How to draw a RGB Color Wheel in Java

Question

I am trying to draw a RGB color wheel in Java, but I can't get the gradient by a circular shape. I just want to draw it on screen, with no user interaction at all.

This is all I have for now:

public void paint (Graphics g){
        super.paint(g);

        int red = 255;
        int green = 0;
        int blue = 0;
        int x1 = 500;
        int y1 = 305;
        int x2 = 500;
        int y2 = 50;

        while (green != 255){
            g.setColor(new Color(red, green, blue));
            green++;
            g.drawLine(x1, y1, x2, y2);
            x2++;
            if (y2 < y1){
                y2++;
            }
        }
        while (red != 0){
            g.setColor(new Color(red, green, blue));
            red--;
            g.drawLine(x1, y1, x2, y2);
            x2--;
            y2++;
        }
        x2 = 500;
        while (blue != 255){
            g.setColor(new Color(red, green, blue));
            blue++;
            g.drawLine(x1, y1, x2, y2);
            x2--;
            if (y2 > y1){
                y2--;
            }
        }

        while (red != 255){
            green--;
            g.setColor(new Color(red, green, blue));
            red++;
            g.drawLine(x1, y1, x2, y2);
            x2++;
            y2--;
        }
    }                
}

Which draws the gradient like this

This is what I want

Answer 1

Here is my solution for drawing a RGB Color Wheel in Java:

public class Main {

    public static void main(String[] args) {
        int rad = 1024;
        BufferedImage img = new BufferedImage(rad, rad, BufferedImage.TYPE_INT_RGB);

        // Center Point (MIDDLE, MIDDLE)
        int centerX = img.getWidth() / 2;
        int centerY = img.getHeight() / 2;
        int radius = (img.getWidth() / 2) * (img.getWidth() / 2);

        // Red Source is (RIGHT, MIDDLE)
        int redX = img.getWidth();
        int redY = img.getHeight() / 2;
        int redRad = img.getWidth() * img.getWidth();

        // Green Source is (LEFT, MIDDLE)
        int greenX = 0;
        int greenY = img.getHeight() / 2;
        int greenRad = img.getWidth() * img.getWidth();

        // Blue Source is (MIDDLE, BOTTOM)
        int blueX = img.getWidth() / 2;
        int blueY = img.getHeight();
        int blueRad = img.getWidth() * img.getWidth();

        for (int i = 0; i < img.getWidth(); i++) {
            for (int j = 0; j < img.getHeight(); j++) {
                int a = i - centerX;
                int b = j - centerY;

                int distance = a * a + b * b;
                if (distance < radius) {
                    int rdx = i - redX;
                    int rdy = j - redY;
                    int redDist = (rdx * rdx + rdy * rdy);
                    int redVal = (int) (255 - ((redDist / (float) redRad) * 256));

                    int gdx = i - greenX;
                    int gdy = j - greenY;
                    int greenDist = (gdx * gdx + gdy * gdy);
                    int greenVal = (int) (255 - ((greenDist / (float) greenRad) * 256));

                    int bdx = i - blueX;
                    int bdy = j - blueY;
                    int blueDist = (bdx * bdx + bdy * bdy);
                    int blueVal = (int) (255 - ((blueDist / (float) blueRad) * 256));

                    Color c = new Color(redVal, greenVal, blueVal);

                    float hsbVals[] = Color.RGBtoHSB(c.getRed(), c.getGreen(), c.getBlue(), null);

                    Color highlight = Color.getHSBColor(hsbVals[0], hsbVals[1], 1);

                    img.setRGB(i, j, RGBtoHEX(highlight));
                } else {
                    img.setRGB(i, j, 0xFFFFFF);
                }
            }
        }

        try {
            ImageIO.write(img, "png", new File("wheel.png"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static int RGBtoHEX(Color color) {
        String hex = Integer.toHexString(color.getRGB() & 0xffffff);
        if (hex.length() < 6) {
            if (hex.length() == 5)
                hex = "0" + hex;
            if (hex.length() == 4)
                hex = "00" + hex;
            if (hex.length() == 3)
                hex = "000" + hex;
        }
        hex = "#" + hex;
        return Integer.decode(hex);
    }
}