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README

      1 PngMinus
      2 --------
      3 (copyright Willem van Schaik, 1999)
      4 
      5 
      6 License
      7 -------
      8 
      9 Permission to use, copy, modify, and distribute this software and
     10 its documentation for any purpose and without fee is hereby granted,
     11 provided that the above copyright notice appear in all copies and
     12 that both that copyright notice and this permission notice appear in
     13 supporting documentation. This software is provided "as is" without
     14 express or implied warranty.
     15 
     16 
     17 Some history
     18 ------------
     19 Soon after the creation of PNG in 1995, the need was felt for a set of
     20 pnmtopng / pngtopnm utilities. Independantly Alexander Lehmann and I
     21 (Willem van Schaik) started such a project. Luckily we discovered this
     22 and merged the two together into pnmtopng.tar.gz, which is available
     23 from a/o ftp://ftp.simplesystems.org/pub/libpng/png/.
     24 
     25 These two utilities have many, many options and make use of most of the
     26 features of PNG, like gamma, alpha, sbit, text-chunks, etc. This makes
     27 the utilities quite complex and by now not anymore very maintainable.
     28 When we wrote these programs, libpng was still in an early stage.
     29 Therefore, lots of the functionality that we put in our software can now
     30 be done using transform-functions in libpng.
     31 
     32 Finally, to compile these programs, you need to have installed and
     33 compiled three libraries: libpng, zlib and netpbm. Especially the latter
     34 makes the whole setup a bit bulky. But that's unavoidable given the many
     35 features of pnmtopng.
     36 
     37 
     38 What now
     39 --------
     40 At this moment libpng is in a very stable state and can do much of the
     41 work done in pnmtopng. Also, pnmtopng needs to be upgraded to the new
     42 interface of libpng. Hence, it is time for a rewrite from the ground up
     43 of pnmtopng and pngtopnm. This will happen in the near future (stay
     44 tuned). The new package will get a different name to distinguish it from
     45 the old one: PngPlus.
     46 
     47 To experiment a bit with the new interface of libpng, I started off with
     48 a small prototype that contains only the basic functionality. It doesn't
     49 have any of the options to read or write special chunks and it will do
     50 no gamma correction. But this makes it also a simple program that is
     51 quite easy to understand and can serve well as a template for other
     52 software developments. (By now there are of course a couple of programs,
     53 like Greg Roelofs' rpng/wpng, that can be used just as good.)
     54 
     55 
     56 Can and can not
     57 ---------------
     58 As this is the small brother of the future PngPlus, I called this fellow
     59 PngMinus. Because I started this development in good-old Turbo-C, I
     60 avoided the use the netpbm library, which requires DOS extenders. Again,
     61 another reason to call it PngMinus (minus netpbm :-). So, part of the
     62 program are some elementary routines to read / write pgm- and ppm-files.
     63 It does not read b&w pbm-files.
     64 
     65 The downside of this approach is that you can not use them on images
     66 that require blocks of memory bigger than 64k (the DOS version). For
     67 larger images you will get an out-of-memory error.
     68 
     69 As said before, PngMinus doesn't correct for gamma. When reading
     70 png-files you can do this just as well by piping the output of png2pnm
     71 to pnmgamma, one of the standard PbmPlus tools. This same scenario will
     72 most probably also be followed in the full-blown future PngPlus, with
     73 the addition of course of the possibility to create gamma-chunks when
     74 writing png-files.
     75 
     76 On the other hand it supports alpha-channels. When reading a png-image
     77 you can write the alpha-channel into a pgm-file. And when creating an
     78 RGB+A png-image, you just combine a ppm-file with a corresponding
     79 pgm-file containing the alpha-channel. When reading, transparency chunks
     80 are converted into an alpha-channel and from there on treated the same
     81 way.
     82 
     83 Finally you can opt for writing ascii or binary pgm- and ppm-files. When
     84 the bit-depth is 16, the format will always be ascii.
     85 
     86 
     87 Using it
     88 --------
     89 To distinguish them from pnmtopng and PngPlus, the utilities are named
     90 png2pnm and pnm2png (2 instead of to). The input- and output-files can
     91 be given as parameters or through redirection. Therefore the programs
     92 can be part of a pipe.
     93 
     94 To list the options type "png2pnm -h" or "pnm2png -h".
     95 
     96 
     97 Just like Scandinavian furniture
     98 --------------------------------
     99 You have to put it together yourself. I did test the software under
    100 MS-DOS with Turbo-C 3.0 and under RedHat Linux 4.2 with gcc. In both
    101 cases I used libpng-1.0.4 and zlib-1.1.3. Later versions should be OK,
    102 however some older libpng versions have a bug in pngmem.c when using
    103 Turbo-C 3.0 (see below).
    104 
    105 You can build it using one of the two makefiles (make -f makefile.###)
    106 or use the batch/script files pngminus.bat / pngminus.sh. This assumes
    107 that you have built the libraries in ../libpng and ../zlib. Using Linux,
    108 make sure that you have built libpng with makefile.std and not
    109 makefile.linux (also called .lnx in earlier versions of libpng). The
    110 latter creates a .so shared-library, while the PngMinus makefile assumes
    111 a normal .a static library.
    112 
    113 If you create a ../pngsuite directory and then store the basn####.png
    114 files from PngSuite (http://www.schaik.com/pngsuite/) in there, you can
    115 test in one go the proper functioning of PngMinus, see png2pnm.bat and
    116 pnm2png.bat (or the .sh versions).
    117 
    118 
    119 Warranty
    120 -------
    121 Please, remember that this was just a small experiment to learn a few
    122 things. It will have many unforeseen features <vbg>. Who said bugs? Use
    123 it when you are in need for something simple or when you want to start
    124 developing your own stuff.
    125 
    126 
    127 The Turbo bug
    128 -------------
    129 ** pngmem.old
    130           hptr = (png_byte huge *)((long)(hptr) & 0xfffffff0L);
    131           hptr += 16L;
    132 ** pngmem.c
    133           hptr = (png_byte huge *)((long)(hptr) & 0xfffffff0L);
    134           hptr = hptr + 16L;
    135 **
    136 
    137 ** pngmem.old
    138           png_ptr->offset_table_ptr[i] = (png_bytep)hptr;
    139           hptr += (png_uint_32)65536L;
    140 ** pngmem.c
    141           png_ptr->offset_table_ptr[i] = (png_bytep)hptr;
    142           hptr = hptr + 65536L;
    143 **
    144 
    145 
    146 The end
    147 -------
    148 Willem van Schaik
    149 mailto:willem at schaik.com
    150 http://www.schaik.com/png/
    151 -------
    152 Oct 1999
    153 
    154