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This is the last chapter on the PNG format It covers the process for creating files in the PNG format, which is essentially the reverse of the one used in the previous chapter to read PNG files
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Overview The basic process for creating a PNG file is fairly simple 1 Write the PNG signature 2 Write the PNG IHDR chunk 3 Create a PLTE chunk if the image requires a palette 4 Compress the image data into a series of IDAT blocks 5 Write an IEND chunk An encoder can be designed so that it adds optional PNG chunks if needed The optional chunks can be either predefined public chunks or application specific However, in most situations the steps listed above are all that is needed With the exception of creating the IDAT blocks, all of the steps listed above are trivial This chapter will deal almost exclusively with storing data in the IDAT chain For information on the other chunks refer to 13
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Deflate Compression Process The previous chapter covered the format of the Deflate compressed data within a chain of IDAT blocks While clearly a compressor uses the same structures for the data a decompressor does, compression is not simply a reversal of decompression The Deflate specification gives an outline of a compression process It recommends that this process be followed because of the patent minefield that surrounds any LZ compression process To implement Deflate compression we need to maintain a 32K or smaller power-of-2 window into the most recently processed uncompressed data bytes, just like the one used with decompression The compression process requires an additional lookahead window into the data yet to be compressed Starting from the beginning of the lookahead buffer we try to find the longest substring that has a match in the LZ77 sliding window Since the longest match allowed by Deflate is 258 bytes, the lookahead window needs to be at least this long to get the longest possible matches Rounding the lookahead window up to the next power of 2 (512) makes wrapping in the window simpler Algorithm 151 illustrates the general compression process for PNG image data This is roughly the inverse of the DecodeBlock function shown in the previous chapter The length and distance values are converted to codes and literal bits using the code also shown in the previous chapter There are two significant omissions in Algorithm 151 In a PNG file the Huffman tables precede the image data, so the encoder needs to generate them first The other missing piece is the method the encoder uses to locate matching strings in the LZ77 windows Finding Matching Strings in the LZ77 Window Finding the best match for the start of the lookahead buffer is the most time-consuming part of compressing PNG files A simple linear search would require 32K searches per string being compressed, which could easily amount to billions of search operations to compress an image file Instead of brute force, the approach recommended by the Deflate specification is to use a hash table where hash values are calculated using 3-byte sequences A hash table is a structure used to store objects that are accessed using a key, when the number of possible key values greatly exceeds the number of table entries at any given time Hash tables are most commonly used with string keys Many compiler implementations use hash tables to store variables defined by a module A typical source module for a compiler may have a few hundred variable names out of the billions upon billions of possibilities During PNG compression we have 32,768 entries with a maximum of 16 million possible values
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