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erincatto / box2d-lite

A small 2D physics engine
http://box2d.org/
MIT License
876 stars 118 forks source link

FeaturePair.value not set correctly? #6

Closed huweiATgithub closed 3 years ago

huweiATgithub commented 3 years ago

Hi Erin, Thank you for your great illustrative code, it is very good for learning rigid body dynamics.

From Arbiter::Update, https://github.com/erincatto/box2d-lite/blob/227b71b6974ea57ab7e96d40f6374287bd6a0e77/src/Arbiter.cpp#L40-L50 I think FeaturePair.value is used to distinguish contacts. While in Collide.cpp where contacts are computed, the only place to set value is in ClipVertex's constructor which set value=0. https://github.com/erincatto/box2d-lite/blob/227b71b6974ea57ab7e96d40f6374287bd6a0e77/src/Collide.cpp#L44-L49 Does it seem like this value is not correctly assigned for its purpose?

Thank you.

bit-hack commented 3 years ago

I have been doing some work on Box2D-Lite and your ticket caught my attention.

It also looked to me that fp.value is only ever set to 0 but thats not the case. If you look at the definition of FeaturePair we can see that its actually a union type so setting inEdge1, etc will also affect value. This seems like just a way to speed up comparisons between all of these char members using type punning.

union FeaturePair
{
  struct Edges
  {
    char inEdge1;
    char outEdge1;
    char inEdge2;
    char outEdge2;
  } e;
  int value;
};
huweiATgithub commented 3 years ago

Oh, I see. Thank you!