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irdalama

OpenGL cant get it to rotate right

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Hey, im trying to rotate objects from the center of their body. I am using visual c++ with the opengl, its rotating from the bottom left corner of the front face (+z normal - righthand coord system) of a cube. I used the glBegin(Gl_QUAD) and drew six faces that ended up as a cube. How can i get my rotate functions to rotate from the center of cube's body instead of rotating from the front face corner(+z normal - right hand coord system)? thanks for any help.

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are you sure your cube's bottom left corner is not at the origin? if it is then that would be where it'll rotate if no tranlation is applied.

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Guest Anonymous Poster
keep in mind though, translating is NOT a lineair image.
this means translate-then-rotate is NOT the same as rotate-then-translate

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The whole answer is:

(1) Translate the mesh so that the wanted center of rotation is rested at the origin 0 (in the same co-ordinate frame as the rotation takes place).

(2) Then rotate.

(3) Translate the mesh back to its original position (obviously just the inverse transformation of (1)), to let the mesh appear where it was intended to appear.


This is the reason why: If looking at an arbitrary rotation matrix applied to a point (i.e. a vertex of a mesh), you'll see that only 1 point in space is guaranteed to rest at its original position, namely the point with all co-ordinates of a value of 0. This is typically called the center (or origin) of rotation. If you want a particular point to be the center of rotation, you have to force that point being at the position 0.

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Quote:
Original post by Anonymous Poster
keep in mind though, translating is NOT a lineair image.
this means translate-then-rotate is NOT the same as rotate-then-translate

It is correct that the order of particular transformations plays a role. That comes from the fact that the matrix product is not commutative. This isn't restricted to translations since all transformations are normally expressed by matrices.

For the transformation I've posted above this means that
T * R * T-1
(where T means the position of the wanted center of rotation and R means the rotation itself) is the correct order for column vectors (as are used by OpenGL). In other words, the order of OpenGL commands is

glTranslate by T
glRotate by R (expressed by a glMultMatrix or 3 glRotates)
glTranslate by negative T
glVertex (i.e. the mesh follows here)

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or just draw the quad with the proper origin right away, this saves two translates. origin the center of the quad, since this will suit every (common) goal for quads.

Also, i personally think that it is very important to keep in mind that you will have to make a choice. basically, everything in relative coords or abs coords. of course an inbetween can also be done, but oversight will even easier be lost.
this is important since it wil have implications on every thing else you want to do. efficiency wise it matters because you'll want the representation which need less calculations

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