added color quantization and floyd steinberg dithering

src/conv.c

@@ -1,6 +1,6 @@
 /*
 libimago - a multi-format image file input/output library.
-Copyright (C) 2010-2021 John Tsiombikas <nuclear@member.fsf.org>
+Copyright (C) 2010-2022 John Tsiombikas <nuclear@member.fsf.org>
 
 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Lesser General Public License as published
@@ -97,9 +97,7 @@ int img_convert(struct img_pixmap *img, enum img_fmt tofmt)
 	}
 
 	if(tofmt == IMG_FMT_IDX8) {
-		/* TODO */
-		fprintf(stderr, "imago: conversions to indexed not implemented yet\n");
-		return 0;
+		return img_quantize(img, 256, 0);
 	}
 
 	img_init(&nimg);

src/imago2.h

@@ -42,6 +42,12 @@ enum img_fmt {
 	NUM_IMG_FMT
 };
 
+enum img_dither {
+	IMG_DITHER_NONE,
+	IMG_DITHER_ORDERED,
+	IMG_DITHER_FLOYD_STEINBERG
+};
+
 struct img_pixmap {
 	void *pixels;
 	int width, height;
@@ -141,6 +147,15 @@ int img_write(struct img_pixmap *img, struct img_io *io);
 /* Converts an image to the specified pixel format */
 int img_convert(struct img_pixmap *img, enum img_fmt tofmt);
 
+/* Quantize an image to a have at most certain maximum number of colors,
+ * converting it to IMG_FMT_IDX8 in the process.
+ * The number of colors must be at most 256.
+ * The last argument defines the dithering algorithm to be used.
+ *
+ * C++: the dither argument is optional and defaults to IMG_DITHER_NONE
+ */
+int img_quantize(struct img_pixmap *img, int maxcol, IMG_OPTARG(enum img_dither dither, IMG_DITHER_NONE));
+
 /* Flip the image vertically or horizontally */
 void img_vflip(struct img_pixmap *img);
 void img_hflip(struct img_pixmap *img);