Map a value from one range to another. In this example map the angle of a line to a hue.

BNRLine.h

#import <Foundation/Foundation.h>

@interface BNRLine : NSObject

@property (nonatomic) CGPoint begin;
@property (nonatomic) CGPoint end;

- (CGFloat)angleInRadians;
- (CGFloat)angleInDegrees;

- (UIColor *)lineColor;
@end

BNRLine.m

#import "BNRLine.h"

@implementation BNRLine

- (CGFloat)angleInRadians
{
    CGPoint begin = self.begin;
    CGPoint end = self.end;
    
    CGFloat dx = end.x - begin.x;
    CGFloat dy = begin.y - end.y;
    CGFloat angle = atan2f(dy, dx);
    if (angle < 0) {
        angle += (2 * M_PI);
    }
    return angle;
}

- (CGFloat)angleInDegrees
{
    return [self angleInRadians] * 180.0 / M_PI;
}

- (CGFloat)hue
{
    return [self mapToNewRangeWithValue:[self angleInRadians]
                                 oldMin:0.0 oldMax:2.0 * M_PI
                                 newMin:0.0 newMax:1.0];
}

- (UIColor *)lineColor
{
    return [UIColor colorWithHue:[self hue] saturation:1.0 brightness:1.0 alpha:1.0];
}

- (CGFloat)mapToNewRangeWithValue:(CGFloat)oldValue
                           oldMin:(CGFloat)oldMin
                           oldMax:(CGFloat)oldMax
                           newMin:(CGFloat)newMin
                           newMax:(CGFloat)newMax
{
    CGFloat oldRangeWidth = (oldMax - oldMin); // width of old range
    CGFloat newRangeWidth = (newMax - newMin); // width of new range
    
    CGFloat distanceFromMinInOldRange = oldValue - oldMin;
    
    CGFloat normalizedValue = distanceFromMinInOldRange / oldRangeWidth; // [0, 1]
    
    CGFloat newValue = normalizedValue * newRangeWidth + newMin; // expand to new range and shift over
    
    return newValue;
};


@end