fix: Wrong RGB -> HSL convertion

[eugene-kulak]

Description

Fixes wrong formula used to convert RGB to HSL color space,
check correct algorithm here (https://www.niwa.nu/2013/05/math-behind-colorspace-conversions-rgb-hsl/)

References

• Add tests for PR #505 with RGB to HSL fix #690

Tasklist

• Add test case(s)
• Ensure all CI builds pass
• Review and approve

[mloskot]

Thanks for the patch.

Could you tell how did you discover the issue, for what input, expected output, a MWE?
In other words, it would be helpful if this PR includes a corresponding test case.

[eugene-kulak]

Hi @mloskot , unfortunately, I don't use boost tests, but here are my GTests:

#definergb2hsl(r, g, b, h, s, l) \
{ \
constfloat abs_error = 0.002; \
using boost::gil::hsl32f_pixel_t; \
using boost::gil::rgb8_pixel_t; \
using boost::gil::hsl_color_space::hue_t; \
using boost::gil::hsl_color_space::lightness_t; \
using boost::gil::hsl_color_space::saturation_t; \
rgb8_pixel_trgb(r, g, b); \
hsl32f_pixel_t hsl; \
\
color_convert(rgb, hsl); \
\
ASSERT_NEAR(h, get_color(hsl, hue_t()), abs_error); \
ASSERT_NEAR(s, get_color(hsl, saturation_t()), abs_error); \
ASSERT_NEAR(l, get_color(hsl, lightness_t()), abs_error); \
}
#definehsl2rgb(h, s, l, r, g, b) \
{ \
constfloat abs_error = 1; \
using boost::gil::blue_t; \
using boost::gil::green_t; \
using boost::gil::hsl32f_pixel_t; \
using boost::gil::red_t; \
using boost::gil::rgb8_pixel_t; \
hsl32f_pixel_thsl(h, s, l); \
rgb8_pixel_t rgb; \
\
color_convert(hsl, rgb); \
\
ASSERT_NEAR(r, get_color(rgb, red_t()), abs_error); \
ASSERT_NEAR(g, get_color(rgb, green_t()), abs_error); \
ASSERT_NEAR(b, get_color(rgb, blue_t()), abs_error); \
}
TEST(ColorSpaceTest, RGBToHSL) {
// blackrgb2hsl(0, 0, 0, 0, 0, 0);
// whitergb2hsl(255, 255, 255, 0, 0, 1);
// redrgb2hsl(255, 0, 0, 0, 1, 0.5);
// limergb2hsl(0, 255, 0, 2 / 6.f, 1, 0.5);
// bluergb2hsl(0, 0, 255, 4 / 6.f, 1, 0.5);
// yellowrgb2hsl(255, 255, 0, 1 / 6.f, 1, 0.5);
// cyanrgb2hsl(0, 255, 255, 3 / 6.f, 1, 0.5);
// magentargb2hsl(255, 0, 255, 5 / 6.f, 1, 0.5);
// silverrgb2hsl(191, 191, 191, 0, 0, 0.75);
// grayrgb2hsl(128, 128, 128, 0, 0, 0.5);
// maroonrgb2hsl(128, 0, 0, 0, 1, 0.25);
// olivergb2hsl(128, 128, 0, 1 / 6.f, 1, 0.25);
// greenrgb2hsl(0, 128, 0, 2 / 6.f, 1, 0.25);
// purplergb2hsl(128, 0, 128, 5 / 6.f, 1, 0.25);
// tealrgb2hsl(0, 128, 128, 3 / 6.f, 1, 0.25);
// navyrgb2hsl(0, 0, 128, 4 / 6.f, 1, 0.25);
}
TEST(ColorSpaceTest, HSLToRGB) {
// blackhsl2rgb(0, 0, 0, 0, 0, 0);
// whitehsl2rgb(0, 0, 1, 255, 255, 255);
// redhsl2rgb(0, 1, 0.5, 255, 0, 0);
// limehsl2rgb(2 / 6.f, 1, 0.5, 0, 255, 0);
// bluehsl2rgb(4 / 6.f, 1, 0.5, 0, 0, 255);
// purplehsl2rgb(5 / 6.f, 1, 0.25, 128, 0, 128);
// tealhsl2rgb(3 / 6.f, 1, 0.25, 0, 128, 128);
// navyhsl2rgb(4 / 6.f, 1, 0.25, 0, 0, 128);
}

[lpranam]

Looks good. Would be nice to add those tests now we don't use boost test so should be easy. Let us know if you can add or not. @eugene-kulak

[mloskot]

@lpranam I think we will have to add the tests on our own
I'm merging this, so this can be followed with porting of the GTest tests - #690