Interpreting the Color Table

The biSize (bcSize) field of the BITMAPINFOHEADER (BITMAPCOREHEADER) structure specifies how many bits define each pixel and specifies the maximum number of colors in the bitmap. Its value affects your interpretation of the color table.

The biSize (bcSize) field can have any of the following values:

Value

Meaning
1
The bitmap is monochrome, and the color table contains two entries. Each bit in the bitmap array represents a pixel. If the bit is clear, the pixel is displayed with the color of the first entry in the color table. If the bit is set, the pixel has the color of the second entry in the table.
4
The bitmap has a maximum of 16 colors. Each pixel in the bitmap is represented by a 4-bit index into the color table. For example, if the first byte in the bitmap is 0x1F, then the byte represents 2 pixels. The first pixel contains the color in the second table entry, and the second pixel contains the color in the 16th table entry.
8
The bitmap has a maximum of 256 colors. Each pixel in the bitmap is represented by a byte-sized index into the color table. For example, if the first byte in the bitmap is 0x1F, then the first pixel has the color of the thirty-second table entry.
24
The bitmap has a maximum of 224 colors. The bmiColors (bmciColors) field is NULL, and each three bytes in the bitmap array represent the relative intensities of red, green, and blue, respectively, of a pixel.

Note on Windows DIBs For Windows 3.0 DIBs, the field of the BITMAPINFOHEADER structure specifies the number of color indexes in the color table actually used by the bitmap. If the biClrUsed field is set to 0, the bit map uses the maximum number of colors corresponding to the value of the field.

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