Technical Specifications 5 of 7
File Structure
A PNG file begins with a fixed eight-byte signature followed by an ordered series of chunks. Every chunk contains a length, a four-byte chunk type, chunk data, and a cyclic redundancy check.
Encoding pipeline
- 1
Pixel rows
The image is represented as scanlines.
- 2
Filtering
Each scanline uses an allowed filter to improve compressibility.
- 3
Compression
Filtered scanlines are compressed with PNG compression method 0.
- 4
IDAT storage
The compressed datastream is divided across one or more IDAT chunks.
- 5
Integrity check
Each chunk carries a CRC for its type and data.
Core chunk structure
| Element | Purpose |
|---|---|
| PNG signature | Identifies the datastream as PNG and helps detect transfer problems |
| IHDR | Defines width, height, bit depth, color type, compression, filtering, and interlacing |
| PLTE | Stores the palette; required for indexed-color images and permitted for some truecolor images |
| IDAT | Contains the filtered and compressed image data across one or more chunks |
| IEND | Marks the end of the PNG datastream |
| Ancillary chunks | May carry transparency, profiles, gamma, text, physical dimensions, timestamps, Exif data, and other information |
Unknown ancillary chunks can often be ignored safely, while unknown critical chunks prevent a decoder from reliably interpreting the image.