Correct color interpolation

Readme.markdown

Android's framework already gives us ArgbEvaluator to animate between two colors. However, it does so with a linear interpolation on the ARGB components of the colors.
Since the RGB components are gamma encoded, a linear interpolation leads to some unseemly color jumps.

We can solve this with an interpolator which convert the start & end color to linear values, interpolate them and then brings them back to gamma-encoded values.

Credits to Romain Guy for the base implementation.

GammaEvaluator.java

import android.animation.TypeEvaluator;
import android.animation.ValueAnimator;

import static java.lang.Math.pow;

public class GammaEvaluator implements TypeEvaluator {

    private static final GammaEvaluator instance = new GammaEvaluator();

    /**
     * Returns an instance of <code>GammaEvaluator</code> that may be used in
     * {@link ValueAnimator#setEvaluator(TypeEvaluator)}. The same instance may
     * be used in multiple <code>Animator</code>s because it holds no state.
     *
     * @return An instance of <code>GammaEvaluator</code>.
     */
    public static GammaEvaluator getInstance() {
        return instance;
    }


    public Object evaluate(float fraction, Object startValue, Object endValue) {
        int startInt = (Integer) startValue;
        float startA = ((startInt >> 24) & 0xff) / 255.0f;
        float startR = ((startInt >> 16) & 0xff) / 255.0f;
        float startG = ((startInt >> 8) & 0xff) / 255.0f;
        float startB = (startInt & 0xff) / 255.0f;

        int endInt = (Integer) endValue;
        float endA = ((endInt >> 24) & 0xff) / 255.0f;
        float endR = ((endInt >> 16) & 0xff) / 255.0f;
        float endG = ((endInt >> 8) & 0xff) / 255.0f;
        float endB = (endInt & 0xff) / 255.0f;

        // convert from sRGB to linear
        startR = EOCF_sRGB(startR);
        startG = EOCF_sRGB(startG);
        startB = EOCF_sRGB(startB);

        endR = EOCF_sRGB(endR);
        endG = EOCF_sRGB(endG);
        endB = EOCF_sRGB(endB);

        // compute the interpolated color in linear space
        float a = startA + fraction * (endA - startA);
        float r = startR + fraction * (endR - startR);
        float g = startG + fraction * (endG - startG);
        float b = startB + fraction * (endB - startB);

        // convert back to sRGB in the [0..255] range
        a = a * 255.0f;
        r = OECF_sRGB(r) * 255.0f;
        g = OECF_sRGB(g) * 255.0f;
        b = OECF_sRGB(b) * 255.0f;

        return Math.round(a) << 24 | Math.round(r) << 16 | Math.round(g) << 8 | Math.round(b);
    }

    /**
     * Opto-electronic conversion function for the sRGB color space
     * Takes a gamma-encoded sRGB value and converts it to a linear sRGB value
     */
    public static float OECF_sRGB(float linear) {
        // IEC 61966-2-1:1999
        return linear <= 0.0031308f ?
               linear * 12.92f : (float) ((pow(linear, 1.0f / 2.4f) * 1.055f) - 0.055f);
    }

    /**
     * Electro-optical conversion function for the sRGB color space
     * Takes a linear sRGB value and converts it to a gamma-encoded sRGB value
     */
    public static float EOCF_sRGB(float srgb) {
        // IEC 61966-2-1:1999
        return srgb <= 0.04045f ? srgb / 12.92f : (float) pow((srgb + 0.055f) / 1.055f, 2.4f);
    }

}

Sample.java

import android.animation.ValueAnimator;
import android.graphics.drawable.ColorDrawable;
import android.support.annotation.ColorInt;

public class Sample {

    public void animate(final ColorDrawable colorDrawable,
                        @ColorInt int colorFrom,
                        @ColorInt int colorTo) {

        ValueAnimator valueAnimator = new ValueAnimator();
        valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override public void onAnimationUpdate(ValueAnimator animation) {
                colorDrawable.setColor((Integer) animation.getAnimatedValue());
            }
        });

        valueAnimator.setIntValues(colorFrom, colorTo);
        valueAnimator.setEvaluator(GammaEvaluator.getInstance());
        valueAnimator.start();
    }
}