154 lines
5.8 KiB
Plaintext
154 lines
5.8 KiB
Plaintext
PngMinus
|
|
--------
|
|
(copyright Willem van Schaik, 1999)
|
|
|
|
|
|
License
|
|
-------
|
|
|
|
Permission to use, copy, modify, and distribute this software and
|
|
its documentation for any purpose and without fee is hereby granted,
|
|
provided that the above copyright notice appear in all copies and
|
|
that both that copyright notice and this permission notice appear in
|
|
supporting documentation. This software is provided "as is" without
|
|
express or implied warranty.
|
|
|
|
|
|
Some history
|
|
------------
|
|
Soon after the creation of PNG in 1995, the need was felt for a set of
|
|
pnmtopng / pngtopnm utilities. Independantly Alexander Lehmann and I
|
|
(Willem van Schaik) started such a project. Luckily we discovered this
|
|
and merged the two together into pnmtopng.tar.gz, which is available
|
|
from a/o ftp://ftp.simplesystems.org/pub/libpng/png/.
|
|
|
|
These two utilities have many, many options and make use of most of the
|
|
features of PNG, like gamma, alpha, sbit, text-chunks, etc. This makes
|
|
the utilities quite complex and by now not anymore very maintainable.
|
|
When we wrote these programs, libpng was still in an early stage.
|
|
Therefore, lots of the functionality that we put in our software can now
|
|
be done using transform-functions in libpng.
|
|
|
|
Finally, to compile these programs, you need to have installed and
|
|
compiled three libraries: libpng, zlib and netpbm. Especially the latter
|
|
makes the whole setup a bit bulky. But that's unavoidable given the many
|
|
features of pnmtopng.
|
|
|
|
|
|
What now
|
|
--------
|
|
At this moment libpng is in a very stable state and can do much of the
|
|
work done in pnmtopng. Also, pnmtopng needs to be upgraded to the new
|
|
interface of libpng. Hence, it is time for a rewrite from the ground up
|
|
of pnmtopng and pngtopnm. This will happen in the near future (stay
|
|
tuned). The new package will get a different name to distinguish it from
|
|
the old one: PngPlus.
|
|
|
|
To experiment a bit with the new interface of libpng, I started off with
|
|
a small prototype that contains only the basic functionality. It doesn't
|
|
have any of the options to read or write special chunks and it will do
|
|
no gamma correction. But this makes it also a simple program that is
|
|
quite easy to understand and can serve well as a template for other
|
|
software developments. (By now there are of course a couple of programs,
|
|
like Greg Roelofs' rpng/wpng, that can be used just as good.)
|
|
|
|
|
|
Can and can not
|
|
---------------
|
|
As this is the small brother of the future PngPlus, I called this fellow
|
|
PngMinus. Because I started this development in good-old Turbo-C, I
|
|
avoided the use the netpbm library, which requires DOS extenders. Again,
|
|
another reason to call it PngMinus (minus netpbm :-). So, part of the
|
|
program are some elementary routines to read / write pgm- and ppm-files.
|
|
It does not read b&w pbm-files.
|
|
|
|
The downside of this approach is that you can not use them on images
|
|
that require blocks of memory bigger than 64k (the DOS version). For
|
|
larger images you will get an out-of-memory error.
|
|
|
|
As said before, PngMinus doesn't correct for gamma. When reading
|
|
png-files you can do this just as well by piping the output of png2pnm
|
|
to pnmgamma, one of the standard PbmPlus tools. This same scenario will
|
|
most probably also be followed in the full-blown future PngPlus, with
|
|
the addition of course of the possibility to create gamma-chunks when
|
|
writing png-files.
|
|
|
|
On the other hand it supports alpha-channels. When reading a png-image
|
|
you can write the alpha-channel into a pgm-file. And when creating an
|
|
RGB+A png-image, you just combine a ppm-file with a corresponding
|
|
pgm-file containing the alpha-channel. When reading, transparency chunks
|
|
are converted into an alpha-channel and from there on treated the same
|
|
way.
|
|
|
|
Finally you can opt for writing ascii or binary pgm- and ppm-files. When
|
|
the bit-depth is 16, the format will always be ascii.
|
|
|
|
|
|
Using it
|
|
--------
|
|
To distinguish them from pnmtopng and PngPlus, the utilities are named
|
|
png2pnm and pnm2png (2 instead of to). The input- and output-files can
|
|
be given as parameters or through redirection. Therefore the programs
|
|
can be part of a pipe.
|
|
|
|
To list the options type "png2pnm -h" or "pnm2png -h".
|
|
|
|
|
|
Just like Scandinavian furniture
|
|
--------------------------------
|
|
You have to put it together yourself. I did test the software under
|
|
MS-DOS with Turbo-C 3.0 and under RedHat Linux 4.2 with gcc. In both
|
|
cases I used libpng-1.0.4 and zlib-1.1.3. Later versions should be OK,
|
|
however some older libpng versions have a bug in pngmem.c when using
|
|
Turbo-C 3.0 (see below).
|
|
|
|
You can build it using one of the two makefiles (make -f makefile.###)
|
|
or use the batch/script files pngminus.bat / pngminus.sh. This assumes
|
|
that you have built the libraries in ../libpng and ../zlib. Using Linux,
|
|
make sure that you have built libpng with makefile.std and not
|
|
makefile.linux (also called .lnx in earlier versions of libpng). The
|
|
latter creates a .so shared-library, while the PngMinus makefile assumes
|
|
a normal .a static library.
|
|
|
|
If you create a ../pngsuite directory and then store the basn####.png
|
|
files from PngSuite (http://www.schaik.com/pngsuite/) in there, you can
|
|
test in one go the proper functioning of PngMinus, see png2pnm.bat and
|
|
pnm2png.bat (or the .sh versions).
|
|
|
|
|
|
Warranty
|
|
-------
|
|
Please, remember that this was just a small experiment to learn a few
|
|
things. It will have many unforeseen features <vbg>. Who said bugs? Use
|
|
it when you are in need for something simple or when you want to start
|
|
developing your own stuff.
|
|
|
|
|
|
The Turbo bug
|
|
-------------
|
|
** pngmem.old
|
|
hptr = (png_byte huge *)((long)(hptr) & 0xfffffff0L);
|
|
hptr += 16L;
|
|
** pngmem.c
|
|
hptr = (png_byte huge *)((long)(hptr) & 0xfffffff0L);
|
|
hptr = hptr + 16L;
|
|
**
|
|
|
|
** pngmem.old
|
|
png_ptr->offset_table_ptr[i] = (png_bytep)hptr;
|
|
hptr += (png_uint_32)65536L;
|
|
** pngmem.c
|
|
png_ptr->offset_table_ptr[i] = (png_bytep)hptr;
|
|
hptr = hptr + 65536L;
|
|
**
|
|
|
|
|
|
The end
|
|
-------
|
|
Willem van Schaik
|
|
mailto:willem@schaik.com
|
|
http://www.schaik.com/png/
|
|
-------
|
|
Oct 1999
|
|
|