Bitmask to Extract GIF Color Table Size

This article explains how to extract the color table size from a GIF descriptor's packed byte using a bitmask. It covers the specific bitmask required for both the Logical Screen Descriptor and the Image Descriptor, details the bitwise layout of the packed byte, and demonstrates the mathematical formula used to compute the total number of colors in the table.

The Bitmask: 0x07

The specific bitmask applied to extract the color table size from the packed byte of a GIF descriptor is 0x07 (hexadecimal), which corresponds to 00000111 in binary or 7 in decimal.

Applying this bitmask isolates the lowest three bits (bits 0 through 2) of the packed field, which define the exponent used to calculate the actual color table size.

Packed Byte Layout in GIF Descriptors

In the GIF87a and GIF89a specifications, color table size information is stored in a packed field byte within two different structures: the Logical Screen Descriptor (for the Global Color Table) and the Image Descriptor (for a Local Color Table). Both structures place the size value in the lowest three bits.

Logical Screen Descriptor Packed Fields

• Bit 7: Global Color Table Flag (1 = present, 0 = absent)
• Bits 4–6: Color Resolution
• Bit 3: Sort Flag (1 = sorted by decreasing importance)
• Bits 0–2: Size of Global Color Table

Image Descriptor Packed Fields

• Bit 7: Local Color Table Flag (1 = present, 0 = absent)
• Bit 6: Interlace Flag
• Bit 5: Sort Flag
• Bits 3–4: Reserved
• Bits 0–2: Size of Local Color Table

How to Calculate the Total Color Count

The 3-bit value extracted using 0x07 does not represent the direct number of entries. Instead, it serves as an exponent \(N\) (ranging from 0 to 7).

The actual number of color entries in the table is calculated using the formula:

\[\text{Table Size} = 2^{(N + 1)}\]

• If bits 0–2 evaluate to 0, the table contains \(2^{(0 + 1)} = 2\) colors.
• If bits 0–2 evaluate to 7, the table contains \(2^{(7 + 1)} = 256\) colors.

Because each color entry in a GIF palette consists of 3 bytes (one byte each for Red, Green, and Blue), the byte length of the color table is the number of colors multiplied by 3.

Implementation Example

In programming languages such as C, Java, Python, or JavaScript, the extraction is executed as follows:

// Extract the raw 3-bit value
unsigned char n = packed_byte & 0x07;

// Compute the number of color entries (2^(N + 1))
int color_count = 1 << (n + 1);

// Compute total size in bytes (3 bytes per RGB entry)
int table_byte_size = color_count * 3;

This operation confirms the exact memory footprint of the palette data immediately following the descriptor.