In GIF image data, what compression code resets the coding dictionary?

In GIF image data, the CLEAR code resets the coding dictionary.

GIF image data uses Lempel–Ziv–Welch (LZW) compression. A decoder begins with codes representing the image’s palette entries, then builds a dictionary of longer pixel strings as it reads the compressed stream. Reusing those strings is what makes GIF files smaller than raw pixel data.

The dictionary has a limited code space. When an encoder needs to restart the process, it emits the CLEAR code. The decoder then discards its accumulated string table and returns to the initial palette-based dictionary. CLEAR is therefore a control code, not a pixel color or an animation command.

A common mix-up is STOP, also called the end-of-information code. STOP ends the image-data stream; it does not reset compression. GIF’s LZW data also includes a minimum code size and packs variable-width codes into sub-blocks of up to 255 bytes. CLEAR may be inserted whenever needed, including before the dictionary becomes full, so encoders can manage dictionary growth and compression efficiency.