How demanding are Physics?

[MorningStarz]

I’m working on a character about 4k+ Verts so far, possibly heading toward 5-6K once I create an outfit.
I would like to add physics to a cape and maybe sleeves as well, btw I’m using blender to model.
So I’m curious, how demanding are Physics? I’m hoping to keep the cape around 200 Verts.

[MSplitz-PsychoK]

Don’t use the same mesh for rendering and physics unless it’s low-poly. I recommend making low-poly versions of your meshes for collision.

Physics isn’t too expensive especially since it’s on a fixed update. When 2 objects collide and push each other into other objects, the collision system has to recalculate for the new collisions that happened because of the previous ones. That is why the game will slow down if you have a large pile of objects all colliding with each other.

With complex shapes, the collision system will pick up more subtle collisions and have to recalculate a lot more often. You especially want to avoid collision meshes with concavities (like a C shape) because parts of other objects would get stuck inside and create a gnarly recalculation nightmare.

That said, Unity has a maximum number of recalculations per physics update before it gives up.

[MorningStarz]

@Gambit MSplitz

Don’t use the same mesh for rendering and physics unless it’s low-poly. I recommend making low-poly versions of your meshes for collision.

Physics isn’t too expensive especially since it’s on a fixed update. When 2 objects collide and push each other into other objects, the collision system has to recalculate for the new collisions that happened because of the previous ones. That is why the game will slow down if you have a large pile of objects all colliding with each other.

With complex shapes, the collision system will pick up more subtle collisions and have to recalculate a lot more often. You especially want to avoid collision meshes with concavities (like a C shape) because parts of other objects would get stuck inside and create a gnarly recalculation nightmare.

That said, Unity has a maximum number of recalculations per physics update before it gives up.

Uuuuh, english? I understand bits n pieces but that’s it.

Umm, for instance, “Don’t use the same mesh for rendering and physics unless it’s low-poly. I recommend making low-poly versions of your meshes for collision.”

and “You especially want to avoid collision meshes with concavities (like a C shape) because parts of other objects would get stuck inside and create a gnarly recalculation nightmare.”

Can you explain those a little more? maybe some references?

[MSplitz-PsychoK]

@MorningStarz

Don’t use the same mesh for rendering and physics unless it’s low-poly.

Low-poly means few polygons or surfaces on the object. Take a look at the following screenshot talking about LOD. The far left model has a ton of verts to render a very detailed object, while the far right model is generally the same shape so collision still feels correct, but it has far fewer verts to the collision system doesn’t have to do a bunch of extra work. http://www.urbanterror.info/cache/images/texts/blogs/3.2.jpg

@MorningStarz

You especially want to avoid collision meshes with concavities

Take a look at the following image. The red shapes have concavities, which are like indents or holes in the object. To give an example of why this is bad, imagine a golf ball gets stuck inside a donut and the donuts hole is too small. The collision system will constantly try to recalculate to figure out how to fit the ball in the donut so that they don’t touch, but it’s impossible. http://udn.epicgames.com/Three/rsrc/Three/FBXStaticMeshPipeline/Convex.gif

[hippocoder]

You’ll have to explain what you mean by “physics”.