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alter implementation of ArdourCanvas::contrasting_text_color() to use gamma-adjusted luminance values rather than simple contrast

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This commit is contained in:
Paul Davis 2014-11-05 11:07:20 -05:00
parent 70e95f951d
commit b02ea3d435
1 changed files with 41 additions and 27 deletions
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68
libs/canvas/utils.cc
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@ -253,35 +253,49 @@ ArdourCanvas::distance_to_segment_squared (Duple const & p, Duple const & p1, Du
return ((dpqx * dpqx) + (dpqy * dpqy));
}
// Inverse of sRGB "gamma" function.
static inline double
inv_gam_sRGB (double c)
{
if (c <= 0.04045) {
return c/12.92;
} else {
return pow(((c+0.055)/(1.055)),2.4);
}
}
// sRGB "gamma" function
static inline int
gam_sRGB(double v)
{
if (v <= 0.0031308) {
v *= 12.92;
} else {
v = 1.055 * pow (v, 1.0 / 2.4) - 0.055;
}
return int (v*255+.5);
}
static double
luminance (uint32_t c)
{
// sRGB luminance(Y) values
const double rY = 0.212655;
const double gY = 0.715158;
const double bY = 0.072187;
double r, g, b, a;
ArdourCanvas::color_to_rgba (c, r, g, b, a);
return (gam_sRGB (rY*inv_gam_sRGB(r) + gY*inv_gam_sRGB(g) + bY*inv_gam_sRGB(b))) / 255.0;
}
uint32_t
ArdourCanvas::contrasting_text_color (uint32_t c)
{
double r, g, b, a;
ArdourCanvas::color_to_rgba (c, r, g, b, a);
static const uint32_t white = ArdourCanvas::rgba_to_color (1.0, 1.0, 1.0, 1.0);
static const uint32_t black = ArdourCanvas::rgba_to_color (0.0, 0.0, 0.0, 1.0);
const double black_r = 0.0;
const double black_g = 0.0;
const double black_b = 0.0;
const double white_r = 1.0;
const double white_g = 1.0;
const double white_b = 1.0;
/* Use W3C contrast guideline calculation */
double white_contrast = (max (r, white_r) - min (r, white_r)) +
(max (g, white_g) - min (g, white_g)) +
(max (b, white_b) - min (b, white_b));
double black_contrast = (max (r, black_r) - min (r, black_r)) +
(max (g, black_g) - min (g, black_g)) +
(max (b, black_b) - min (b, black_b));
if (white_contrast > black_contrast) {
/* use white */
return ArdourCanvas::rgba_to_color (1.0, 1.0, 1.0, 1.0);
} else {
/* use black */
return ArdourCanvas::rgba_to_color (0.0, 0.0, 0.0, 1.0);
}
return (luminance (c) < 0.50) ? white : black;
}