Interpretation of PNG files (1): past and present lives of png/apng format

PNG The past and present life of the format

png Is a lossless compressed bit image format , It was designed to try to replace GIF and TIFF File format , At the same time, add some GIF Features that file formats don't have .PNG Use from LZ77 Derived lossless data compression algorithm ——LZW patent (Lempel-Ziv-Welch) The compression algorithm is patented by Unisys hold ,Unisys Limit GIF Use on free software , That's why PNG The birth of format .

GIF The patent owner is the manufacturer Unisys Is in 1994 year 12 The levy was announced in June GIF Royalties on patents , stay 94 Before the announcement in , This patent is from 1985 After its adoption in, it has not really been enforced .  about Adobe For such software manufacturers , Pay the authorization fee to use GIF There is no problem with patents , But for developers who provide free software , This charging condition forms a limitation .  Original design PNG The motivation of the format is to respond to the urgency of the market ,W3C The organization's Liley Express . Originally everyone used GIF Of , But suddenly we can't continue to use , So the market wants to launch other alternative formats . 

PNG The format is 8 position 、24 position 、32 There are three forms , among 8 position PNG Supports two different forms of transparency （ Index transparency and alpha transparent ）,24 position PNG Transparency is not supported ,32 position PNG stay 24 Bit added 8 Bit transparent channel , So it can show 256 Level of transparency .

PNG8 and PNG24 The numbers behind represent this PNG Format can index and store color values at most .8 representative 2 Of 8 To the power of 256 color , and 24 Then represent 2 Of 24 To the power of 1600 Many colors .PNG 32 stay 24 Bit added 8 Bit transparent channel , So it can show 256 Level of transparency

PNG The advantages of

about PNG This image storage format , It has two characteristics ： Lossless compression and support transparent effects .

• because PNG Document adoption LZ77 Algorithm derived algorithm to compress , The result is a high compression ratio , No loss of data . It uses a special encoding method to mark repeated data , Therefore, it has no effect on the color of the image , It is impossible to produce color loss , In this way, it can be saved repeatedly without reducing the image quality .
• PNG You can define... For the original image 256 A transparency level , So that the edge of the color image can be smoothly fused with any background , So as to completely eliminate the jagged edges . This function is GIF and JPEG There is no the .

PNG The development history

1995 Early years ,Unisys According to it, the company is GIF Used in the format LZW Software patents for data compression algorithms began to be commercially charged , To avoid patent impact , Used to represent a single image PNG、 Used to represent animation MNG(Multiple-image Network Graphics/ Multi image network graphics ) The graphics file format is created at the same time .

1996 year 6 Month proposed PNF（Portable Network Frame） The draft , That year 8 The month was renamed MNG（Multiple-image Network Graphics）.

1996 year 7 month 1 Japan ,PNG Of 1.0 Version specification release , Later called RFC 2083 standard , And in 1996 year 10 month 1 Day to day W3C Suggest .

1999 year 8 month ,Unisys The company further suspended support for free software and non-commercial software developers GIF Patent free license , So that PNG Format gets more attention —— Everyone doesn't think of this patent fee , Like MP3 And OGG

1998 year 12 month 31 Promulgated by the PNG Of 1.1 edition , Slightly modified and added three new data block definitions

1999 year 8 month 11 Promulgated by the PNG Of 1.2 edition , Added another data block

2001 year 1 month 31 Promulgated by the MNG standard 1.0 edition ,MNG It's from PNG The hands of the development team , But because of the complex structure MNG library , The use process will consume a lot of resources , In the early days, only a few browsers supported ,Chrome、IE、Opera、Safari Never supported .Chrome and Opera Are promoting their own WebP

2003 year 11 month 10,PNG Current version of international standards （ISO/IEC 15948:2003） Release , And as W3C It is recommended that , And PNG1.2 The version is only slightly different .

2004 end of the year ,PNG Animation extension of ——APNG, To be mentioned —— from Mozilla Two from the company Mozilla The programmer Stuart Parmenter and Vladimir Vukićević Jointly design APNG. This is a relative to MNG Simpler animation implementation scheme , Don't recognize APNG Format PNG The decoder can play back at least the first ordinary picture normally PNG The picture . however APNG, It's always a tragedy , Even if Mozilla Community , stay 2007 Years ago, I also refused to recognize this miserable baby .

2007 year 3 month 23 Japan ,Mozilla In hindsight , stay Mozilla Firefox 3.0 in For the first time APNG Format .

2007 year 4 month 20 Japan ,Mozilla hope APNG Can become an official standard , therefore PNG Organize a vote , With a final 8：10 The number of votes rejected APNG Into the official standard , because PNG The organization is determined to continue to promote MNG, But this does not affect Mozilla Continue to support APNG.

PNG vsJPEG2000 vsGIF

There were few options for the format of early network pictures ,GIF It's almost the only option （GIF - 1987、JPEG - 1992、PNG - 1996、APNG - 2004、WebP - 2010）

PNG I think it's just GIF A royalty free enhanced version of . Someone once predicted ,2003 year 6 month 20 Japan , One of the most popular drawing file formats on the network GIF The patent will expire ,PNG He will die at the end of his life ——  That's because of all evil IE Yes PNG Half hanging support .IE6、7 After death ,PNG Not getting better ？

—— Want to use PNG Translucent function of , but 2003 year IE Browser occupied 90% market , Most people start from IE The result seen on the is quite ugly .

JPEG2002 As JPEG Upgraded version , It supports both lossy compression and lossless compression . But the transparent format is still PNG Dominate the mainstream .

In fact, early browsers did not support PNG Images ;JPEG and GIF Is the mainstream image format . because GIF Color depth limit , Images with color transitions in web pages are all used JPEG. No matter what ,JPEG Compression will cause slight blur of the image . and PNG It can be as accurate as possible under the corresponding color depth , Keep the image file small at the same time .PNG It has gradually become a better choice for small gradient images , Many browsers are already on PNG With good support

PNG Embedded systems are not included in the specification EXIF（ Exchangeable image file format ） Standard for image data , For example, the image taken by the digital camera . and TIFF,JPEG 2000, DNG All support EXIF.

Before written 《JPEG/Exif/TIFF Format interpretation (1):JEPG Analysis of picture compression and storage principle 》, Interested can be a little .

Reference article ：

GIF When the patent expires PNG Format bedtime ？ www.blueidea.com/news/other/2003/1112.asp

APNG Those things https://aotu.io/notes/2016/11/07/apng/

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