[Question] Keep existing contacts on shape updates

[Fxlr8]

Hi, I am making a top-down view multiplayer game and I have a lot of objects on my map. The map is bigger than the player's screen, so one player can only see a part of a map.

When I am sending updates to the player I want to send him only the objects he can see to prevent him from cheating by zooming out. The most convenient solution that I found was to create a screen-sized sensor shape around the player. When this sensor begins a contact with an object, this means the player can see it and I can send it to the client. When the contact ends this means that the object disappeared from the player's screen and should be removed from the client's state.

This was a perfect solution, which saved me from storing arrays of everything a player can see and compare it with the new one on every tick to find what was added/removed from the screen. But, unfortunately player field of view can change over time. Shapes can't be updated in box2d. This means i have to delete an old screen shape and create a new one. When this happens, every contact with this shape is deleted and created again. Is there a solution for this case? Bodies involved in this contact didn't change, only their shapes did. What i want here is to track contacts by bodies not by shapes. I don't need any normals or something, only a boolean are they still touching or not.

[flyover]

Try modifying the shape connected to the fixture then call fixture.Synchronize() using the transform from fixture.GetBody() for both transforms.

const shape: box2d.b2PolygonShape = fixture.GetShape() as box2d.b2PolygonShape;
shape.SetAsBox(...);
fixture.Synchronize(fixture.GetBody().GetTransform(), fixture.GetBody().GetTransform());

I suppose the b2Fixture:Synchronize method could be changed to use this as the default arguments.

[Fxlr8]

This works perfectly, thank you!

[jcyuan]

cool, I actually have the same question, I have to change shapes when the character is performing into another state for example fro run to slide, I can successfully modify the vertices of the shapes but it seems the changes will be applied in next frame tick.
let me try the Synchronize method, thank you.

[jcyuan]

it works perfect. thanks.

[Fxlr8]

By the way, creating a sensor for every player's screen was a terrible idea performance-wise. You should prevent every single body from sleeping so it could detect contacts with the sensor. And it becomes even worse when you have to change that shape's size or position. It causes a lot of resources to recalculate all the contacts of that sensor. I returned for AABB queries and everything is super fast.