biClrUsed

biClrUsed is a field in the BITMAPINFOHEADER structure used by the Windows BMP (bitmap) file format. It is a 32-bit unsigned integer that indicates how many color table entries are actually used by the bitmap. The field is relevant for palettized or indexed color images, where the image stores color indices into a palette rather than direct color values.

In practice, biClrUsed applies when biBitCount is 8 or less. The color table (palette) for such images can contain up to 2^biBitCount entries. biClrUsed tells how many of those palette entries are used by the bitmap. If biClrUsed is zero, the bitmap is assumed to use the maximum number of colors for the given bit count (for example, 2 for 1 bpp, 4 for 2 bpp, 16 for 4 bpp, or 256 for 8 bpp), and all palette entries may be significant.

If biBitCount is greater than 8, the biClrUsed field is not meaningful and should be zero; in practice it is ignored by decoders for non-indexed images.

biClrImportant is a related field that indicates how many color indices are considered important for displaying the bitmap. Decoders may preserve these important colors during palette reductions or quantization, while others may ignore less important entries.

Overall, biClrUsed helps specify the actual color information used from the palette, enabling more efficient storage and potentially simpler color-management for indexed BMP images.