PLTE

PLTE is a chunk in the Portable Network Graphics (PNG) image format that defines the color palette used by indexed-color images (color type 3). It is a critical chunk; its presence is required for color type 3.

The PLTE chunk data consists of a sequence of RGB triplets, each 3 bytes, specifying the red, green, blue components of a palette entry. The number of entries ranges from 1 to 256, and image pixel data stores indices into this palette rather than explicit color values.

The PLTE chunk must appear before the first IDAT chunk in the PNG stream. If a tRNS chunk accompanies PLTE, it defines per-entry transparency by providing alpha values for corresponding palette entries, allowing some colors to be fully or partially transparent.

In color types 0 (grayscale) and 2 (truecolor) PLTE is not used; images of those types either do not include PLTE or ignore it. If a PLTE chunk is present with a non-indexed color type, it is considered invalid.

Technical details include that the PLTE chunk length is three times the number of palette entries, and the three bytes per entry are in the order red, green, blue with each value in the range 0–255. The maximum palette size is 256 entries (768 bytes of data).

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