XLIB Programming Manual, Rel. 5, Third Edition

7.5 Allocating Private Colors

In colormaps of the PseudoColor or DirectColor visual classes, a client can allocate read/write cells. Read/write colorcells should be allocated when:

• The application draws something whose color must be changed dynamically without redrawing it. For example, in a color mixing program, the palette must be drawn in colors that change frequently. If this were done with read-only colors, cells would have to be allocated and freed frequently and the palette area redrawn with each new color. However, with read/write colorcells, the steps of allocation and color setting are separate, so that the color of an already allocated cell can be changed at will. Anything drawn using the pixel value of this colorcell will change color immediately when the RGB values in the colorcell are changed.

• The application needs to overlay graphics on top of other graphics in such a way that the overlayed graphics can be erased without disturbing the underlying graphics. For example, in a Computer Aided Design (CAD) package for chip design, it is often useful to overlay the various layers of a chip in different colors on the screen. When one of the layers is removed, you want to avoid having to redraw all the underlying layers. How to do this by allocating read/write cells will be described.

• The system has a huge colormap, and the application needs to set a large number of colorcells. The calls for manipulating read/write colorcells allow you to manipulate multiple cells per call, ...