Bitmap上下合成图片

合成两张图片，上下叠加的效果：

/**
     * 把两个位图覆盖合成为一个位图，以底层位图的长宽为基准
     *
     * @param backBitmap  在底部的位图
     * @param frontBitmap 盖在上面的位图
     * @return
     */
    public static Bitmap mergeBitmap(Bitmap backBitmap, Bitmap frontBitmap) {

        if (backBitmap == null || backBitmap.isRecycled()
                || frontBitmap == null || frontBitmap.isRecycled()) {
            return null;
        }
        Bitmap bitmap = backBitmap.copy(Bitmap.Config.ARGB_8888, true);
        Canvas canvas = new Canvas(bitmap);
        Rect baseRect = new Rect(0, 0, backBitmap.getWidth(), backBitmap.getHeight());
        Rect frontRect = new Rect(0, 0, frontBitmap.getWidth(), frontBitmap.getHeight());
        canvas.drawBitmap(frontBitmap, frontRect, baseRect, null);
        return bitmap;
    }