diff --git a/contrib/pngminus/CHANGES.txt b/contrib/pngminus/CHANGES.txt
new file mode 100644
index 0000000..4e64e5e
--- /dev/null
+++ b/contrib/pngminus/CHANGES.txt
@@ -0,0 +1,13 @@
+
+pnm2png / png2pnm --- conversion from PBM/PGM/PPM-file to PNG-file
+copyright (C) 1999-2019 by Willem van Schaik <willem at schaik dot com>
+
+version 1.0 - 1999.10.15 - First version.
+        1.1 - 2015.07.29 - Fixed leaks (Glenn Randers-Pehrson)
+        1.2 - 2017.04.22 - Add buffer-size check
+        1.3 - 2017.08.24 - Fix potential overflow in buffer-size check
+                           (Glenn Randers-Pehrson)
+        1.4 - 2017.08.28 - Add PNGMINUS_UNUSED (Christian Hesse)
+        1.5 - 2018.08.05 - Fix buffer overflow in tokenizer (Cosmin Truta)
+        1.6 - 2018.08.05 - Improve portability and fix style (Cosmin Truta)
+        1.7 - 2019.01.22 - Change license to MIT (Willem van Schaik)
diff --git a/contrib/pngminus/LICENSE.txt b/contrib/pngminus/LICENSE.txt
new file mode 100644
index 0000000..00878a9
--- /dev/null
+++ b/contrib/pngminus/LICENSE.txt
@@ -0,0 +1,22 @@
+
+pnm2png / png2pnm --- conversion from PBM/PGM/PPM-file to PNG-file
+
+copyright (C) 1999-2019 by Willem van Schaik <willem at schaik dot com>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+The software is provided "as is", without warranty of any kind, express or
+implied, including but not limited to the warranties of merchantability,
+fitness for a particular purpose and noninfringement. In no event shall the
+authors or copyight holders be liable for any claim, damages or other
+liability, whether in an action of contract, tort or otherwise, arising from,
+out of or in connection with the software or the use or other dealings in the
+software.
diff --git a/contrib/pngminus/Makefile b/contrib/pngminus/Makefile
new file mode 100644
index 0000000..7eb503a
--- /dev/null
+++ b/contrib/pngminus/Makefile
@@ -0,0 +1,57 @@
+# Makefile for PngMinus (png2pnm and pnm2png)
+# Linux / Unix
+
+#CC = cc
+CC = gcc
+LD = $(CC)
+
+RM = rm -f
+
+PNGINC = -I../..
+PNGLIB_SHARED = -L../.. -lpng
+PNGLIB_STATIC = ../../libpng.a
+
+ZINC = -I../../../zlib
+ZLIB_SHARED = -L../../../zlib -lz
+ZLIB_STATIC = ../../../zlib/libz.a
+
+CPPFLAGS = $(PNGINC) $(ZINC)
+CFLAGS =
+LDFLAGS =
+LIBS_SHARED = $(PNGLIB_SHARED) $(ZLIB_SHARED)
+LIBS_STATIC = $(PNGLIB_STATIC) $(ZLIB_STATIC)
+
+EXEEXT =
+#EXEEXT = .exe
+
+# dependencies
+
+all: png2pnm$(EXEEXT) pnm2png$(EXEEXT) png2pnm-static$(EXEEXT) pnm2png-static$(EXEEXT)
+
+png2pnm.o: png2pnm.c
+	$(CC) -c $(CPPFLAGS) $(CFLAGS) png2pnm.c
+
+pnm2png.o: pnm2png.c
+	$(CC) -c $(CPPFLAGS) $(CFLAGS) pnm2png.c
+
+png2pnm$(EXEEXT): png2pnm.o
+	$(LD) $(LDFLAGS) -o png2pnm$(EXEEXT) png2pnm.o $(LIBS_SHARED) -lm
+
+pnm2png$(EXEEXT): pnm2png.o
+	$(LD) $(LDFLAGS) -o pnm2png$(EXEEXT) pnm2png.o $(LIBS_SHARED) -lm
+
+png2pnm-static$(EXEEXT): png2pnm.o
+	$(LD) $(LDFLAGS) -o png2pnm-static$(EXEEXT) png2pnm.o $(LIBS_STATIC) -lm
+
+pnm2png-static$(EXEEXT): pnm2png.o
+	$(LD) $(LDFLAGS) -o pnm2png-static$(EXEEXT) pnm2png.o $(LIBS_STATIC) -lm
+
+clean:
+	$(RM) png2pnm.o
+	$(RM) pnm2png.o
+	$(RM) png2pnm$(EXEEXT)
+	$(RM) pnm2png$(EXEEXT)
+	$(RM) png2pnm-static$(EXEEXT)
+	$(RM) pnm2png-static$(EXEEXT)
+
+# End of makefile for png2pnm / pnm2png
diff --git a/contrib/pngminus/README b/contrib/pngminus/README.txt
similarity index 62%
rename from contrib/pngminus/README
rename to contrib/pngminus/README.txt
index 23614a8..f7f6ecb 100644
--- a/contrib/pngminus/README
+++ b/contrib/pngminus/README.txt
@@ -1,26 +1,16 @@
 PngMinus
 --------
-(copyright Willem van Schaik, 1999)
+(copyright Willem van Schaik, 1999-2019)
 
 
-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. Independently 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/.
+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
@@ -35,8 +25,8 @@
 features of pnmtopng.
 
