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color-util: simplify hsv_to_rgb, fix rgb_to_hsv negative-hue wrap

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In hsv_to_rgb, restructure the conversion around the sector index
k = (int)(h/60) and fractional offset f = h/60 - k. The auxiliary
x value becomes c * (k & 1 ? 1.0 - f : f) and the six branches turn
into a switch on k. This drops the two xfmod() calls that were doing
the modulo work, in exchange for a single assert(h >= 0 && h < 360) —
all in-tree callers satisfy this and never relied on the wrap.

In rgb_to_hsv, the two xfmod() calls were no-ops (their arguments
were always within the divisor's magnitude). The trailing
xfmod(*ret_h, 360) appeared to be wrapping negative hues from the
r-max branch back into [0, 360), but fmod is sign-preserving so it
never did. Drop the no-ops and add an explicit +360 wrap so magenta
(1, 0, 1) now yields h ≈ 300 instead of -60.

Extend the tests to cover all six primary/secondary colors at sector
boundaries, all six sector midpoints (to catch any future inversion
of the ramp direction), the h-near-360 edge of the last sector, and
the rgb_to_hsv negative-wrap path via magenta. Switch the new and
existing integer-channel checks to ASSERT_EQ from tests.h; the
double-typed h/s/v range checks stay on ASSERT_TRUE since the
ASSERT_* comparison macros only support integer types.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
Daan De Meyer
2026-05-18 21:17:38 +00:00
parent bcb7cf4a61
commit 7d2925fcde
2 changed files with 120 additions and 46 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/shared/color-util.c
+23 -17
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@@ -32,13 +32,15 @@ void rgb_to_hsv(double r, double g, double b,
if (ret_h) {
if (delta > 0) {
if (r >= max_color)
*ret_h = 60 * fmod((g - b) / delta, 6);
*ret_h = 60 * (g - b) / delta;
else if (g >= max_color)
*ret_h = 60 * (((b - r) / delta) + 2);
else if (b >= max_color)
else /* b >= max_color */
*ret_h = 60 * (((r - g) / delta) + 4);
*ret_h = fmod(*ret_h, 360);
/* The r-max branch produces (-60, 60); fold the negative half up. */
if (*ret_h < 0)
*ret_h += 360;
} else
*ret_h = NAN;
}
@@ -49,29 +51,33 @@ void hsv_to_rgb(double h, double s, double v,
double c, x, m, r, g, b;
assert(h >= 0 && h <= 360);
assert(s >= 0 && s <= 100);
assert(v >= 0 && v <= 100);
assert(ret_r);
assert(ret_g);
assert(ret_b);
h = fmod(h, 360);
c = (s / 100.0) * (v / 100.0);
x = c * (1 - fabs(fmod(h / 60.0, 2) - 1));
m = (v / 100) - c;
if (h >= 0 && h < 60)
r = c, g = x, b = 0.0;
else if (h >= 60 && h < 120)
r = x, g = c, b = 0.0;
else if (h >= 120 && h < 180)
r = 0.0, g = c, b = x;
else if (h >= 180 && h < 240)
r = 0.0, g = x, b = c;
else if (h >= 240 && h < 300)
r = x, g = 0.0, b = c;
else
r = c, g = 0.0, b = x;
/* Split h into sector index k ∈ [0, 6] and fractional offset f ∈ [0, 1) within the sector.
* Within each sector exactly one of {r, g, b} equals c, one equals 0, and the third (x)
* ramps linearly between them — ascending in even sectors, descending in odd. h == 360 is
* the cyclic boundary equivalent to h == 0, and maps to sector 0. */
double seg = h / 60.0;
int k = (int) seg % 6;
double f = seg - (int) seg;
x = c * (k & 1 ? 1.0 - f : f);
switch (k) {
case 0: r = c; g = x; b = 0.0; break;
case 1: r = x; g = c; b = 0.0; break;
case 2: r = 0.0; g = c; b = x; break;
case 3: r = 0.0; g = x; b = c; break;
case 4: r = x; g = 0.0; b = c; break;
default: r = c; g = 0.0; b = x; break;
}
*ret_r = (uint8_t) ((r + m) * 255);
*ret_g = (uint8_t) ((g + m) * 255);