feat(cli/quantize): preserve transparency

v0.2.0

README.md

@@ -6,7 +6,7 @@ HiColor is a program for converting images to 15- and 16-bit RGB color, the colo
 
 I wrote this program because I wanted to create images with the characteristic high-color look, and nothing seemed to support high color.  It implements its own simple [file format](format.md) and converts between this format and PNG.  It can also convert normal PNG to normal 32-bit PNG with only high color color values.  (This simulates a roundtrip through HiColor without creating a temporary file.)  To reduce the quantization error (the difference between the original and the high-color pixel), HiColor uses the period-appropriate [Bayer ordered dithering](https://bisqwit.iki.fi/story/howto/dither/jy/#StandardOrderedDitheringAlgorithm) algorithm.  Dithering can be disabled with a command line flag.  HiColor files have the extension `.hic` or `.hi5` for 15-bit and `.hi6` for 16-bit.
 
-Quantized images compress better when their originals, so HiColor may serve as a less-lossy alternative to the 256-color [pngquant](https://pngquant.org/).  However, unlike pngquant, it does not preserve transparency.
+Quantized images compress better when their originals, so HiColor may serve as a less-lossy alternative to the 256-color [pngquant](https://pngquant.org/).  Quantizing a PNG file to PNG preserves transparency (but does not quantize the alpha channel).  Conversion to and from the HiColor format does not preserve transparency.
 
 The program is written in C with minimal dependencies and builds as a static binary by default.  It works at least on Linux and Windows 98 Second Edition, 2000 Service Pack 4, XP, and 7.
 

cli.c

@@ -159,6 +159,10 @@ bool png_quantize(
     }
 
     cp_image_t quant_png_img = rgb_to_cp(meta, rgb_img);
+    /* Restore the alpha channel. */
+    for (uint32_t i = 0; i < (uint32_t) png_img.w * (uint32_t) png_img.h; i++) {
+        quant_png_img.pix[i].a = png_img.pix[i].a;
+    }
     if (!cp_save_png(dest, &quant_png_img)) {
         fprintf(stderr, "can't save PNG\n");
         goto clean_up_quant_image;

hicolor.h

@@ -6,7 +6,7 @@
 #include <stdio.h>
 
 #define HICOLOR_BAYER_SIZE 8
-#define HICOLOR_LIBRARY_VERSION 101
+#define HICOLOR_LIBRARY_VERSION 200
 
 /* Types. */
 

tests/hicolor.test

@@ -137,6 +137,11 @@ tcltest::test data-integrity-1.1 {roundtrip} -constraints gm -body {
     exec gm compare -metric rmse photo.png temp.png
 } -match glob -result {*Total: 0.01*}
 
+tcltest::test data-integrity-2.1 {alpha roundtrip} -constraints gm -body {
+    hicolor quantize alpha.png alpha-q.png
+    exec gm compare -metric rmse alpha.png alpha-q.png
+} -match regexp -result {Total: 0.0+ }
+
 
 incr failed [expr {$tcltest::numTests(Failed) > 0}]
 tcltest::cleanupTests