Okay, so the first thing you need to do is setup libpng, which should come with the needed zlib code. You'll likely have to build these yourself, and of course move the correct files over. To my knowledge, you don't need to dynamically link to these if you don't want to. Don't quote me on that; you should probably read the licenses. Monkey doesn't use libpng for decoding, it uses stb (On the GLFW targets, anyway), which does not support encoding, therefore, we'll have to use libpng or a similar library. Project files and make-files are already provided by libpng, but if you want to build a different version of zlib, you'll want to grab CMake. Getting CMake is probably a good idea anyway. Library setup is per-compiler, so if you want to use both GCC/MinGW and MSVC, you'll have to set them both up. After that, external C++ code can be used through Monkey on its C++ based targets. MinGW/GCC is currently untested, so if you're using it/them, tell me if it causes major errors. Non-C++ targets are not supported by the code I'm posting (This may change in the future). They likely have their own ways of doing this, to begin with. Anyway, on to the actual code:
Here's a repository on GitHub. That code has only been tested with MSVC, at the moment. I'm basically just too lazy to compile zlib and libpng again. This was originally based on the code provided here. That being said, the original version found on that page is pretty awful, not to mention inefficient.
I'm not going to do the work for the specific situation you have, but that should allow you to re-save the image-data you retrieve with the 'opengl.gles11' module's 'LoadImageData' command. Array support is on the to-do list, but Monkey's not as open about them. That module was written with somewhat limited knowledge of libpng, so it may be inefficient.
This put me through way too much hell to implement. Seriously, I went an hour debugging everything, just to find out that I wrote "= NULL" instead of "== NULL". That was it. I can't tell if Monkey's made me pick of bad habits, or I honestly think C++ should use '=' for both assignment and comparison. I got so angry righting this module, you have no idea. AND IT WAS ALL BECAUSE OF ONE MISSING CHARACTER. Not to mention that the documentation for libpng is all over the place, so I had to debug it and make guesses about the names of the proper destruction commands; there shouldn't be any memory leaks now. That module is licensed under the MIT license, so you should be good to use it for commercial projects. Since this is currently C++ only, the targets you have at your disposal are mainly desktop targets (Could be useful for tools). iOS might also work, assuming you can get libpng working on it.
Well, anyway, I hope this helps. I know this doesn't put several images into an atlas (Something you might want to use an external tool for), but it should prove useful.
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