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libsdl2_image/external/libpng-1.6.37/contrib/pngminus/README.txt
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PngMinus
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--------
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(copyright Willem van Schaik, 1999-2019)
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Some history
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------------
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Soon after the creation of PNG in 1995, the need was felt for a set of
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pnmtopng / pngtopnm utilities. Independently Alexander Lehmann and I
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(Willem van Schaik) started such a project. Luckily we discovered this
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and merged the two, which later became part of NetPBM, available from
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SourceForge.
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These two utilities have many, many options and make use of most of the
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features of PNG, like gamma, alpha, sbit, text-chunks, etc. This makes
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the utilities quite complex and by now not anymore very maintainable.
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When we wrote these programs, libpng was still in an early stage.
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Therefore, lots of the functionality that we put in our software can now
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be done using transform-functions in libpng.
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Finally, to compile these programs, you need to have installed and
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compiled three libraries: libpng, zlib and netpbm. Especially the latter
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makes the whole setup a bit bulky. But that's unavoidable given the many
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features of pnmtopng.
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What now (1999)
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---------------
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At this moment libpng is in a very stable state and can do much of the
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work done in pnmtopng. Also, pnmtopng needs to be upgraded to the new
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interface of libpng. Hence, it is time for a rewrite from the ground up
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of pnmtopng and pngtopnm. This will happen in the near future (stay
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tuned). The new package will get a different name to distinguish it from
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the old one: PngPlus.
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To experiment a bit with the new interface of libpng, I started off with
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a small prototype that contains only the basic functionality. It doesn't
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have any of the options to read or write special chunks and it will do
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no gamma correction. But this makes it also a simple program that is
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quite easy to understand and can serve well as a template for other
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software developments. By now there are of course a couple of programs,
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like Greg Roelofs' rpng/wpng, that can be used just as good.
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Can and can not
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---------------
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As this is the small brother of the future PngPlus, I called this fellow
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PngMinus. Because I started this development in good-old Turbo-C, I
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avoided the use the netpbm library, which requires DOS extenders. Again,
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another reason to call it PngMinus (minus netpbm :-). So, part of the
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program are some elementary routines to read / write pgm- and ppm-files.
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It does not handle B&W pbm-files, but instead you could do pgm with bit-
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depth 1.
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The downside of this approach is that you can not use them on images
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that require blocks of memory bigger than 64k (the DOS version). For
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larger images you will get an out-of-memory error.
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As said before, PngMinus doesn't correct for gamma. When reading
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png-files you can do this just as well by piping the output of png2pnm
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to pnmgamma, one of the standard PbmPlus tools. This same scenario will
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most probably also be followed in the full-blown future PngPlus, with
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the addition of course of the possibility to create gamma-chunks when
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writing png-files.
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On the other hand it supports alpha-channels. When reading a png-image
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you can write the alpha-channel into a pgm-file. And when creating an
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RGB+A png-image, you just combine a ppm-file with a corresponding
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pgm-file containing the alpha-channel. When reading, transparency chunks
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are converted into an alpha-channel and from there on treated the same
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way.
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Finally you can opt for writing ascii or binary pgm- and ppm-files. When
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the bit-depth is 16, the format will always be ascii.
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Using it
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--------
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To distinguish them from pnmtopng and PngPlus, the utilities are named
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png2pnm and pnm2png (2 instead of to). The input- and output-files can
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be given as parameters or through redirection. Therefore the programs
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can be part of a pipe.
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To list the options type "png2pnm -h" or "pnm2png -h".
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Just like Scandinavian furniture
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--------------------------------
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You have to put it together yourself. I developed the software on MS-DOS
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with Turbo-C 3.0 and RedHat Linux 4.2 with gcc. In both cases I used
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libpng-1.0.4 and zlib-1.1.3. By now (2019) it is twenty years later and
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more current versions are OK.
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The makefile assumes that the libpng libraries can be found in ../.. and
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libz in ../../../zlib. But you can change this to for example ../libpng
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and ../zlib. The makefile creates two versions of each program, one with
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static library support and the other using shared libraries.
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If you create a ../pngsuite directory and then store the basn####.png
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files from PngSuite (http://www.schaik.com/pngsuite/) in there, you can
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test the proper functioning of PngMinus by running pngminus.sh.
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Warranty
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-------
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Please, remember that this was just a small experiment to learn a few
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things. It will have many unforeseen features <vbg> ... who said bugs? Use
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it when you are in need for something simple or when you want a starting
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point for developing your own stuff.
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The end
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-------
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Willem van Schaik
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mailto:willem at schaik dot com
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http://www.schaik.com/png/
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Oct 1999, Jan 2019
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