 
-What now
---------
+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
@@ -49,8 +39,8 @@
 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.)
+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
@@ -60,7 +50,8 @@
 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.
+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
@@ -96,58 +87,34 @@
 
 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 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.
 
-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.
+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 in one go the proper functioning of PngMinus, see png2pnm.bat and
-pnm2png.bat (or the .sh versions).
+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 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;
-**
+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.com
+mailto:willem at schaik dot com
 http://www.schaik.com/png/
--------
-Oct 1999
+
+Oct 1999, Jan 2019
 
diff --git a/contrib/pngminus/makefile.std b/contrib/pngminus/makefile.std
deleted file mode 100644
index 14e25cd..0000000
--- a/contrib/pngminus/makefile.std
+++ /dev/null
@@ -1,66 +0,0 @@
-# Makefile for PngMinus (png2pnm and pnm2png)
-# Linux / Unix
-
-#CC=cc
-CC=gcc
-LD=$(CC)
-
-RM=rm -f
-
-#PNGPATH = /usr/local
-#PNGINC = -I$(PNGPATH)/include/libpng16
-#PNGLIB = -L$(PNGPATH)/lib -lpng16
-#PNGLIBS = $(PNGPATH)/lib/libpng16.a
-PNGINC = -I../..
-PNGLIB = -L../.. -lpng
-PNGLIBS = ../../libpng.a
-
-#ZPATH = /usr/local
-#ZINC = -I$(ZPATH)/include
-#ZLIB = -L$(ZPATH)/lib -lz
-#ZLIBS = $(ZPATH)/lib/libz.a
-ZINC = -I../../../zlib
-ZLIB = -L../../../zlib -lz
-ZLIBS = ../../../zlib/libz.a
-
-CPPFLAGS=$(PNGINC) $(ZINC)
-CFLAGS=
-LDLIBS=$(PNGLIB) $(ZLIB)
-LDLIBSS=$(PNGLIBS) $(ZLIBS)
-C=.c
-O=.o
-L=.a
-E=
-
-# dependencies
-
-#all: png2pnm$(E) pnm2png$(E)
-all: png2pnm$(E) pnm2png$(E) png2pnm-static$(E) pnm2png-static$(E)
-
-png2pnm$(O): png2pnm$(C)
-	$(CC) -c $(CPPFLAGS) $(CFLAGS) png2pnm$(C)
-
-png2pnm$(E): png2pnm$(O)
-	$(LD) $(LDFLAGS) -o png2pnm$(E) png2pnm$(O) $(LDLIBS) -lm
-
-png2pnm-static$(E): png2pnm$(O)
-	$(LD) $(LDFLAGS) -o png2pnm-static$(E) png2pnm$(O) $(LDLIBSS) -lm
-
-pnm2png$(O): pnm2png$(C)
-	$(CC) -c $(CPPFLAGS) $(CFLAGS) pnm2png$(C)
-
-pnm2png$(E): pnm2png$(O)
-	$(LD) $(LDFLAGS) -o pnm2png$(E) pnm2png$(O) $(LDLIBS) -lm
-
-pnm2png-static$(E): pnm2png$(O)
-	$(LD) $(LDFLAGS) -o pnm2png-static$(E) pnm2png$(O) $(LDLIBSS) -lm
-
-clean:
-	$(RM) png2pnm$(O)
-	$(RM) pnm2png$(O)
-	$(RM) png2pnm$(E)
-	$(RM) pnm2png$(E)
-	$(RM) png2pnm-static$(E)
-	$(RM) pnm2png-static$(E)
-
-# End of makefile for png2pnm / pnm2png
diff --git a/contrib/pngminus/makefile.tc3 b/contrib/pngminus/makefile.tc3
deleted file mode 100644
index 6a2f4b9..0000000
--- a/contrib/pngminus/makefile.tc3
+++ /dev/null
@@ -1,38 +0,0 @@
-# Makefile for PngMinus (png2pnm and pnm2png)
-# TurboC++ 3.0
-
-CC=tcc -Ic:\tc3\inc
-LD=tcc -Lc:\tc3\lib
-LB=tlib
-RM=del
-CP=copy
-MODEL=l
-CPPFLAGS=-I..\libpng -I..\zlib
-CFLAGS=-O -m$(MODEL)
-LDFLAGS=-m$(MODEL) -L..\libpng -L..\zlib
-C=.c
-O=.obj
-L=.lib
-E=.exe
-
-# dependencies
-
-all: png2pnm$(E) pnm2png$(E)
-
-png2pnm$(O): png2pnm$(C)
-        $(CC) -c $(CPPFLAGS) $(CFLAGS) png2pnm$(C)
-
-png2pnm$(E): png2pnm$(O)
-        $(LD) $(LDFLAGS) png2pnm$(O) libpng$(L) zlib$(L)
-
-pnm2png$(O): pnm2png$(C)
-        $(CC) -c $(CPPFLAGS) $(CFLAGS) pnm2png$(C)
-
-pnm2png$(E): pnm2png$(O)
-        $(LD) $(LDFLAGS) pnm2png$(O) libpng$(L) zlib$(L)
-
-clean:
-        $(RM) *$(O)
-        $(RM) *$(E)
-
-# End of makefile for png2pnm / pnm2png
diff --git a/contrib/pngminus/png2pnm.c b/contrib/pngminus/png2pnm.c
index 67d5430..5fef7ed 100644
--- a/contrib/pngminus/png2pnm.c
+++ b/contrib/pngminus/png2pnm.c
@@ -1,20 +1,9 @@
 /*
  *  png2pnm.c --- conversion from PNG-file to PGM/PPM-file
- *  copyright (C) 1999,2017,2018 by Willem van Schaik <willem at schaik.com>
+ *  copyright (C) 1999-2019 by Willem van Schaik <willem at schaik dot com>
  *
- *  version 1.0 - 1999.10.15 - First version.
- *          1.1 - 2017.04.22 - Add buffer-size check (Glenn Randers-Pehrson)
- *          1.2 - 2017.08.24 - Fix potential overflow in buffer-size check
- *                             (Glenn Randers-Pehrson)
- *          1.3 - 2017.08.28 - Add PNGMINUS_UNUSED (Christian Hesse)
- *          1.4 - 2018.08.05 - Improve portability and fix style (Cosmin Truta)
- *
- *  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.
+ *  This software is released under the MIT license. For conditions of
+ *  distribution and use, see the LICENSE file part of this package.
  */
 
