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/*
* index_to_hue.c
*
* This file provides the function
*
* double index_to_hue(int index);
*
* which maps the nonnegative integers to a set of easily distinguishable
* hues. The rule for computing the hue is to write the index in binary,
* reverse the order of the digits, and prepend a decimal point. Here are
* the first few values:
*
* index hue
* decimal binary binary decimal fraction
* 0 0 0.000 0.000 0
* 1 1 0.100 0.500 1/2
* 2 10 0.010 0.250 1/4
* 3 11 0.110 0.750 3/4
* 4 100 0.001 0.125 1/8
* 5 101 0.101 0.625 5/8
* 6 110 0.011 0.375 3/8
* 7 111 0.111 0.875 7/8
*/
#include "kernel.h"
double index_to_hue(
int index)
{
/*
* To maximize speed and avoid unnecessary roundoff error,
* compute the hue as a rational number num/den, and divide
* to get the floating point equivalent at the last moment.
*/
unsigned int num,
den;
num = 0;
den = 1;
while (index)
{
num <<= 1;
den <<= 1;
if (index & 0x1)
num++;
index >>= 1;
}
return ( (double)num / (double)den );
}
double horoball_hue(
int index)
{
/*
* The index_to_hue() colors don't look so nice for horoballs,
* mainly because the yellowish green color looks sickly.
* Here we provide hand chosen colors for up to six colors,
* and use index_to_hue() to interpolate thereafter.
* These colors look nice on my monitor, but they could
* look different on other monitors. And, of course, beauty
* is in the eye of the beholder.
*/
const static int base_hue[6] = { 0, /* red */
3, /* cyan */
2, /* green */
4, /* blue */
5, /* magenta */
1 }; /* yellow */
return (base_hue[index%6] + index_to_hue(index/6)) / 6.0;
}
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