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When I import my wavefront obj model into unity and then draw lines over it with the same coordinates in the obj file, the x coordinate is negated.

I don't see any option in the importer that might be doing that. And I'm using the same localToWorldMatrix and the same coordinate data in the .obj file. Hmmm



GL.Color(new Color(0, 1, 0));




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The difference between a r/h coordinate system & a l/h coordinate system is that the x axis is negated. I wonder if the object was created in a modeling app that uses one system while your development framework uses the other.

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Look at the mesh instead of the game object in the inspector and see the orientation. Most of the times, exporters apply an overall transform.

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i checked that. the orientation is (0,0,0). besides, i'm using transform.localToWorldMatrix to draw my wireframe so that wouldn't matter anyways – milkplus Oct 14 '12 at 23:07

I was seeing a similar issue. To investigate further I added an editor script like this, to log the vertices of the mesh I applied it to:

using UnityEngine;
using System.Collections;
using System;

public class TestScript : MonoBehaviour 
    void OnEnable()
        Mesh mesh = GetComponent<MeshFilter>().mesh;
        Vector3[] vertices = mesh.vertices;
        foreach (var vertex in vertices)
            Debug.Log(String.Format("{0} {1} {2}", vertex.x, vertex.y, vertex.z));

This logged coordinates like this:

-42.4 -6.608938 -1.6
-42   -6.579293 -1.6
-42.4 -6.652683 -1.2
-42.4 -6.608938 -1.6

Whereas my original .obj file had vertices like this:

v 42.4000015258785 -6.60893774032594 -1.60000002384146
v 42.4000015258785 -6.65268325805652 -1.20000004768452
v 42.0000000000008 -6.57929277420054 -1.60000002384146
v 42.0000000000004 -6.57929277420055 -1.60000002384106

So as Steve H suggested, it does look like Unity is negating the X values while importing. It must be assuming that the .obj file is right-handed, and so it converts to Unity's left-handed coordinate system by negating the X.

It looks like Unity also does some re-ordering / optimisation of the mesh, as the order and number of the vertices in Unity is not the same as the original .obj file either.

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