Use the old libpng API so it will work on pre-1.6 installations.

The simplified API is so much nicer, but 1.2 is still extremely common.

Makefile

@@ -1,24 +1,24 @@
 all: encode render enumerate merge
 
-PNG_CFLAGS=$(shell pkg-config libpng16 --cflags)
-PNG_LDFLAGS=$(shell pkg-config libpng16 --libs)
+PNG_CFLAGS=
+PNG_LDFLAGS=-lpng
 
 ENCODE_OBJS = encode.o util.o
 RENDER_OBJS = render.o util.o graphics.o
 ENUMERATE_OBJS = enumerate.o util.o
 MERGE_OBJS = merge.o util.o
 
 encode: $(ENCODE_OBJS)
-	$(CC) -g -Wall -O3 -o $@ $^ -lm $(PNG_LDFLAGS)
+	$(CC) -g -Wall -O3 -o $@ $^ -lm
 
 render: $(RENDER_OBJS)
 	$(CC) -g -Wall -O3 -o $@ $^ -lm -lz $(PNG_LDFLAGS)
 
 enumerate: $(ENUMERATE_OBJS)
-	$(CC) -g -Wall -O3 -o $@ $^ -lm $(PNG_LDFLAGS)
+	$(CC) -g -Wall -O3 -o $@ $^ -lm
 
 merge: $(MERGE_OBJS)
-	$(CC) -g -Wall -O3 -o $@ $^ -lm $(PNG_LDFLAGS)
+	$(CC) -g -Wall -O3 -o $@ $^ -lm
 
 .c.o:
 	$(CC) -g -Wall -O3 $(PNG_CFLAGS) -c $<

graphics.c

@@ -6,9 +6,10 @@
 #include "util.h"
 #include "graphics.h"
 
-#if PNG_LIBPNG_VER < 10600
-#error libpng >= 1.6 is required
-#endif
+static void fail(png_structp png_ptr, png_const_charp error_msg) {
+	fprintf(stderr, "PNG error %s\n", error_msg);
+	exit(EXIT_FAILURE);
+}
 
 void out(double *src, double *cx, double *cy, int width, int height, int transparency, double gamma, int invert, int color, int color2, int saturate, int mask) {
 	unsigned char *buf = malloc(width * height * 4);
@@ -131,16 +132,31 @@ void out(double *src, double *cx, double *cy, int width, int height, int transpa
 		}
 	}
 
-	png_image image;
+	unsigned char *rows[height];
+	for (i = 0 ; i < height; i++) {
+		rows[i] = buf + i * (4 * width);
+	}
+
+	png_structp png_ptr;
+	png_infop info_ptr;
 
-	memset(&image, 0, sizeof image);
-	image.version = PNG_IMAGE_VERSION;
-	image.format = PNG_FORMAT_RGBA;
-	image.width = width;
-	image.height = height;
+	png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, fail, fail, fail);
+	if (png_ptr == NULL) {
+		fprintf(stderr, "PNG failure (write struct)\n");
+		exit(EXIT_FAILURE);
+	}
+	info_ptr = png_create_info_struct(png_ptr);
+	if (info_ptr == NULL) {
+		png_destroy_write_struct(&png_ptr, NULL);
+		fprintf(stderr, "PNG failure (info struct)\n");
+		exit(EXIT_FAILURE);
+	}
 
-	png_image_write_to_stdio(&image, stdout, 0, buf, 4 * width, NULL);
-	png_image_free(&image);
+	png_set_IHDR(png_ptr, info_ptr, width, height, 8, PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+	png_set_rows(png_ptr, info_ptr, rows);
+	png_init_io(png_ptr, stdout);
+	png_write_png(png_ptr, info_ptr, 0, NULL);
+	png_destroy_write_struct(&png_ptr, &info_ptr);
 
 	free(buf);
 }