PNG: The Definitive Guide

Chapter 8. PNG Basics

The fundamental building block of PNG images is the chunk. With the exception of the first 8 bytes in the file (and we'll come back to those shortly), a PNG image consists of nothing but chunks.

8.1. Chunks

Chunks were designed to be easily tested and manipulated by computer programs, easily detected by human eyes, and reasonably self-contained. Every chunk has the same structure: a 4-byte length (in “big-endian” format, as with all integer values in PNG streams), a 4-byte chunk type, between 0 and 2,147,483,647 bytes of chunk data, and a 4-byte cyclic redundancy check value (CRC). This is diagrammed in Figure 8-1.

Figure 8-1: PNG chunk structure.

The data field is straightforward; that's where the interesting bits (if any) go; specific content will be discussed later, as each chunk is described. The length field refers to the length of the data field alone, not the chunk type or CRC. The CRC, on the other hand, covers both the chunk-type field and the chunk data and is always present, even when there is no chunk data. Note that the combination of length fields and CRC values is already sufficient to check the basic integrity of a PNG file! The only missing information--not including the contents of the first 8 bytes in the file--is the exact algorithm (or “polynomial”) used for the CRC. That turns out to be identical to the CRC used by gzip and many popular ...