Custom Character Controller in Unity: Part 6 – Ground Detection

Despite having written five posts about the Super Character Controller, up until now I’ve only briefly touched on the issue of ground detection. Knowing what your controller is standing on is a hugely important topic, since a great many of your player’s actions will depend on what kind of ground he is standing on, if any at all. Good ground detection can make all the difference between a smooth playing experience and a terrible one.

An example of bad ground detection

So what do we want to know about the ground beneath our character? We definitely need to know how far away it is. We’ll want to know if our character has his “feet” touching the surface of ground or if he’s 6 meters above it. We also will want to know the location of the point on the surface of the ground directly below us, as this is important for ground clamping, highlighted in a previous post. Thirdly, we will want to know the normal of the surface directly below us, a direction represented by a Vector3. And lastly it will be necessary that we know the GameObject that the ground belongs to, to allow us to retrieve any attached components to the object that may be relevant to grounding. (In the Super Character Controller, we retrieve a SuperCollisionType component that describes properties on the object).

Looking at an earlier post surveying the Unity Physics API, the most obvious solution to our problem comes in the form of Physics.Raycast. We can fire a ray directly downwards at the ground below us. Through the RaycastHit structure we can retrieve the contacted point, the distance traveled, the normal of the surface, and the object the ray collided with. This seems to fulfill all of our requirements at first glance, but looking more closely reveals a significant problem.

Our character controller is represented by a series of spheres to form a capsule, which means that when he is standing on directly flat ground, the nearest point on the surface of the ground will be exactly radius distance from the center of the lower sphere in the capsule. This is fine, but problems appear once the character is standing on a slope. If we now fire a raycast directly downwards (as before) the point we contact is no longer the closest point to our “feet.” This will cause issues with our ground clamping method (among other things).

Ray is cast directly downwards from the center of the lowest sphere in the controller. As the controller is standing on a slope, the point directly below is NOT correct, and when the controller is clamped to that point it introduces an error where the controller is slightly clipping into the slope. For steeper slopes, this error will become more pronounced

Luckily, we have a savior in the form of Physics.SphereCast. Instead of casting a thin ray downwards, we cast a sphere (if it wasn’t already clear enough in the name). This will solve the above issue by ensuring that our controller is properly aligned to the surface below us, regardless of it’s normal.

While SphereCast works extremely well in representing our controller’s “feet,” it comes with a few issues. The first is one you will encounter regardless of what you’re using SphereCast for: when the SphereCast contacts the edge of a collider (rather than a face directly on) the hit.normal that is returned is the interpolation of the two normals of the faces that are joined to that edge. You can think of it similar to the Vector3.Lerp method, with the normals of the two faces being your from and to values. The t value would then be represented by how far the contacted point was from the center of the sphere that contacted the surface; if it hit dead center, the from and to would be equally weighted with a t value of 0.5, and as you move along the edge it would increase or decrease.

Animation demonstrating SphereCast hit.normal interpolation. Yellow wire sphere is the origin point; red is the target. The green vector represents the hit.normal. Notice how as the SphereCast moves over the edge of the box, the hit.normal is interpolated between the two normals of the joining faces

I discovered fairly early on that it was necessary to know the actual normal of the surface you are standing on, rather than just the interpolated normal. To solve the problem presented by SphereCast’s interpolation, I would follow up the SphereCast with two separate Raycasts aimed at each of the two joining faces which would retrieve for me the correct normals. (In the Super Character Controller’s ProbeGround method, these are called nearHit and farHit, representing the normals of the closest and furthest face from the center of the controller, respectively.)

The next issue with SphereCast is specific to using it for ground detection but highly essential to ensuring proper accuracy. We’ve been assuming that anything the SphereCast collides with is valid ground that our character can stand on (or slide or, in the case of some slopes for certain games). In practice however this is not always true! We can reasonably assume that the physical surfaces of a game world (that the controller collides with) can be divided into ground and walls, with the idea that only the ground surfaces should be detected by ground probing methods (like the SphereCast defined above). The simplest way to partition the world into ground and walls would be to say that any surface with any angle (relative to the Vector3.up of the world) less than 90 degrees is ground, while surfaces angled 90 and above are walls and cannot be detected as ground. Therefore we should ensure that we never mistakenly detect a wall in our ground probing method. Naively it looks like this problem is implicitly solved by the nature of our ground probing: we are casting a SphereCast directly downwards, which means it should (in theory) never contact a 90 degrees wall. However, very often the normals of the walls in a game world will only be near 90 degrees, or somewhere between 85 and 90. We want to treat these 85 degree angle surfaces as walls, meaning that they should be ignored in our SphereCast.

