# Why does unity obj import flip my x coordinate?

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.PushMatrix();
GL.MultMatrix(transform.localToWorldMatrix);

CreateMaterial();
lineMaterial.SetPass(0);

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

GL.Vertex(p1);
GL.Vertex(p2);

GL.Vertex(p2);
GL.Vertex(p3);

//...

GL.End();
GL.PopMatrix();


1. The actual OBJ file format specification declares that, "A right-hand coordinate system is used to specify the coordinate locations."

2. Unity uses a left-hand coordinate system.

3. Conversion from right-handed to left-handed is accomplished by negating the coordinates on any axis (it doesn't matter which one).

4. Unity negates the X-coordinates to convert the right-handed OBJ data to left-handed data.

As far as I can tell, this is not documented by Unity. But, if you look at that bug report and the dialog I had with Unity's rep, it is the intended behavior.

Note that none of this depends on any modeling app. I did my tests with a file I created in Notepad. It all comes down to the OBJ file being specified as using a right-hand system, Unity using a left-hand system, and Unity negating the X-coordinate to convert from one to the other.

• Thank you so much for this! You saved me hours of additional debugging! – Verdagon Feb 10 at 23:52

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.

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.

• 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;

[ExecuteInEditMode]
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.