IHDR

IHDR is the first chunk in the Portable Network Graphics (PNG) file format after the eight-byte signature. It defines the image’s basic properties and is required in every valid PNG. The data section of the IHDR chunk is 13 bytes long and contains, in order: width (4 bytes, unsigned, big-endian), height (4 bytes, unsigned, big-endian), bit depth (1 byte), color type (1 byte), compression method (1 byte), filter method (1 byte), and interlace method (1 byte).

The width and height must be nonzero. The bit depth and color type encode how pixel data is stored: common combinations include bit depths of 8 or 16 with color types 0 (grayscale), 2 (truecolor RGB), 3 (indexed color), 4 (grayscale with alpha), and 6 (truecolor with alpha). The compression method must be 0 (deflate/inflate), the filter method must be 0 (adaptive filtering), and the interlace method is 0 for no interlacing or 1 for Adam7 interlacing.

IHDR is a critical chunk and must occur before subsequent data chunks such as IDAT, which carry the image data, and IEND. The chunk type 'IHDR' is stored as four ASCII letters and is followed by a 4-byte CRC that covers both the chunk type and its data. The specifications also require that there is exactly one IHDR chunk per image.