Our controller is flush up against an 85 degree angle wall. Due to this slight angle in the wall, our SphereCast’s contact point is at the yellow X marker, rather than the surface directly below us. This will cause our controller to believe he is standing on a steep slope, rather than safely on flat ground

The most obvious solution would be to use Physics.SphereCastAll. It would ideally collide with both the steep wall and the flat ground, and we could iterate through all contact points to decide what we are standing on. Unfortunately, SphereCastAll only picks up a single contact point per object, so if the ground and wall are part of the same mesh collider, this solution will not work.

Trying something simpler, we could just reduce the radius of the SphereCast by a small amount to account for the above error. And for very slight errors, this does solve some problems, but not all. We still need to find a way to reliably retrieve the proper ground when flush up against a steep slope.

[ Note: In the Super Character Controller, the angle of a “steep slope” is defined as the value StandAngle in the SuperCollisionType component attached to each object the controller collides with. ]

To do this, let’s make the assumption that there is some sort of ground beneath us, and that if there was not a steep slope blocking us our SphereCast would have contacted it. To retrieve this ground, we can Raycast down the steep slope our SphereCast hit. This is primarily to verify that there is some sort of ground there, and (more importantly) to retrieve it’s normal.

Initial SphereCast contact point marked in yellow. Because we have contacted a steep slope, we Raycast (shown in red) down the slope to detect the ground that is actually beneath us (Raycast contact point marked in purple)

This tells us what’s below us, but because we used a Raycast, and not a proper SphereCast, we once again run into the problem presented earlier, where the shape of our ground detection does not correctly align with the shape of the bottom of the controller. Given the information we have (the normal of the surface below us), can we somehow transform our Raycast data into an approximation of SphereCast data? Yessir.

When you SphereCast downwards from the bottom of the controller and contact a surface, there exists a relationship between the normal of the surface and the point on the sphere that the contact takes place. When we Raycast down the steep slope have the normal of the proper ground surface, so it’s our job to find the point along the bottom of the controller that a SphereCast would connect with. We also have the planar direction (from top view) towards the point (given by the direction of the slope below us), allowing us to tackle this problem in 2-dimensional space (finding a point on a circle, as opposed to a sphere) and then convert our result to 3-dimensional space.

Animation showing how the contact point of a SphereCast is directly related to the normal of the surface it collides with. SphereCast origin in yellow, contact in red, with the contact point marked in light blue. Notice how as the slope becomes steeper the point moves further up and along the edge of the red circle

Since we are attempting to find a point in 2d space, we are looking for two values which we can call x and y. If calculated properly, x and y will describe a point along the position of our circle. Referring to the diagram above as an example, we would be given the normal of the ground surface, and our job would be to calculate the position of the light blue point.

Luckily, this really isn’t all that difficult. Referring back to grade 8 math, we can use the Sine and Cosine functions to calculate the x and y positions, with the normal of the surface being the angle we pass in.

x = Mathf.Sin(groundAngle);
y = Mathf.Cos(groundAngle);

Pretty handy. Note that in Unity the Sine and Cosine methods require you to pass in the angle in radians, so ensure you convert your angles beforehand.

Calculating the contact point (light blue) from the angle of the green slope using the Sine and Cosine functions

We can now use these values to find our point by adding them to the position of our controller (multiplied by it’s radius). Effectively, we’ve now converted our Raycast data in SphereCast data, and nobody is the wiser.

[ Note: In the Super Character Controller, the method for approximating SphereCast data based off a surface normal is called SimulateSphereCast. ]

This sums up the current technique I am using for ground detection (in the Beta controller). Unlike previous articles, the question of finding a solution to the problem of detecting ground is fairly open—the above is by no means the optimal solution, although I’ve found it works very well in practice. I plan to write a follow-up to this showing how to actual use the data we’ve worked so hard to retrieve, since it’s a somewhat involved process.

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