ref: 8acb84ea7f21a581349458a39ec3f889fd525cc5
parent: 562b29e102cdfa019b0f9d79e105f3f7d7e7bb0e
author: Ben Harris <bjh21@bjh21.me.uk>
date: Tue Dec 13 18:28:35 EST 2022
Rewrite mkhighlight to handle a wider range of colours Before, it worked by scaling all the colour components, which worked reasonably for colours in the vicinity of white, but failed badly on a black background. Now it generates the highlight and lowlight colours by mixing the background colour with white and black respectively. Where there isn't enough headroom, it adjusts the background by mixing in a negative amount of white or black, which makes sense geometrically if not in paint. There is still a problem that green and yellow colours don't end up with bright enough highlights. I think this is because the code doesn't understand that green is brighter than red or blue.
--- a/misc.c
+++ b/misc.c
@@ -3,6 +3,7 @@
*/
#include <assert.h>
+#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
@@ -197,30 +198,78 @@
return ret;
}
+/* Utility functions for colour manipulation. */
+
+static float colour_distance(const float a[3], const float b[3])
+{+ return (float)sqrt((a[0]-b[0]) * (a[0]-b[0]) +
+ (a[1]-b[1]) * (a[1]-b[1]) +
+ (a[2]-b[2]) * (a[2]-b[2]));
+}
+
+static void colour_mix(const float src1[3], const float src2[3],
+ float p, float dst[3])
+{+ int i;
+ for (i = 0; i < 3; i++)
+ dst[i] = src1[i] * (1.0F - p) + src2[i] * p;
+}
+
void game_mkhighlight_specific(frontend *fe, float *ret,
int background, int highlight, int lowlight)
{- float max;
+ static const float black[3] = { 0.0F, 0.0F, 0.0F };+ static const float white[3] = { 1.0F, 1.0F, 1.0F };+ float db, dw;
int i;
-
/*
- * Drop the background colour so that the highlight is
- * noticeably brighter than it while still being under 1.
+ * New geometric highlight-generation algorithm: Draw a line from
+ * the base colour to white. The point K distance along this line
+ * from the base colour is the highlight colour. Similarly, draw
+ * a line from the base colour to black. The point on this line
+ * at a distance K from the base colour is the shadow. If either
+ * of these colours is imaginary (for reasonable K at most one
+ * will be), _extrapolate_ the base colour along the same line
+ * until it's a distance K from white (or black) and start again
+ * with that as the base colour.
+ *
+ * This preserves the hue of the base colour, ensures that of the
+ * three the base colour is the most saturated, and only ever
+ * flattens the highlight and shadow to pure white or pure black.
+ *
+ * K must be at most sqrt(3)/2, or mid grey would be too close to
+ * both white and black. Here K is set to sqrt(3)/6 so that this
+ * code produces the same results as the former code in the common
+ * case where the background is grey and the highlight saturates
+ * to white.
*/
- max = ret[background*3];
- for (i = 1; i < 3; i++)
- if (ret[background*3+i] > max)
- max = ret[background*3+i];
- if (highlight >= 0 && max * 1.2F > 1.0F) {- for (i = 0; i < 3; i++)
- ret[background*3+i] /= (max * 1.2F);
+ const float k = sqrt(3)/6.0F;
+ if (lowlight >= 0) {+ db = colour_distance(&ret[background*3], black);
+ if (db < k) {+ for (i = 0; i < 3; i++) ret[lowlight*3+i] = black[i];
+ if (db == 0.0F)
+ colour_mix(black, white, k/sqrt(3), &ret[background*3]);
+ else
+ colour_mix(black, &ret[background*3], k/db, &ret[background*3]);
+ } else {+ colour_mix(&ret[background*3], black, k/db, &ret[lowlight*3]);
+ }
}
-
- for (i = 0; i < 3; i++) {- if (highlight >= 0)
- ret[highlight * 3 + i] = ret[background * 3 + i] * 1.2F;
- if (lowlight >= 0)
- ret[lowlight * 3 + i] = ret[background * 3 + i] * 0.8F;
+ if (highlight >= 0) {+ dw = colour_distance(&ret[background*3], white);
+ if (dw < k) {+ for (i = 0; i < 3; i++) ret[highlight*3+i] = white[i];
+ if (dw == 0.0F)
+ colour_mix(white, black, k/sqrt(3), &ret[background*3]);
+ else
+ colour_mix(white, &ret[background*3], k/dw, &ret[background*3]);
+ /* Background has changed; recalculate lowlight. */
+ if (lowlight >= 0)
+ colour_mix(&ret[background*3], black, k/db, &ret[lowlight*3]);
+ } else {+ colour_mix(&ret[background*3], white, k/dw, &ret[highlight*3]);
+ }
}
}