# OpenGL MaxScript transforms

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leandro    122
Hello, I'm trying to write a 3DSMax MaxScript exporter to save to a custom format geometry, keyframes, material and hierarchies. I had no problems with geometry, materials and keyframes but I messed it up with hierarchies. I can build the hierarchy but when I add keyframes to it translations rotations and/or scales looks wrong. I've tryied to work out the offset transform, etc. that 3DXMax uses for transforms, but I couldn't get anything. Can anyone tell me how to proper manipulate the transforms of objects without messing the hierarchies relative position, transformations, etc. I need the transforms in object space, but it would be good to know how to get them in world, etc. spaces. Also, how do I transform left-handed rotations to right-handed (Or is it the other way about? I don't remember right now, but I want to use OpenGL). Thanks a lot.

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Motorherp    613
Not sure about the hierarchies thing, I've not gotten that far with my 3ds parser yet, but to tranform between the different world axes is fairly simple. To begin with 3dMax has its world axes on their side, ie: z looks up and y looks into the screen, so first you have to multiply all your verts by the following matrix to get the y axis going up:

1, 0, 0
0, 0, 1
0, 1, 0

Only problem now is that the z axis is flipped due to the difference in handed coord systems. This is fixed now by simply negating all the verts' z coords. Just one more thing that you might not have encountered yet. When you rotate your meshes in 3ds it doesn't update the mesh normals for you so they'll end up reflecting light incorrectly in OpenGL or DirectX. Hence before applying the world axis correction above to the normals you first have to multiply them by the meshes rotation matrix. Hope this helps

PS: Just occured to me, with the normals above I'm talking about when you export to an .ASE file. It might be different when working from within the package, I'm not sure.

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Teknofreek    331
To get an object's transform in local space, that is relative to it's parent, you multiply it's transform by the inverse of it's parent's transform. For example, to collect the local transforms for all of the objects in the scene you could do this:

(    local SceneTransforms = #()    for Obj in Objects do    (        local ObjLocalTransform = Obj.transform        if( Obj.parent != undefined ) then        (            ObjLocalTransform = Obj.transform * ( inverse Obj.parent.transform )        )        append SceneTransforms ObjLocalTransform    ))

Hope that helps!

-John

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Guest Anonymous Poster
Tanks a lot. I'll try and update my exporter. Now I have another starting point for reading and experimenting

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leandro    122
Ups, that was me but forgot to logg-in.

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