PNG file format

PNG file structure analysis (I: Understand the PNG file storage format)

 

Preface

We all know that during the development of the mobile phone application program for j2's, we can use images in PNG format (even on some mobile phones, we can only use images in PNG format), although using images can add many highlights to our application, however, only images in PNG format are supported, which limits the possibility of further improvement (in fact, it should be said that the processing capability on the mobile phone platform is limited ). In midp2 or APIs provided by some vendors (such as Nokia), drawpixels/getpixels methods are provided. These methods further improve the flexibility of developers in processing images. However, we need to use. 0 implementation of this type of method is still whimsical, so in order to achieve more advanced applications, we must fully tap the potential of PNG.

PNG file structure

For a PNG file, the file header is always described by fixed Bytes:

Decimal number	137 80 78 71 13 10 26 10
Hexadecimal number	89 50 4E 47 0d 0a 1A 0a

The first byte 0x89 is out of the ASCII character range to prevent some software from processing PNG files as text files. The remaining parts of the file are composed of more than three PNG data blocks (chunk) in a specific order. Therefore, a standard PNG file structure should be as follows:

PNG file flag

PNG data block

......

PNG data block

PNG data block (chunk)

PNG defines two types of data blocks. One is critical chunk, which is a standard data block, and the other is ancillary chunks ), this is an optional data block. Key data blocks define four standard data blocks. Each PNG file must contain them. PNG read/write software must also support these data blocks. Although the PNG file specification does not require the PNG encoding/Decoding of optional data blocks, the specification advocates the support of optional data blocks.

The following table lists the types of data blocks in PNG. The key data blocks are distinguished by the dark background.

Data blocks in PNG file format
Data Block symbol	Data Block name	Multiple Data blocks	Optional	Location restrictions
Ihdr	File header data block	No	No	First
CHRM	Base color and white point data block	No	Yes	Before plte and idat
Gama	Image gamma data block	No	Yes	Before plte and idat
Sbit	Sample valid BIT data block	No	Yes	Before plte and idat
Plte	Color Palette data block	No	Yes	Before idat
Bkgd	Background color data block	No	Yes	Before idat after plte
Hist	Image histogram data block	No	Yes	Before idat after plte
TRNS	Transparent data block	No	Yes	Before idat after plte
Offs	(Private Public Data blocks)	No	Yes	Before idat
Phys	Physical pixel size data block	No	Yes	Before idat
Scal	(Private Public Data blocks)	No	Yes	Before idat
Idat	Image Data Block	Yes	No	Continuous with other idat
Time	Last modification time data block of the image	No	Yes	Unlimited
Text	Text Information Data Block	Yes	Yes	Unlimited
Ztxt	Compressing text data blocks	Yes	Yes	Unlimited
Frac	(Private Public Data blocks)	Yes	Yes	Unlimited
Gifg	(Private Public Data blocks)	Yes	Yes	Unlimited
Gift	(Private Public Data blocks)	Yes	Yes	Unlimited
Gifx	(Private Public Data blocks)	Yes	Yes	Unlimited
Iend	Image end data	No	No	Last data block

For the sake of simplicity, we assume that in the PNG file we use, the four data blocks are stored in the above Order, and only appear once.

Data Block Structure

In a PNG file, each data block consists of four parts:

Name	Bytes	Description
Length)	4 bytes	Specifies the length of the data field in a data block. The length cannot exceed (231-1) bytes.
Chunk type code (data block type code)	4 bytes	The data block type code consists of ASCII letters (A-Z and A-Z)
Chunk data (data block data)