storage space. See color depth, color palette and color lookup table.


Color Palette Index
The pixels in the image contain index numbers that point to the RGB value in the color lookup table. The RGB values are the ones used by the display system.







24-Bit vs. 8-Bit
Indexed color images can look nearly identical to their 24-bit originals, because the 256 most frequently used colors are identified. The 256-color palette (top) was created in Photoshop from the original 24-bit image (left) to generate the image as 8 bits (right).


24-Bit vs. 8-Bit
Indexed color images can look nearly identical to their 24-bit originals, because the 256 most frequently used colors are identified. The 256-color palette (top) was created in Photoshop from the original 24-bit image (left) to generate the image as 8 bits (right).
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References in periodicals archive ?
The color map begins with green, but then runs in the opposite direction from the color map in Figure 1: the solder mask defects rise with altitude to yellow, then to red.
In fact, says Soucy, "operators can set up inspection projects even if they don't have access to a measurement device or a real 3D dataset of a part." For instance, operators can simulate measured object components, data alignments, data color maps, geometry control tables, 3D-scene snapshots and inspection reports before the real measurement task.
Figure 3 shows a juxtaposition of the (in)famous RGB rainbow color map and an alternative HCL rainbow.
Once Hy-Ten received the color map from Laser Design, it sent the suggested changes to the mold maker, Concept Tool and Die in Goffstown, N.H.
Color screen shot examples blend with good-sized color map examples as tutorials consider working with data from both private sector and government sources.
The pocket identification guide includes a color map of the country; range maps; color illustrations by fellow Costa Rican birder Robert Dean (e.g., of anatomical features, adult vultures and raptors in flight); a glossary; taxonomic notes; references; and a comprehensive bird list.
The Touchscreen is a PC-based solution that offers on-screen touch-activated controls and a customized color map of resident rooms.
It performs draft analysis to identify undercuts and zero-draft areas though a user-defined color map of draft angles.
* A color map of ownership plots on the lower, heavily forested shoulders of nearby Mt.
The PC-based Graphical Touchscreen Master offers on-screen touch-activated controls and a color map floor plan of patient rooms.
The team's color map suggests that the oldest stars in the Cartwheel formed about 100 million years ago and lie just outside the inner ring -- two indications that the galaxy was virtually starless before a tiny intruder apparently collided with it more than 100 million years ago.
The text is heavily illustrated with color photos, summary charts, maps, and infographics on every page, plus a two-page color map of the US, and a color illustration on the cover.