Xu Xin illegal Serve in Detail

i dont know where i heard it but i remember someone saying you cant hide the ball but can until the contact of the ball so you can throw the ball and cover it but when it makes contact with your bat it has to be visible

From the opponents position,I think the ball is visible.
However,because the camera is at an angle,it cannot see the ball as Xu's arm is blocking the camera.

It's tough to do a video analysis and photo analysis from this angle. As someone who does livestream footage I know that the angle from the camera can be deceptive. For instance the two standing rules in service are that the service must be visible to the opponent/umpires satisfaction and the hand must exit the space between the ball and net indefinitely upward after the toss.

The problem with this is that once the ball falls below head height it's perfectly legal for you to bend your body and head over the ball when contacting the ball so long as the opponent/umpire is satisfied it does not restrict view from the opponent. This means that to us and the live audience it's perfectly legal to hide the ball from our view. I've seen services many times look illegal on camera, but when I myself an standing behind the opponent I can see the service fine. It's all about perspective. This is also the main reason why players in a righty lefty situation stand more toward the middle of the table to better see the contact of the ball.

However I'd also like to point out that I have no doubt that Xu Xin's service has been illegal but to someone like Ma Long it's pretty hard to deceive one of your practice partners.

The contact point is visible for the players but he is throwing it too much towards him making the serve illegal but most umpires dot really care about this

Well done Dan, illegal serve no question. Ball is not thrown near vertical and he masks bat/ball contact with his body. We discussed the same issue with Wang Hao some time ago. Both of these points are not called into question by where his opponent stands. This is a deliberate action to gain an unfair advantage in my opinion. Like you I do not blame Xu Xin but I believe it is the responsibility of the officials to ensure the game is played correctly.