2D physics interpolation

[paulocoutinhox]

Hi,

In latest Godot they announce "2D physics interpolation":
https://godotengine.org/releases/4.3/#highlights-2d-physics-interpolation

physics-interpolation-2d.webm

How we can do the same in Axmol?

Thanks.

[aismann]

Look at cpp-tests
Chipmunk / Box2D Testbed examples

[DelinWorks]

it's pretty simple, when you set any object position to the physics engine's position you have to keep track of the last delta time, typically in a fixed physics update it would be 60 frames a second, which is 0.0166666666666667 delta

you then store a variable accumPhysicsDt that every frame (game loop update) you add deltaTime to it
each time you update the physics engine you reset that variable accumPhysicsDt to 0.

then you map the value from 0 to the fixed delta time which is 0.01666~ to the old object's position and the new one, keep in mind that you have to keep track of the old position which is pretty simple.

kinda like this

lastDt is 0.01666~
iox and ioy are the old positions
x and y are the new positions

Vec2 PhysicsWorld::getInterpolatedDynamicPosition(DynamicShape* s)
{
    float x = Math::map(accumPhysicsDt, 0.0f, lastDt, s->iox, s->x);
    float y = Math::map(accumPhysicsDt, 0.0f, lastDt, s->ioy, s->y);

    return Vec2(x, y);
}

you could implement it in a way where you store the future position but then you have to fiddle around with velocity and stuff that are so confusing, with this technique you have 1 frame delay because of the old position and it depends whether it's fine or not, with higher physics updates the issue becomes less of a worry.

[DelinWorks]

I see in the godot implementation they too use the 1 frame delay technique which suggests it's not really a problem