CGFloat RRLuminanceFromRGBComponents(const CGFloat *rgb)
{
    // 0.3086 + 0.6094 + 0.0820 = 1.0
    return 0.3086f*rgb[0] + 0.6094f*rgb[1] + 0.0820f*rgb[2];
}

@interface RRCausticColorMatcher : NSObject
{
    CGFloat causticHue;
        // Yellow by default.
    CGFloat graySaturationThreshold;
        // Saturation level at which colours appear grey. Below this level,
        // matcher response snaps to pure caustic.
    CGFloat causticSaturationForGrays;
        // Defines the caustic saturation for grey colours. Grey colours fall
        // below the grey saturation threshold. When saturation drops too low,
        // everything looks grey.
    CGFloat redHueThreshold;
        // Colours at this threshold and above match to default caustics rather
        // than default magenta for blues.
    CGFloat blueHueThreshold;
        // Triggers a switch to magenta caustics. Hues at blue and beyond
        // display magenta-modulated caustics by default.
    CGFloat blueCausticHue;
        // Magenta by default. Magenta caustics for blue colours.
    CGFloat causticFractionDomainFactor;
        // Expands or contracts the caustic fraction's domain. With factor equal
        // to 1, non-caustic and caustic hues blend according to the cosine of
        // their difference. Smaller the difference, greater the amount of
        // caustic hue. Defaults to 1.4 meaning that the point of absolutely no
        // caustic blending occurs at 1/1.4 difference from caustic hue. Try
        // plotting cos(x*pi*1.4) in the -1,1 interval.
    CGFloat causticFractionRangeFactor;
        // Scales the caustic fraction which without a factor outputs a blending
        // fraction between 0 and 1 in favour of the caustic blend. Defaults to
        // 0.6 which scales down the amount of caustic hue-and-brightness by
        // that amount.
}

- (NSColor *)matchForColor:(NSColor *)aColor;
    // Matches the given colour. Answers a matching caustic colour. The result
    // shifts hue and brightness towards yellow. Saturation remains unchanged.
- (void)matchForHSB:(const CGFloat *)hsb caustic:(CGFloat *)outHSB;
    // Does the work.

@property(assign) CGFloat causticHue;
@property(assign) CGFloat graySaturationThreshold;
@property(assign) CGFloat causticSaturationForGrays;
@property(assign) CGFloat redHueThreshold;
@property(assign) CGFloat blueHueThreshold;
@property(assign) CGFloat blueCausticHue;
@property(assign) CGFloat causticFractionDomainFactor;
@property(assign) CGFloat causticFractionRangeFactor;

@end

// Encapsulates an optimising generic Exponential Function where
//      y=1-(exp(x*-c)-exp(-c))/(1-exp(-c))
// and where 0<c is a general coefficient describing the exponential
// curvature. The function's input domain lies within the 0..1 interval, its
// output range likewise. The implementation optimises by pre-computing those
// constant terms depending only on the coefficient whenever the coefficient
// changes. Repeated evaluation takes less computing time thereafter.
struct RRExponentialFunction
{
    float coefficient;
    float exponentOfMinusCoefficient;
    float oneOverOneMinusExponentOfMinusCoefficient;
};

typedef struct RRExponentialFunction RRExponentialFunction;

void RRExponentialFunctionSetCoefficient(RRExponentialFunction *f, float c);
float RRExponentialFunctionEvaluate(RRExponentialFunction *f, float x);

@interface RRGlossCausticShader : NSObject
{
    struct RRGlossCausticShaderInfo *info;
    RRCausticColorMatcher *matcher;
}

- (void)drawShadingFromPoint:(NSPoint)startingPoint toPoint:(NSPoint)endingPoint inContext:(CGContextRef)aContext;

- (void)update;
    // Send -update after changing one or more parameters. Setters do not
    // automatically update the shader. This is by design. It applies to the
    // caustic colour matcher too. Change anything? Send an update. Otherwise,
    // if the setters automatically update, multiple changes trigger unnecessary
    // multiple updates. It's a small optimisation.
    // Updating follows the dependency chain. Caustic colour depends on
    // non-caustic colour along with all the caustic colour matcher's tuneable
    // configuration settings. Updating also re-computes the gloss. Gloss
    // derives from non-caustic colour luminance, among other things.

//---------------------------------------------------------------------- setters

- (void)setExponentialCoefficient:(float)c;
- (void)setNoncausticColor:(NSColor *)aColor;
    // Converts aColor to device RGB colour space. The resulting colour
    // components become the new non-caustic colour. This setter, like all
    // others, does not automatically readjust the dependencies. Invoke -update
    // after adjusting one or more settings.
- (void)setGlossReflectionPower:(CGFloat)powerLevel;
    // Assigns a new power level to the gloss reflection.
- (void)setGlossStartingWhite:(CGFloat)whiteLevel;
    // White levels range between 0 and 1 inclusive. Gloss starting white levels
    // typically have higher values compared to ending white level.
- (void)setGlossEndingWhite:(CGFloat)whiteLevel;

//---------------------------------------------------------------------- getters

- (float)exponentialCoefficient;
- (NSColor *)noncausticColor;
    // Returns the non-caustic colour.
- (CGFloat)glossReflectionPower;
- (CGFloat)glossStartingWhite;
- (CGFloat)glossEndingWhite;

// Key-value coding automatically gives access to colour matching for caustic
// colours. Special note though, changing caustic matcher thresholds does not
// (repeat not) automatically adjust the shader's caustic colour. You need to
// update the shader when ready.
// Currently, the matcher property offers read-only access. You cannot set the
// matcher! However, perhaps future versions will allow setting in order to
// override the default caustic matching behaviour. Developers might want to
// customise the colour matching algorithmically as well as by tweaking
// parameters.
@property(readonly) RRCausticColorMatcher *matcher;

@end