You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
121 lines
4.8 KiB
121 lines
4.8 KiB
4 years ago
|
PngMinus
|
||
|
|
--------
|
||
|
|
(copyright Willem van Schaik, 1999-2019)
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
Some history
|
||
|
|
------------
|
||
|
|
Soon after the creation of PNG in 1995, the need was felt for a set of
|
||
|
|
pnmtopng / pngtopnm utilities. Independently Alexander Lehmann and I
|
||
|
|
(Willem van Schaik) started such a project. Luckily we discovered this
|
||
|
|
and merged the two, which later became part of NetPBM, available from
|
||
|
|
SourceForge.
|
||
|
|
|
||
|
|
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 (1999)
|
||
|
|
---------------
|
||
|
|
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 handle B&W pbm-files, but instead you could do pgm with bit-
|
||
|
|
depth 1.
|
||
|
|
|
||
|
|
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 developed the software on MS-DOS
|
||
|
|
with Turbo-C 3.0 and RedHat Linux 4.2 with gcc. In both cases I used
|
||
|
|
libpng-1.0.4 and zlib-1.1.3. By now (2019) it is twenty years later and
|
||
|
|
more current versions are OK.
|
||
|
|
|
||
|
|
The makefile assumes that the libpng libraries can be found in ../.. and
|
||
|
|
libz in ../../../zlib. But you can change this to for example ../libpng
|
||
|
|
and ../zlib. The makefile creates two versions of each program, one with
|
||
|
|
static library support and the other using shared libraries.
|
||
|
|
|
||
|
|
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 the proper functioning of PngMinus by running pngminus.sh.
|
||
|
|
|
||
|
|
|
||
|
|
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 a starting
|
||
|
|
point for developing your own stuff.
|
||
|
|
|
||
|
|
|
||
|
|
The end
|
||
|
|
-------
|
||
|
|
Willem van Schaik
|
||
|
|
mailto:willem at schaik dot com
|
||
|
|
http://www.schaik.com/png/
|
||
|
|
|
||
|
|
Oct 1999, Jan 2019
|
||
|
|
|