contrib/pngminus/README.txt - third_party/libpng - Git at Google

 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
	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