The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

PNG Signature

So chunk names encode additional information that is primarily useful if the chunk is not recognized. The remainder of this book will be concerned with known chunks, but before we turn to those, there is one more component of PNG files that has to do with the unknown: the PNG file signature. As noted earlier, the first 8 bytes of the file are not, strictly speaking, a chunk.[58] They are a critical component of a PNG file, however, since they allow it to be identified as such regardless of filename. But the PNG signature bytes are more than a simple identifier code: they were cleverly designed to allow the most common types of file-transfer corruption to be detected. Web protocols these days typically ensure the correct transfer of binary files such as PNG images, but older transfer programs like the venerable command-line FTP (File Transfer Protocol) often default to text-mode or ``ASCII'' transfers. The unsuspecting user who transfers a PNG image or other binary file as text is practically guaranteed of destroying it. The same is true of the user who extracts a PNG file from a compressed archive in text mode or who emails it without some form of ``ASCII armor'' (such as MIME Base64 encoding or Unix uuencoding).

[58] They can be thought of as such, however, since their length is known (8 bytes), their position and purpose are known (beginning of the file; signature), and their CRC is implied (the 8 bytes are constant, so effectively they are their own CRC).

The 8-byte PNG file signature can detect this sort of problem because it simulates a text file in some respects. The 8 bytes are given in Table 8-1.

Table 8-1. PNG Signature Bytes

ASCII Interpretation
137 A byte with its most significant bit set (``8-bit character'')
80 P
78 N
71 G
13 Carriage-return (CR) character, a.k.a. CTRL-M or ^M
10 Line-feed (LF) character, a.k.a. CTRL-J or ^J
26 CTRL-Z or ^Z
10 Line-feed (LF) character, a.k.a. CTRL-J or ^J

The first byte is used to detect transmission over a 7-bit channel--for example, email transfer programs often strip the 8th bit, thus changing the PNG signature. The 2nd, 3rd, and 4th bytes simply spell ``PNG'' (in ASCII, that is). Bytes 5 and 6 are end-of-line characters for Macintosh and Unix, respectively, and the combination of the two is the standard line ending for DOS, Windows, and OS/2. Byte 7 (CTRL-Z) is the end-of-file character for DOS text files, which allows one to TYPE the PNG file under DOS-like operating systems and see only the acronym ``PNG'' preceded by one strange character, rather than page after page of gobbledygook. Byte 8 is another Unix end-of-line character.

Text-mode transfer of a PNG file from a DOS-like system to Unix will strip off the carriage return (byte 5); the reverse transfer will replace byte 8 with a CR/LF pair. Transfer to or from a Macintosh will strip off the line feeds or replace the carriage return with a line feed, respectively. Either way, the signature is altered, and in all likelihood the remainder of the file is irreversibly damaged.

Note that the 9th, 10th, and 11th bytes are guaranteed to be 0 (that is, the ASCII NUL character) by the fact that the first chunk is required to be IHDR, whose first 4 bytes are its length--a value that is currently 13 and, according to the spec, will never change. (Instead, ``new chunk types will be added to carry new information.'') The fact that the 0 bytes in the length come first is another benefit of the big-endian integer format, which stores the high-order bytes first. Since NUL bytes are also often stripped out by text-mode transfer protocols, the detection of damaged PNG files is even more robust than the signature alone would suggest.

Last Update: 2010-Nov-26