HLS to RGB conversion is incorrect.

[gnachman]

For example, h=0, l=100, s=50 gives an RGB value of (128, 127, 128). Please see my fork for a correct implementation of hls_to_rgb.

[j4james]

@gnachman I don't think the implementation in your fork is correct either - the hue gradient looks wrong to me. But the original hls_to_rgb implementation can be fixed with a fairly simple patch:

diff --git a/src/fromsixel.c b/src/fromsixel.c
index 46f4059..6125bc0 100644
--- a/src/fromsixel.c
+++ b/src/fromsixel.c
@@ -110,10 +110,10 @@ hls_to_rgb(int hue, int lum, int sat)
     }
 
     /* https://wikimedia.org/api/rest_v1/media/math/render/svg/17e876f7e3260ea7fed73f69e19c71eb715dd09d */
-    max = lum + sat * (1.0 - (lum > 50 ? (((lum << 2) / 100.0) - 1.0): - (2 * (lum / 100.0) - 1.0))) / 2.0;
+    max = lum + sat * (1.0 - (lum > 50 ? (2 * (lum / 100.0) - 1.0): - (2 * (lum / 100.0) - 1.0))) / 2.0;
 
     /* https://wikimedia.org/api/rest_v1/media/math/render/svg/f6721b57985ad83db3d5b800dc38c9980eedde1d */
-    min = lum - sat * (1.0 - (lum > 50 ? (((lum << 2) / 100.0) - 1.0): - (2 * (lum / 100.0) - 1.0))) / 2.0;
+    min = lum - sat * (1.0 - (lum > 50 ? (2 * (lum / 100.0) - 1.0): - (2 * (lum / 100.0) - 1.0))) / 2.0;
 
     /* sixel hue color ring is rotated -120 degree from one generally used today. */
     hue = (hue + 240) % 360;

This patch was based off the libsixel/libsixel fork, but I think it should work on this repo too.

But on that note, if you're planning on using libsixel for anything serious, you probably should be using the libsixel/libsixel fork, because it has a bunch of fixes that haven't been applied to this repo. You'll also find a number of patches in the open issue list which may be worth applying, depending on what part of the library you plan to use. Both repos have been abandoned now, but the other one has at least been active for longer.

[gnachman]

@j4james Thanks for pointing that out. I was not aware of libsixel/libsixel. I've updated my fork to include its fixes but to stick with the old build system.

[saitoha]

@gnachman @j4james The proposed patch has been incorporated in dc9cfe4. Thank you!