how to get the contact point from TOI? about velcrophysics HOT 5 CLOSED

TOI sweeps the shape along a path to the point of collision. Distance.ComputeDistance() is used by TOI and faster than TOI if you only need the point of collision and normal.

from velcrophysics.

Hey, thanks for your answer! Not sure I understand though, how would I get the distance considering I'm doing a sweep? As I understand it CalculateTimeOfImpact iterates a bunch of times along the sweep, while ComputeDistance only takes transforms.
Currently this is what I'm doing:

TimeOfImpact.CalculateTimeOfImpact(out _output, input);

#region compute the contact point at TOI
// FIXME: this is super redundant and inefficient as CalculateTimeOfImpact() already does all this!

if (_output.State == TOIOutputState.Touching ||
    (_output.State == TOIOutputState.Overlapped && _overlapPolicy == ShapeCastOverlapPolicy.Accept)) // NOTE: not sure ComputeDistance() returns something valid in this case...
{
	Sweep sweepA = input.SweepA;
	Sweep sweepB = input.SweepB;
	sweepA.Normalize();
	sweepB.Normalize();
	TransformF xfA, xfB;
	sweepA.GetTransform(out xfA, _output.T);
	sweepB.GetTransform(out xfB, _output.T);

	distanceInput.ProxyA = input.ProxyA;
	distanceInput.ProxyB = input.ProxyB;
	distanceInput.TransformA = xfA;
	distanceInput.TransformB = xfB;
	distanceInput.UseRadii = false;

	DistanceOutput distance;
	SimplexCache cache;
	Distance.ComputeDistance(out distance, out cache, distanceInput);
	_contactPoint = distance.PointA;
}
else
{
	_contactPoint = new Vector2();
}

#endregion

... thoughts?

from velcrophysics.

Right, now I understand.

The internals try to limit the size of input/output structs and caches such that more of it will fit in the CPU cache. On most systems it is much faster to just run a single iteration of the distance function rather than storing a lot of (mostly) unused values filling up the cache.

It does not store the original value of the distance function in the cache or output as it is not needed for the engine itself. However, you can try and store the original values from the distance function in TOIOutput if you want to omit that one distance call.

from velcrophysics.

OK, that's what I thought, that it could just be added to TOIOutput, but I haven't studied the loop closely enough that I felt sure where exactly to do that.
Do you think adding more data to TOIOutput would have a negative impact on cache performances for the general case? I don't have a good grasp on how heavily CalculateTimeOfImpact is used by the engine itself... not even sure it matters for my game as I mostly use the engine for collision detection, not real physics (no non-kinematic bodies, forces etc).

from velcrophysics.

Honestly, it probably wouldn't. However, if we take that approach everywhere it would have a big impact.

This engine was built many years ago and both computers & compilers have changed a lot since. Nowadays I have to use Intel VTune Amplifier to get the real numbers as pipelining, prefetching, cache coherence etc. all have a huge impact on tight loops. For the most part, a simple profiler is enough though.

from velcrophysics.