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# OpenGL Converting opengl to d3d coords

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I''ve written a d3d conversion of an opengl md3 parser. The problem is, as I understand it, all opengl (and md3) coords are Lefthanded and d3d''s are right handed (or is it vice-versa?). What do I need to convert an opengl vertex into a d3d one? I know this has been discussed before, but the search engine was of no use. Sorry. Should be an easy one! Thanks for the help, Onnel

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I''m quite sure OpenGL can use left handed AND right handed coords. Just choose the one you want (I don''t remember how to do this, check your doc).

I suppose D3D8 can do this too.

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Simply negate the Z coordinate to transform between a left-handed and a right-handed coordinate system.
In a left handed coordinate system the positive Z axis points 'into' the screen, while in a right-handed coordinate system the positive Z axis 'comes out of' the screen.

The positive X axis always point to the right and the positive Y axis always point upwards.

Hold you right hand in front of your face, with your thumb (X axis) pointing to the right and your index finger (Y axis) pointing upwards, then point your middle finger (Z axis) towards your face. This is where the name 'right handed coordinate system' comes from.

Now hold your left hand in front of your face, similarly with your index finger (Y axis) pointing upwards and your thumb (X axis) pointing to the right, now your middle finger (Z axis) points away from your face. Hence the name 'left handed coordinate system'.

So, you see, there isn't really much difference between the two. IIRC OpenGL by default uses an RHS (Right handed system), while Direct3D uses an LHS. To make either one use the other system, simply scale your world matrix with (1, 1, -1) before applying your transformations and rotations etc...

[EDIT: I realised that I had given an incorrect scale vector]

Edited by - Dactylos on July 31, 2001 6:33:29 AM

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DAC,

Thanks for the good explanation! Does this even truly apply to the way coordinates are stored then, or does it only apply to transformations done on coords? If it doesn''t effect the way coords and stored, and front how I understand what you wrote, it doesn;t, then there is no problem loading the exact same vertex data into OPENGL or D3D.

Is this correct?

Are there any other issues that would cause problems loading index and vertex data which was originally stored for use with opengl into d3d? Winding issues (ordering of vertices CW vs. CCW), or anything else?

Thanks!
Onnel

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If you load a model created for an LHS into an application that uses an RHS, then the model will appear mirrored in the XY plane (the Z coordinate is in effect implicitly negated; so to speak ).

You should decide whether to use an RHS or an LHS in your engine and then load the vertices etc in the exact same way, but each frame you would automatically mirror the world matrix in the XY plane (in other words, scale by (1, 1, -1) as I mentioned above) if you are using the rendering API for which your chosen coordinate system isn't the default.

ex.

If you choose to always work with a right handed coordinate system (I'm assuming here that OpenGL uses a right handed system and Direct3D uses a left handed system. This might not be correct however, please look this up, or try it out. If I have it backwards, then simply do it 'the opposite way' from what I show here).

When rendering with the OpenGL API you don't have to do anything, because it uses a right handed system by default. so simply apply all the translations/rotations/etc and then render your model.

When rendering with the Direct3D API, you should apply a scale(1, 1, -1) to the world matrix (or the view matrix) every frame, before doing any transformations and/or rendering. This should ideally be done in your BeginFrame() function or equivalent (the one where you call d3d_device->BeginScene() etc...), so that afterward all translations and rotations and whatever the application using your engine performs are in effect performed in a right handed coordinate system.

If you don't apply this scale your models will appear mirrored (eg. an enemy wielding a sword in his right hand would appear correctly under one API, while the sword would appear in his left hand under the other API).

Of course there is also the possibility that I'm totally off and that both OpenGL and Direct3D use the same kind of coordinate system. If so, then just ignore what I've said (it's of course still good to know).

Umm.... Feels like I'm mostly ranting by now, so I'll stop...

[EDIT: I realised that I had given an incorrect scale vector]

Edited by - Dactylos on July 31, 2001 6:34:31 AM

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This may be a little of the topic,
but I think both OpenGL and D3D can use both RH and LH systems.

I use RH for both APIs. It''s just the question of projection matrix and face culling. To use RH for d3d call D3DXPerspectiveRH (instead of D3DXPrespectiveLH).

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That's a convenience function that performs what I have described above (in addition to setting up a perspective projection matrix). Though, I claimed that you should apply the scale to the world matrix, perhaps I was wrong, and the projection matrix is the one you should scale.

I was not aware such a function existed, since I mostly dabble in OpenGL. What I have described above is however the 'theory' behind using different coordinate systems in the two APIs. AFAIK there is no such convenience function in OpenGL and you must explicitly perform a mirror in the XY plane (if you want a nonstandard coord-system).

The culling mode can be changed in both APIs to use either CW or CCW.

Edited by - Dactylos on July 31, 2001 10:37:44 AM

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