Bitmap上下合成圖片

合成兩張圖片，上下疊加的效果：

/**
     * 把兩個點陣圖覆蓋合成為一個點陣圖，以底層點陣圖的長寬為基準
     *
     * @param backBitmap  在底部的點陣圖
     * @param frontBitmap 蓋在上面的點陣圖
     * @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;
    }