| 1 | PngMinus
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| 2 | --------
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| 3 | (copyright Willem van Schaik, 1999-2019)
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| 4 |
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| 5 |
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| 6 |
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| 7 | Some history
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| 8 | ------------
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| 9 | Soon after the creation of PNG in 1995, the need was felt for a set of
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| 10 | pnmtopng / pngtopnm utilities. Independently Alexander Lehmann and I
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| 11 | (Willem van Schaik) started such a project. Luckily we discovered this
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| 12 | and merged the two, which later became part of NetPBM, available from
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| 13 | SourceForge.
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| 14 |
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| 15 | These two utilities have many, many options and make use of most of the
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| 16 | features of PNG, like gamma, alpha, sbit, text-chunks, etc. This makes
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| 17 | the utilities quite complex and by now not anymore very maintainable.
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| 18 | When we wrote these programs, libpng was still in an early stage.
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| 19 | Therefore, lots of the functionality that we put in our software can now
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| 20 | be done using transform-functions in libpng.
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| 21 |
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| 22 | Finally, to compile these programs, you need to have installed and
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| 23 | compiled three libraries: libpng, zlib and netpbm. Especially the latter
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| 24 | makes the whole setup a bit bulky. But that's unavoidable given the many
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| 25 | features of pnmtopng.
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| 26 |
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| 27 |
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| 28 | What now (1999)
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| 29 | ---------------
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| 30 | At this moment libpng is in a very stable state and can do much of the
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| 31 | work done in pnmtopng. Also, pnmtopng needs to be upgraded to the new
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| 32 | interface of libpng. Hence, it is time for a rewrite from the ground up
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| 33 | of pnmtopng and pngtopnm. This will happen in the near future (stay
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| 34 | tuned). The new package will get a different name to distinguish it from
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| 35 | the old one: PngPlus.
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| 36 |
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| 37 | To experiment a bit with the new interface of libpng, I started off with
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| 38 | a small prototype that contains only the basic functionality. It doesn't
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| 39 | have any of the options to read or write special chunks and it will do
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| 40 | no gamma correction. But this makes it also a simple program that is
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| 41 | quite easy to understand and can serve well as a template for other
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| 42 | software developments. By now there are of course a couple of programs,
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| 43 | like Greg Roelofs' rpng/wpng, that can be used just as good.
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| 44 |
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| 45 |
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| 46 | Can and can not
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| 47 | ---------------
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| 48 | As this is the small brother of the future PngPlus, I called this fellow
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| 49 | PngMinus. Because I started this development in good-old Turbo-C, I
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| 50 | avoided the use the netpbm library, which requires DOS extenders. Again,
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| 51 | another reason to call it PngMinus (minus netpbm :-). So, part of the
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| 52 | program are some elementary routines to read / write pgm- and ppm-files.
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| 53 | It does not handle B&W pbm-files, but instead you could do pgm with bit-
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| 54 | depth 1.
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| 55 |
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| 56 | The downside of this approach is that you can not use them on images
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| 57 | that require blocks of memory bigger than 64k (the DOS version). For
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| 58 | larger images you will get an out-of-memory error.
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| 59 |
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| 60 | As said before, PngMinus doesn't correct for gamma. When reading
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| 61 | png-files you can do this just as well by piping the output of png2pnm
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| 62 | to pnmgamma, one of the standard PbmPlus tools. This same scenario will
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| 63 | most probably also be followed in the full-blown future PngPlus, with
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| 64 | the addition of course of the possibility to create gamma-chunks when
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| 65 | writing png-files.
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| 66 |
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| 67 | On the other hand it supports alpha-channels. When reading a png-image
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| 68 | you can write the alpha-channel into a pgm-file. And when creating an
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| 69 | RGB+A png-image, you just combine a ppm-file with a corresponding
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| 70 | pgm-file containing the alpha-channel. When reading, transparency chunks
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| 71 | are converted into an alpha-channel and from there on treated the same
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| 72 | way.
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| 73 |
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| 74 | Finally you can opt for writing ascii or binary pgm- and ppm-files. When
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| 75 | the bit-depth is 16, the format will always be ascii.
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| 76 |
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| 77 |
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| 78 | Using it
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| 79 | --------
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| 80 | To distinguish them from pnmtopng and PngPlus, the utilities are named
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| 81 | png2pnm and pnm2png (2 instead of to). The input- and output-files can
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| 82 | be given as parameters or through redirection. Therefore the programs
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| 83 | can be part of a pipe.
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| 84 |
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| 85 | To list the options type "png2pnm -h" or "pnm2png -h".
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| 86 |
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| 87 |
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| 88 | Just like Scandinavian furniture
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| 89 | --------------------------------
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| 90 | You have to put it together yourself. I developed the software on MS-DOS
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| 91 | with Turbo-C 3.0 and RedHat Linux 4.2 with gcc. In both cases I used
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| 92 | libpng-1.0.4 and zlib-1.1.3. By now (2019) it is twenty years later and
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| 93 | more current versions are OK.
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| 94 |
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| 95 | The makefile assumes that the libpng libraries can be found in ../.. and
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| 96 | libz in ../../../zlib. But you can change this to for example ../libpng
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| 97 | and ../zlib. The makefile creates two versions of each program, one with
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| 98 | static library support and the other using shared libraries.
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| 99 |
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| 100 | If you create a ../pngsuite directory and then store the basn####.png
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| 101 | files from PngSuite (http://www.schaik.com/pngsuite/) in there, you can
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| 102 | test the proper functioning of PngMinus by running pngminus.sh.
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| 103 |
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| 104 |
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| 105 | Warranty
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| 106 | -------
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| 107 | Please, remember that this was just a small experiment to learn a few
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| 108 | things. It will have many unforeseen features <vbg> ... who said bugs? Use
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| 109 | it when you are in need for something simple or when you want a starting
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| 110 | point for developing your own stuff.
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| 111 |
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| 112 |
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| 113 | The end
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| 114 | -------
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| 115 | Willem van Schaik
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| 116 | mailto:willem at schaik dot com
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| 117 | http://www.schaik.com/png/
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| 118 |
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| 119 | Oct 1999, Jan 2019
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| 120 |
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