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OpenGL Porting my engine from OpenGL to Direct3D

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gamma_strahler    122
Hi, in order to port my engine from OpenGl to Direct3D, i also need to convert polygonal data from counterclockwise to clockwise winding order, to be correctly displayed with Direct3D. But i´m not sure if it is enough to just reverse the winding order or do i need to rewrite all of my graphics and physic algorithms (collision detection, csg, bsp) so that they use the correct calculation of normals? would be very nice if you could give me some advices and tips on this topic. thanks Gammastrahler

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xycsoscyx    1150
You shouldn't need to convert your data at all. The main differences between OGL and D3D is the handedness, but that can be corrected by properly setting up your transformation matrices. Using Vertex Shaders actually makes it simpler because you can make sure things transform normally (even if you're using some odd axis alignment, like the old school Z axis == up).

You definitely shouldn't need to adjust your physics or anything, if nothing else, I say handle the conversion from OGL-space to D3D-space in the transformation step, it'll simplify things (since everything else can then stay the same).

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_neutrin0_    241
Original post by gamma_strahler

in order to port my engine from OpenGl to Direct3D,

i also need to convert polygonal data from counterclockwise to clockwise winding order, to be correctly displayed with Direct3D.

That is not true. What made you think that? You will have to change the OpenGL API calls to Direct3D calls, but you will not have to change the winding of your polygons. The right and left handedness are mere conventions. To say that OpenGL is right-handed and Direct3D is left-handed is not really true. Yes they both default to different conventions but its just a matter of setting the matrices correctly to make one behave exactly like the other. It is actually up to you to decide the handedness of the data you are using in your application, and create matrices that transform that data in the correct way. Direct3D offers APIs to create both types eg.
D3DXMatrixPerspectiveFovLH - Left handed.
D3DXMatrixPerspectiveFovRH - Right handed.

Better yet, if you have the option, use your own matrix library for both APIs. That way the matrices are always created correctly.

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My D3D engine uses an OpenGL-style coordinate system, and all of the winding issues are just handled in the transform pipe, as the others here have mentioned.

Even if that approach weren't feasible for some reason, you could just invert your culling mode between the GL & D3D implementations (CW vs CCW) to get what you need.

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