Cycle R,G,B vales as HUE?

Question

I am using SFML and it has a color function that takes values in RGB. example.. (255,0,0). I would like to be able to cycle these numbers though a loop so that the displayed colour cycles though the hue...

So if I am using (76,204,63) the function will adjust those 3 numbers. So I need the function to intake rgb convert to HSV and then return rgb.

Any ideas how I would go about this?

the sfml code I wish to use is...

_sprite.setColor(76,204,63); This will set the sprite to a colour... I ma trying to work out how once that is done with those numbers to cycle the colour though the hue.

Answer 1

With a bit googling I've found this answer and converted the code to C++ with SFML in mind. I'm casting around pretty badly, so feel free to make it better. I guess it should even be possible to replace the 3x3 array.

sf::Uint8 clampAndConvert(float v)
{
    if(v < 0)
        return 0;
    if( v > 255)
        return 255;
    return static_cast<sf::Uint8>(v);
}

sf::Color RGBRotate(sf::Color old, float degrees)
{
    float cosA = cos(degrees*3.14159265f/180);
    float sinA = sin(degrees*3.14159265f/180);
    float rot = 1.f/3.f * (1.0f - cosA) + sqrt(1.f/3.f) * sinA;

    float rx = old.r * (cosA + (1.0f - cosA) / 3.0f) + old.g * rot + old.b * rot;
    float gx = old.r * rot + old.g * (cosA + 1.f/3.f*(1.0f - cosA)) + old.b * rot;
    float bx = old.r * rot + old.g * rot + old.b * cosA + 1.f/3.f * (1.0f - cosA);

    return sf::Color(clampAndConvert(rx), clampAndConvert(gx), clampAndConvert(bx), old.a);
}

Edit: Removed unnecessary casts.

Edit: Got rid of the matrix.

Edit: As I've noticed the code doesn't really work as wanted, but here's a hardcoded solution that works perfectly, just isn't that compact and nice.

#include <SFML/Graphics.hpp>

int main()
{
    sf::RenderWindow Screen (sf::VideoMode (800, 600, 32), "Game", sf::Style::Close);
    Screen.setFramerateLimit(60);

    sf::RectangleShape rect(sf::Vector2f(350.f, 350.f));
    rect.setPosition(150, 150);

    int dr = 0;
    int dg = 0;
    int db = 0;

    sf::Uint8 r = 255, g = 0,  b = 0;

    while (Screen.isOpen())
    {
        sf::Event Event;
        while (Screen.pollEvent (Event))
        {
            if (Event.type == sf::Event::Closed)
                Screen.close();
        }

        r += dr;
        g += dg;
        b += db;

        if(r == 255 && g == 0 && b == 0)
        {
            dr = 0; dg = 1; db = 0;
        }

        if(r == 255 && g == 255 && b == 0)
        {
            dr = -1; dg = 0; db = 0;
        }

        if(r == 0 && g == 255 && b == 0)
        {
            dr = 0; dg = 0; db = 1;
        }

        if(r == 0 && g == 255 && b == 255)
        {
            dr = 0; dg = -1; db = 0;
        }

        if(r == 0 && g == 0 && b == 255)
        {
            dr = 1; dg = 0; db = 0;
        }

        if(r == 255 && g == 0 && b == 255)
        {
            dr = 0; dg = 0; db = -1;
        }

        rect.setFillColor(sf::Color(r, g, b));

        Screen.clear();
        Screen.draw(rect);
        Screen.display();
    }

    return 0;
}

Answer 2

Convert RGB to HSL or HSV, modify the hue, then convert the result back to RGB.

Answer 3

Jerry's answer above is one correct way. If you don't care about preserving luminance (which if you do - don't use HSV, either), you can simply rotate your RGB color along the R=G=B axis. This is just a matrix multiply and saves you the conversion to and from HLS or HSV space.