AddGradient.java

    public static Bitmap addGradient(Bitmap src, @ColorInt int start, @ColorInt int end, int alpha) {
        int w = src.getWidth();
        int h = src.getHeight();
        Bitmap overlay = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(overlay);

        canvas.drawBitmap(src, 0, 0, null);

        Paint paint = new Paint();
        LinearGradient shader = new LinearGradient(0, 0, 0, h, start, end, Shader.TileMode.CLAMP);
        paint.setShader(shader);
        paint.setStyle(Paint.Style.FILL);
        paint.setAlpha(alpha);
        canvas.drawRect(0, 0, w, h, paint);

        return overlay;
    }