How GIF Color Tables Store RGB Values

This article explains how RGB values are formatted and structured within the color tables of a GIF file. You will learn the exact byte-level layout of these color palettes, how red, green, and blue components are ordered, and how the indexed color system references these definitions to display pixel data accurately.

The Indexed Color Architecture

The Graphics Interchange Format (GIF) does not store 24-bit true-color data directly inside individual pixels. Instead, it relies on an indexed color system limited to a maximum of 256 colors per frame. The actual color information is stored in either a Global Color Table (GCT), which applies to the entire file, or a Local Color Table (LCT), which applies to a specific image frame.

Byte-Level Storage of RGB Values

Within both global and local color tables, colors are defined as pure, uncompressed RGB triplets. Each color entry consists of exactly three sequential bytes (24 bits total):

1. Byte 1: Red value (0 to 255)
2. Byte 2: Green value (0 to 255)
3. Byte 3: Blue value (0 to 255)

The table stores these triplets as a continuous stream of raw binary data. There are no delimiters, padding, or separating bytes between colors. For example, a palette with three colors is structured consecutively as:

[R0, G0, B0] [R1, G1, B1] [R2, G2, B2]

Color Table Sizing

The number of colors in a table is determined by a 3-bit field located in the file's Logical Screen Descriptor (for a GCT) or the Image Descriptor (for an LCT).

• The value \(N\) (from 0 to 7) indicates the table size via the formula \(2^{N+1}\).
• The total number of colors ranges from 2 (\(N=0\)) to 256 (\(N=7\)).
• The total byte size of the color table is always \(3 \times 2^{N+1}\) bytes.

Consequently, a 256-color table consumes exactly 768 bytes (\(256 \times 3\)).

How Pixels Reference RGB Entries

The raster data of a GIF consists of a stream of index values rather than color values. When an image decoder encounters an index (for instance, index 4), it calculates the memory offset into the active color table:

\[\text{Offset} = \text{Index} \times 3\]

At that offset, the decoder reads the three consecutive bytes representing the red, green, and blue channels and renders the corresponding pixel on the screen.

Handling Transparency

Standard GIF color tables do not contain an alpha or opacity channel; they are strictly 24-bit RGB. Transparency is handled separately using a Graphic Control Extension block. This block identifies a specific palette index to be treated as completely transparent. When the decoder encounters this designated index during rendering, it skips drawing the pixel, preserving the background color or underlying frame.