Find Direction of Exterior Walls of GLTF Model

Questions
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1

Hello, I loaded a gltf house model in threejs. The house has walls, what i want to know is what direction the exterior wall face is pointing. For example i have a 4 walled house, i want to know which direction the front of the house is facing. The walls that I have, they have a thickness. So there is one side of the wall towards the inside of the house, and the other is towards the exterior side. So how can i detect which direction the exterior of the wall is pointing?

-x, +x, -z, or +z. Is this possible?

2

I already posted a similar question, but i do not like the way i worded the question.

3

There are several ways, but it all depends on the model. I will start with 4 methods, other people may add the rest:

  • check whether the designer of the model has followed some rules that will help you, for example, if the house is designed so that the local X axis of each wall points outwards, then you can use it to find exterior direction
  • cast a ray from the center of the house towards the wall – the first hit point is internal, the second is external, the raycaster will give the normal vector and this is the direction – this will not work well if there are other objects on the wall that you cannot isolate
  • cast a ray but only check the first hit point, take its normal vector, flip it and you will have the external direction
  • take the tangent vector of the long side of the wall, rotate it horizontally 90° and check whether it points towards the center of the house or away of the center; of course, you only check just the sign of a dot product; +1 means external, -1 means internal
4

thank you, i want to try method 2 or 3, but how would this work with models like this?

image
image439×550 224 Bytes

5

It would not. They would work for 4-wall houses with X/Z orientations – this is what you described in your first message.

For a shape like the image, it might be better to use the odd-even algorithm. Pick a point in front of the wall (either inside or outside, you do not know yet). Cast a ray in any direction and count how many walls are intersected. Odd number of walls = the point is internal; even number of walls = the point is external.

Note that this algorithm would not work if there are holes in the walls. So, you should treat doors and windows are part of the walls.

6

Okay, I’m trying to understand this algorithm.
Let’s say we go back to the four walls. If I choose the left wall and the ray points to the right, it will detect the right wall. If the ray points left, it will detect the left wall. How can I use this information to determine the direction of the exterior?

And what if the ray points upward and detects the back wall? How can I use that information?

7

hold on i might have figured it out sorry, ill update what i figure out

8

I’ve figured it out, thanks to your guidance.

Here’s the approach I took: for each wall, I assigned four raycasters in the positive x, negative x, positive z, and negative z directions. When one of these raycasters detects the wall that generated it, I remove the other three raycasters. Afterward, I check how many other walls intersect with the active raycaster (excluding the wall that generated it). Then, I use the algorithm you explained to process the intersections.