/**
 * Creates a 'ghost' bitmap version of the given source drawable (ideally a BitmapDrawable).
 * In the ghost bitmap, the RGB values take on the values from the 'color' argument, while
 * the alpha values are derived from the source's grayscaled RGB values. The effect is that
 * you can see through darker parts of the source bitmap, while lighter parts show up as
 * the given color. The 'invert' argument inverts the computation of alpha values, and looks
 * best when the given color is a dark.
 */
private Bitmap createGhostIcon(Drawable src, int color, boolean invert) {
    int width = src.getIntrinsicWidth();
    int height = src.getIntrinsicHeight();
    if (width <= 0 || height <= 0) {
        throw new UnsupportedOperationException("Source drawable needs an intrinsic size.");
    }

    Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bitmap);
    Paint colorToAlphaPaint = new Paint();
    int invMul = invert ? -1 : 1;
    colorToAlphaPaint.setColorFilter(new ColorMatrixColorFilter(new ColorMatrix(new float[]{
            0, 0, 0, 0, Color.red(color),
            0, 0, 0, 0, Color.green(color),
            0, 0, 0, 0, Color.blue(color),
            invMul * 0.213f, invMul * 0.715f, invMul * 0.072f, 0, invert ? 255 : 0,
    })));
    canvas.saveLayer(0, 0, width, height, colorToAlphaPaint, Canvas.ALL_SAVE_FLAG);
    canvas.drawColor(invert ? Color.WHITE : Color.BLACK);
    src.setBounds(0, 0, width, height);
    src.draw(canvas);
    canvas.restore();
    return bitmap;
}