getting the normal of a plane

Unity Engine
1

I am running on Android. I have a program where I place a plane on the floor in an AR application at the location of a placement indicator (placement indicator follows AR planes and is placed based on a raycast from the center of screen)

Since I cannot guarantee that the plane will be placed with no rotation I can’t guarantee that a Vector3.up is the normal of the place. How do I get the normal to the plane?

placedPlane = GameObject.CreatePrimitive(PrimitiveType.Plane);
placedPlane.transform.SetPositionAndRotation(placementIndicator.transform.position, placementIndicator.transform.rotation);
debugLog("placed plane's rotation: " + placedPlane.transform.rotation);
Plane planeComponent = placedPlane.GetComponent<Plane>();
Vector3 normal = new Vector3(planeComponent.normal.x, planeComponent.normal.y, planeComponent.normal.z);
debugLog("plane's normal vector: " + normal.ToString());

I have created a debug “console window” at the bottom of the screen. debugLog(string) simply add that text to a new line in its scrollview.

The first debug log reports back a non zero rotation, which is not surprising because AR planes are not going to be perfect.

The second debug log gives a (0,0,0) value for the normal vector always.

An example output is:
placed plan’s rotation: (0.0, -0.9, 0.0, -0.4)
normal vector: (0.0, 0.0, 0.0)

My guess is that I am grabbing the Plane component incorrectly, because a normal vector should at least have some value somewhere. And as far as I can tell you can’t check if you failed to get a Plane object with getComponent because you can’t check a Plane object == null.

How should I get the normal vector of a plane?

2

Bingo! So a UnityEngine.Plane is not a component that can be .GetComponent-ed. It is a purely mathematical construct, a “Representation of a plane in 3D space,” as the Unity docs say when you hover over it.

A UnityEngine.Plane is also a struct, not a class, which is also why it can’t be compared to null.

When you GameObject.CreatePrimitive a PrimitiveType.Plane, that makes a GameObject with a MeshRenderer, MeshFilter, the flat plane mesh, etc. There is no actual UnityEngine.Plane structure per se to be gotten and the two are completely unrelated.

However, given that we know it’s a flat mesh that Unity provides, the normal to it will always be transform.up.

transform.up is different from Vector3.up, which is a constant unchanging (0,1,0).

transform.up is the local “up” for any given object at its current transform, and since we know the plane mesh is flat, that’s the normal too, coincidentally.

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3

Thanks, this cleared things up quite a bit.

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