Images that are less than 8 bits per pixel require a palette, which is an array of color values. The value of each pixel in one of these images is an index into the palette. (Images that are 16 bits per pixel or higher store the color values directly in the image, as pixel values.)

The number of colors in a palette is limited to the highest possible index value. For example, if an image is 8 bits per pixel, it can have a palette of 256 colors, which is the number of different colors that can be referenced by using the pixel values as 8-bit indexes.

If an image file is less than 16 bits per pixel, the file contains a palette, and when you load the image into a bitmap, the palette is also loaded into the LEAD bitmap handle structure.

Fixed Palettes and Optimized Palettes

A Fixed Palette is a single standard palette that contains colors uniformly covering the entire range of possible colors. This is useful if you must convert an arbitrary set of images to use the same palette, particularly if you don't know in advance all of the images which will share the palette, or if you must display multiple images simultaneously on a display device with a bit-depth of 8-bits (256 colors) or less (see Palette Shifts below).

LEADTOOLS gives you the option to convert an image using a Fixed Palette (actually changing the image and image's palette), or to display an image using a fixed palette to paint to the device (without changing the original image). LEADTOOLS supports the LEAD fixed palette and the Netscape fixed palette, which are shown below:
LEADTOOLS also gives you the option of creating a an Optimized Palette , or a palette with the best possible colors for a particular image or set of images. Shown at right is a True Color image which has been converted to 8-bits/pixel (256 color) using both an optimized palette (automatically generated from the original image data) and a fixed palette (for comparison).

Palatte Shifts

So why would you use a fixed palette if optimized palette is going to give you closer color matching? Fixed palettes come in very handy when you have to display several images simultaneously on a device that can only display 256 colors or less. Shown at right is a capture of two images displayed while Windows is in 256 color display mode, both with and without using fixed palettes. When fixed palettes are not used, whichever image is last painted controls what palette the entire display uses. When fixed palettes are used, both images display properly regardless of which image was last painted.