The CRC can be used to check for corruption of the data. This field can be of zero length.Ī 4-byte CRC (Cyclic Redundancy Code) calculated on the preceding bytes in the chunk, including the chunk type field and the chunk data fields, but not including the length field. The data bytes appropriate to the chunk type, if any. Zero is a valid length.Ī sequence of 4 bytes defining the chunk type. The length counts only the data field, not itself, the chunk type, or the CRC. Raise Exception( 'Invalid PNG Signature') Reading chunksĪfter the PNG signature comes a series of chunks, until the end of the file.Įach chunk has the following fields: LengthĤ-byte unsigned integer giving the number of bytes in the chunk's data field. We start off by reading the first 8 bytes of the file and checking that it matches: PngSignature = b'\x89PNG\r\n\x1a\n' if f.read(len(PngSignature)) != PngSignature: The first 8 bytes of a valid PNG file is the PNG signature, which is the byte string b'\x89PNG\r\n\x1a\n'. We will use struct for parsing the PNG file, and zlib for decompressing image data later. To start off, we'll import the libraries we need, and open the PNG file for reading: import struct Save this as basn6a08.png in your working directory. Starting offįor this exercise, we shall try to decode this RGBA PNG Image from PngSuite ( LICENSE): In other words, a typical RGBA image where each pixel is a R,G,B,A tuple and each sample is one byte.Īs we can see, these restrictions aren't that bad and you should be able to decode some of your favorite images using this simple decoder. We only support 8 bit truecolor with alpha.We shall place some restrictions on our input PNG images: To avoid burdening ourselves with implementation details, ![]() Which has zlib bindings available in its standard library that will make the job much easier for us. The PNG Spec might seem daunting at first,īut it's actually surprisingly easy to write a simple PNG decoderĪs long as we place some restrictions on our input PNG files, ![]() 14th October 2019 Writing a (simple) PNG decoder might be easier than you think
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