Based on the differences of hue and perceived brightness of foreground color foreground and background color bg, this method checks and possibly adjusts the given foreground color such that its hue stays unchanged, but its brightness is adapted to make it better perceivable on the background.

Description

Demo Code

/*//w  w w .  j a va2s .  c  o m

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import java.awt.Color;

public class Main{
    private static final float[] PERCEIVED_BRIGHTNESS = { 0.299f, 0.587f,
            0.114f };
    private static final float MIN_CONTRAST_TO_BACKGROUND = 0.30f;
    /**
     * Based on the differences of hue and perceived brightness of foreground color
     * <code>fg</code> and background color <code>bg</code>, this method checks and
     * possibly adjusts the given foreground color <code>fg</code> such that its hue
     * stays unchanged, but its brightness is adapted to make it better perceivable
     * on the background.
     * @param fg foreground color
     * @param bg background color
    //    * @param minContrast the minimum desired contrast (0 to 0.5)
     * @return unchanged or adjusted fg
     */
    public static Color getContrastColor(Color fg, Color bg) {
        float bgb = ColorHelper.perceivedBrightness(bg);
        float fgb = ColorHelper.perceivedBrightness(fg);

        float contrast = Math.abs(bgb - fgb);
        if (contrast > MIN_CONTRAST_TO_BACKGROUND)
            return fg;

        float[] hsbBG = new float[3];
        Color.RGBtoHSB(bg.getRed(), bg.getGreen(), bg.getBlue(), hsbBG);
        float[] hsbFG = new float[3];
        Color.RGBtoHSB(fg.getRed(), fg.getGreen(), fg.getBlue(), hsbFG);

        double hueDif = Math.abs(hsbFG[0] - hsbBG[0]);
        if (hueDif > 0.5)
            hueDif = 1.0 - hueDif;

        float saturationFactor = 1 - Math.max(hsbFG[1], hsbBG[1]);
        float brightnessFactor = Math.abs(fgb + bgb - 1);
        float hueDifferenceFactor = (float) Math.cos(Math.PI * hueDif * 3);

        float neededContrast = MIN_CONTRAST_TO_BACKGROUND
                * Math.max(saturationFactor,
                        Math.max(brightnessFactor, hueDifferenceFactor));

        if (contrast > neededContrast)
            return fg;

        boolean darken = (fgb > bgb) ? (fgb + neededContrast > 1f) : (fgb
                - neededContrast > 0f);

        return createColor(fg, darken ? bgb - neededContrast : bgb
                + neededContrast);
    }
    /**
     * This is a color's perceived brightness by the human eye
     * @param c
     * @return brightness from 0.0 to 1.0
     */
    public static float perceivedBrightness(Color c) {
        return (c == null) ? 1.0f
                : (PERCEIVED_BRIGHTNESS[0] * c.getRed()
                        + PERCEIVED_BRIGHTNESS[1] * c.getGreen() + PERCEIVED_BRIGHTNESS[2]
                        * c.getBlue()) / 255f;
    }
    /**
     * Creates a new color with the hue taken from color <code>c</code>, but adjusted
     * in brightness to match the desired perceived brightness.
     * @param c
     * @param perceivedBrightness
     * @return
     */
    public static Color createColor(Color c, float perceivedBrightness) {
        float[] cc = c.getRGBComponents(null);

        float pb = perceivedBrightness(c);
        if (pb == 0f)
            return new Color(pb, pb, pb, cc[3]);

        float f = perceivedBrightness / perceivedBrightness(c);
        float surplusBrightness = 0f;
        float sum = 0f;
        for (int i = 0; i < 3; i++) {
            cc[i] *= f;
            if (cc[i] < 1f) {
                sum += PERCEIVED_BRIGHTNESS[i];
            } else {
                surplusBrightness += (cc[i] - 1f) * PERCEIVED_BRIGHTNESS[i];
                cc[i] = 1f;
            }
        }
        if (surplusBrightness != 0) {
            float remainingBrightness = 0f;
            for (int i = 0; i < 3; i++) {
                if (cc[i] < 1f) {
                    cc[i] += surplusBrightness / sum;
                    if (cc[i] > 1f) {
                        remainingBrightness += (cc[i] - 1f)
                                * PERCEIVED_BRIGHTNESS[i];
                        cc[i] = 1f;
                    }
                }
            }
            if (remainingBrightness != 0f) {
                for (int i = 0; i < 3; i++) {
                    if (cc[i] < 1f) {
                        cc[i] += remainingBrightness
                                / PERCEIVED_BRIGHTNESS[i];
                        if (cc[i] > 1f) {
                            cc[i] = 1f;
                        }
                    }
                }
            }
        }

        return new Color(cc[0], cc[1], cc[2], cc[3]);
    }
}
Description

Demo Code

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