Decompress PNG using zlib

Question

Hi All,

How can I use zlib library to decompress a PNG file? I have no idea how to read a PNG file. When I opened a PNG file in TextPad(a text editor), the data was not at all understandable. Is it because a PNG file is compressed?

Any help is greatly appreciated. I need to read a PNG file using a C program and gcc compiler.

Regards, darkie

Answer 1

Why not use libpng? The PNG file format is fairly simple, but there are many different possible variations and encoding methods and it can be fairly tedious to ensure you cover all of the cases. Something like libpng handles all the conversion and stuff for you automatically.

Answer 2

You should probably read up on how a binary file-format works and use a hex-editor instead of a text-editor to look at the files. Generally you should use libpng to handle png-files as stated earlier but if you want to decode it yourself you have alot of reading to do.

I recommend reading this http://www.libpng.org/pub/png/book/chapter13.html

Answer 3

If this is a homework assignment and you really are only restricted to the standard C library, you to be looking at the official PNG file format specification: http://www.w3.org/TR/PNG/. However, are you sure you really need to be decoding the PNG file? If all you need to do is display it somehow, you're headed on the wrong path.

It will be rather complex and time consuming to write a decoder for any general PNG file, but not too bad for simple ones. In fact, because the PNG format allows for pieces of it to be compressed, to do it with only standard C libraries would require you to implement gzip decompress (a reasonable homework assignment for a mid-level undergrad course, but my guess is that you would have spent a lot of discussing compression algoirthms before this was assigned to you)

However, it isn't terribly difficult if you restrict yourself to non-compressed, non-interlaced PNG files. I wrote a decoder once in Python that handled only the easy cases in a couple of hours, so I'm sure it'll be doable in C.

Answer 4

I've code once a basic Java library for reading/writing PNG files: http://code.google.com/p/pngj/

It does not support palleted images, but apart from that is fairly complete, simple and the code has no external dependencies (i.e. it only uses the standard JSE API, which includes zip decompression). And the code is available. I guess you could port it to C with not much effort.