SAOimage display of M-51

SAOimage Color Palette

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Most color workstations use an 8-bit color map, making it possible to choose a palette of 256 colors and to map each pixel on the screen to one of the 256 possible colors. Each of the 256 colors in the palette can be any color specified by its red, green, and blue intensities. By contrast, IBM PC's commonly offer palettes of 4 or 16 colors selected from a restricted group of colors. At the other extreme, 24 bit displays allow each pixel to be mapped directly to virtually any color, defined by its red, green, and blue intensities (using 8 bits for each). If you are using a display with more than 8 bit planes, check out this advice.

X11 uses a color reservation system by which each application reserves a portion of the palette for its own use. Color reservation prevents an application from changing the colors of other applications present on the screen. Typically, SAOimage will be able to reserve as many as 240 color entries in the palette. However, if you (or your window manager) have tailored your X environment by specifying a variety of unusual colors for terminal windows, the clock, and other commonly used applications, there will be fewer unreserved colors in the palette. Conversely, if you are running an SAOimage which has reserved all of the available colors, you may be unable to bring up a new application which expects to be able to grab new colors of its own. A color in the palette is often referred to as a color cell (referring to its reservation) or color level (referring to its index between 0 and 255).

SAOimage uses color cells for more than just rendering the image. The adjustable cursor and the region cursors must also be referenced to colors in the color palette. When the cursor is drawn, pixels in the display are set to the value of the cursor color, overwriting the original image data. If no special care was taken, the image would gradually be erased as the cursor was moved about the display. To avoid this problem, SAOimage uses either of two strategies for tracking the cursor while it is in motion.

With one strategy, while the cursor is moving, or being adjusted, the bits in the displayed image are simply reversed to represent the cursor, and flipped back when the cursor moves away. This doesn't draw the cursor in its correct color, but, in most cases, it produces a visible cursor. When the cursor adjustment ends (the mouse button is released), the entire image is redrawn with the cursor in its new position. This same strategy is used to track the cursor on halftone displays.

The other strategy actually reserves half of the colors just for drawing the cursor. In this alternative strategy, each image color level has a corresponding level in the palette which has the cursor color. The image color levels differ from those of the cursor by one bit, which can be set and unset to represent either the cursor or the image. Drawing the cursor simply involves manipulating the distinguishing bit in display memory. In this mode, cursor can be tracked smoothly in its correct color without erasing the image data. This system is analogous to reserving "planes" of display memory to be treated as overlay planes and image planes (X does not as yet recognize true hardware overlay planes). The advantage of the "plane" reservation strategy is that cursor tracking is visually flawless. The disadvantage is that fewer than half of the color levels are available for rendering the image. Fortunately, it is easy to switch between the two modes (using the "ovlay" button), so one can choose whichever coloring strategy is most appropriate for the current activity.

[Color Palette] [Color Maps] [Color Table Manipulation]