Weighing in on the side of simplicity is the fact that there is no need for a lot of platform-specific code, particularly platform-specific graphical code--unless, of course, the application already is graphical. In general, there is also no need for a special progressive mode; writing a PNG file, or almost any image format, for that matter, is more or less progressive by nature, although some complexity creeps in when the image is interlaced. Writing PNGs is more explicitly complex when it comes to dealing with ancillary information like text annotations, timestamps, and so forth.
The fundamental building block of PNG images is the chunk. 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: This is diagrammed in Figure 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! That turns out to be identical to the CRC used by gzip and many popular archiving programs; it is described in detail in Section 3.
The chunk type is possibly the most unusual feature. It is specified as a sequence of binary values, which just happen to correspond to the upper- and lowercase ASCII letters used on virtually every computer in the Western, non-mainframe world.
Since it is far more convenient and readable to speak in terms of text characters than numerical sequences, the remainder of this book will adopt the convention of referring to chunks by their ASCII names.
Chunk types or names are usually mnemonic, as in the case of the IHDR or image header chunk. In addition, however, each character in the name encodes a single bit of information that shows up in the capitalization of the character.
From left to right, the four extra bits are interpreted as follows: To put it another way, adding 32 to an uppercase character code gives you the code for its lowercase version.
The second character indicates whether the chunk is public uppercase or private. Public chunks are those defined in the specification or registered as official, special-purpose types.
But a company may wish to encode its own, application-specific information in a PNG file, and private chunks are one way to do that. The case bit of the third character is reserved for use by future versions of the PNG specification.
It must be uppercase for PNG 1. It indicates whether an editing program encountering an unknown ancillary chunk  can safely copy it into the new file lowercase or not uppercase.
If an unknown chunk is marked unsafe to copy, then it depends on the image data in some way.
It must be omitted from the new image if any critical chunks have been modified in any way, including the addition of new ones or the reordering or deletion of existing ones.
Note that if the program recognizes the chunk, it may choose to modify it appropriately and then copy it to the new file. Also note that unsafe-to-copy chunks may be copied to the new file if only ancillary chunks have been modified--again, including addition, deletion, and reordering--which implies that ancillary chunks cannot depend on other ancillary chunks.
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: As noted earlier, the first 8 bytes of the file are not, strictly speaking, a chunk.
But the PNG signature bytes are more than a simple identifier code: The unsuspecting user who transfers a PNG image or other binary file as text is practically guaranteed of destroying it. 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 Showing changed files with 34, additions and 0 deletions. Show comments View php. If you need to write a private chunk that you want to appear before the PLTE chunk when PLTE is present, you can write the PNG info in two steps, and insert code to write your own chunk between them.
The png Module ¶ Pure Python PNG Reader/Writer In this case each letter abbreviates a single channel: L is for Luminance or Luma or Lightness which is the channel used in greyscale images; R, G Write an array in flat row flat pixel format as a PNG file on the output file.
See also write() method. Mar 09, · My Media - Your Music and Video on the Roku DVP Remember me.
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Read and Write png file without change file size in C (libpng) Ask Question. There is an example project in libpng which has claimed to read and write image without loss, it's correct in pixel values, but change the structure of file (such as number of IDATs, optional chunks and etc) png file size.
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