biBitCount

biBitCount is a field in the BITMAPINFOHEADER structure used by the Windows API to describe the number of bits used to represent each pixel in a bitmap. It defines the color depth of the image and helps determine how pixel data is stored and how a color palette is applied.

Common values for biBitCount include 1, 4, 8, 16, 24, and 32. Bitmaps with 1, 4, or 8 bits per pixel are typically palette-based, meaning the actual colors come from a color table that follows the header. The palette size is 2 raised to the power of biBitCount. Bitmaps with 16, 24, or 32 bits per pixel usually store direct color data, with 24-bit images using three bytes per pixel (BGR) and 32-bit images often including an alpha channel (BGRA). However, 16- and 32-bit formats may employ BI_BITFIELDS compression to define red, green, and blue channel masks, rather than a simple color table.

The biBitCount value thus affects both memory layout and color representation. For paletted formats, the color table must be present and its size depends on biBitCount; for higher-color formats, a color table is typically absent unless masks are used. The field works in concert with additional header fields such as biCompression and biSizeImage to describe how the bitmap is stored on disk or in memory.

In practice, biBitCount appears in various Windows bitmap variants, including the classic BITMAPINFOHEADER and newer DIB headers, preserving the same core meaning: it specifies how many bits are allocated per pixel and, consequently, how colors are defined and indexed.