 #include <stdio.h>
diff --git a/contrib/pngminus/pngminus.bat b/contrib/pngminus/pngminus.bat
index 911bb8d..fa60e7d 100755
--- a/contrib/pngminus/pngminus.bat
+++ b/contrib/pngminus/pngminus.bat
@@ -1,4 +1,4 @@
-make -f makefile.tc3
+make
 call png2pnm.bat
 call pnm2png.bat
 
diff --git a/contrib/pngminus/pngminus.sh b/contrib/pngminus/pngminus.sh
index 2a0a9d8..cc9b4a5 100755
--- a/contrib/pngminus/pngminus.sh
+++ b/contrib/pngminus/pngminus.sh
@@ -1,5 +1,5 @@
 #!/bin/sh
-make -f makefile.std
+make
 sh png2pnm.sh
 sh pnm2png.sh
 
diff --git a/contrib/pngminus/pnm2png.c b/contrib/pngminus/pnm2png.c
index 3d4c6c5..a2c2551 100644
--- a/contrib/pngminus/pnm2png.c
+++ b/contrib/pngminus/pnm2png.c
@@ -1,22 +1,9 @@
 /*
  *  pnm2png.c --- conversion from PBM/PGM/PPM-file to PNG-file
- *  copyright (C) 1999,2015,2017,2018 by Willem van Schaik <willem at schaik.com>
+ *  copyright (C) 1999-2019 by Willem van Schaik <willem at schaik dot com>
  *
- *  version 1.0 - 1999.10.15 - First version.
- *          1.1 - 2015.07.29 - Fixed leaks (Glenn Randers-Pehrson)
- *          1.2 - 2017.04.22 - Add buffer-size check
- *          1.3 - 2017.08.24 - Fix potential overflow in buffer-size check
- *                             (Glenn Randers-Pehrson)
- *          1.4 - 2017.08.28 - Add PNGMINUS_UNUSED (Christian Hesse)
- *          1.5 - 2018.08.05 - Fix buffer overflow in tokenizer (Cosmin Truta)
- *          1.6 - 2018.08.05 - Improve portability and fix style (Cosmin Truta)
- *
- *  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.
+ *  This software is released under the MIT license. For conditions of
+ *  distribution and use, see the LICENSE file part of this package.
  */
 
 #include <stdio.h>