# Problems with "stickiness"

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In the game I am working on you can have objects that are sticky.

I have modeled this by catching collision events and processing them.

The strength of the bond is proportional to the overlap area of the two objects and this works very well indeed for objects with planar faces.

However for spherical or curved objects, not so much.

Thinking about it in the general case, if you were to place a sphere on top of a cube. The overlap area would be infinitely small.

Indeed when  I look at the numbers coming out of my physics system, the overlap area is very small indeed. This is an artifact of the mesh being high poly. I want to keep the poly count high on these objects as they just look awful with a low poly count.

I am looking for inspiration.

I really want to avoid having a special case for curved objects. It creates all kinds of problems for things like hemispheres and introduces extra processing in a part of the code I need to keep speedy.

Any ideas guys?

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Can you not have high-poly objects for rendering and lower-poly physics spheres if the physics geometry is what is driving your "stickiness"...?

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First off, I would recommend not using highly tessellated shapes in your physics simulation. Generally it's a best practice to have very low face counts, and use different geometries for physics than compared to rendering. This way we have high poly rendering, but physics is happy with the larger faces (which are not visible to the player). There are mesh-simplification tools that can take a high-poly mesh and simplify it to be more fit for physics simulation.

If this isn't an option then I would just hard-code special cases for highly tessellated shapes. Circles would just have a different code path than cubes. But maybe there isn't a good way to do special cases, if so then here is the approach I would take:

When you gather a collision event (with shape A and shape B) from a curved surface, or tessellated surface, I would take the contact plane from the physics engines. This will be a very good frame of reference. Project the vertices of each shape from 3D to 2D in the contact plane, but only project points to the contact frame of reference if they are within PROJECT_TOLERANCE distance, which is a tunable parameter.

Now we have a 2D area from shape A and another from shape B, only on the contact plane. These areas will approximate how much overlap a real-life physical contact would have. I would take these two area values and mix them together with this formula:

float Mix( float a, float b )
{
return sqrt( a * b );
}


The purpose of the Mix function is to make sure that if either A or B contribute a super tiny area -- which could happen if a sharp edge of say a cube hits a flat plane -- the overall mix result used to calculate stickyness is very small. However if A and B are roughly equal, the mix result will be close to A and B.

To compute the convex hull in 2D there is a very fast routine within Box2D (here it is) that you can copy. If you have physics shapes that are implicitly defined (like a sphere represented by a point and radius, or a capsule with 2 points and radius), then a dedicated "projected area" function can probably be created. In other words, computing the 2D area of a projected sphere is simple enough to be solved directly, without projecting a bunch of triangles.

This kind of algorithm is generally more of an art than a science since we are dealing with idealized rigid bodies, but want them to behave more intuitively like real-life objects. Naturally many kinds of corners can be cut. This is just my take on the problem.

Edit: Thought of another way to approximate the contact area. In Game Programming Gems 6 Erin Catto has a good article on clipping 3D meshes to a clipping plane, and then calculating the resulting volume. In our case the contact plane can be expanded into two separate planes, where we offset the contact plane by + and - PROJECT_TOLERANCE. Take shape A and clip it against the positive plane, take shape B and clip it against the positive plane. Take the two resulting volumes, mix them together, and use that as the input value to stickyness. Personally I would still prefer the 2D projection since it's a little simpler, and can handle implicit shapes easily.

Edited by Randy Gaul

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I get the overlap area out of the physics system anyway. Glancing at the code it seems to be using planar equations to calculate the value.

We have separate collision meshes from the visible mesh, but if we reduce the poly count of the collision mesh to a point that the objects stick together, then they don't behave properly in the general case. Throw one onto a flat plane and the don't do a weeble impersonation (weebles wobble but they don't fall down) they just jerk around until they settle on a face of the collision mesh. Usually at a very unnatural angle.

I thought about having two collision meshes and switching between them depending on the game state, but that's not practical in our engine.

Thanks for the ideas though.

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Using the area of overlap seems reasonable enough.

I would try having a slightly larger version of the collision shape to determine how much area to use for stickiness / adhesion.

How to do it will depend on the physics library you're using, but I know for example in Bullet you can use a btGhostObject to detect that two objects are intersecting without actually doing any collision response.

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Hi Aken,

Can't do that in this physics engine I'm afraid, nice idea though.

I'm starting to think I have no choice but to have a special case for these objects. Nothing I have tried works.

Cheers

Paul

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