A grayscale image uses only shades of gray to represent an image.
Black-and-white photographs can use a virtually unlimited number of shades of gray, but most computers can display only 16 or 256. To grayscale is to convert a continuous-tone image, like a black-and-white photograph, to an image made up of pixels. Grayscaling is different from dithering, which uses either black or white pixels next to one another to simulate shades of gray. In grayscaling, each individual pixel can be a different shade of gray.
Image maps are images that have several links geographically mapped onto it.
For example, an image map of a photograph of the Beatles might enable you to click on Ringo and receive a page describing his drumming abilities. Click on George, and receive a file about how Eric Clapton stole Patti Boyd. One thing to remember about image maps is that they are a purely visual form of navigation, so if your visitor isn’t loading the images, they’ll never know where to click. For this reason, you should always include text links under the images as an alternative way to navigate.
Producing images for the web invariably means minimizing the number of colors (and therefore the file size), and the index color system is another step in this squishing process. With a 216-color palette loaded, Photoshop will map an image to those colors when you move it into index color mode. While this helps the compression and allows you to choose bit depth, it also makes the colors dither, or shift numerically, to the palette. One way to compensate for dithering in the index mode is to use a histogram, which is basically a bar graph of each color’s frequency in the image. In most image-processing programs, you can manipulate the histogram and determine how much weight to give certain colors in the resulting palette.
Many image-processing programs, like Adobe Photoshop, allow you to build images in layers. These layers are created one at a time and placed on top of each other to assemble the whole image. While the file is a pile of little layered images, you can manipulate each layer individually and look at how each change will alter the completed picture.
In computer graphics, a color look-up table, or CLUT, is the set of available colors for a given application.
For example, a 24-bit system can display 16 million unique colors, but a given program would use only 256 of them at a time if the display is in 256-color mode. The CLUT in this case would consist of the 16 million colors, but the program’s palette would contain only the 256-color subset. To avoid dithering (i.e., varying the pattern of dots in an image) on 8-bit machines, you should only use colors from a predesignated CLUT.