旋转其中一个后，如何将两个对象移向相同方向？

Question

I am making a 3D tank (just for learning 3d programming) (using this tutorials: http://www.spacesimulator.net/wiki/index.php?title=3d_Engine_Programming_Tutorials ) I made two 3ds models, the body of the tank, and the turret. Now I can move with my tank, I can play with the lights etc. and I can rotate the turret. But if I rotate the turret, and then I wanted to move the tank in any direction, the turret move into its own direction, and the body too.

For example: -> this is the direction of the tank, and <- this is the direction of the turret after the rotation. So in this example, the tank will go to right, and the turret will go to left (while i am only pushing the right button). I want the turret to go with my tank's body, to the right.

Of course without rotation, its working. I guess, matrix is the key of this problem, but i dont know what to do.

ps.: my source is almost the same as in the tutorial: "Print fonts using display lists"

void keyboard_s (int p_key, int p_x, int py)
{
switch (p_key)
{
case GLUT_KEY_UP:
ObjTranslate(&object[0],0,0.2,0);
ObjTranslate(&object[1],0,0.2,0);
break;
case GLUT_KEY_DOWN:
ObjTranslate(&object[0],0,-0.2,0);
ObjTranslate(&object[1],0,-0.2,0);
break;
case GLUT_KEY_LEFT:
ObjRotate(&object[0],0,0,3);
ObjRotate(&object[1],0,0,3);
break;
case GLUT_KEY_RIGHT:
ObjRotate(&object[0],0,0,-3);
ObjRotate(&object[1],0,0,-3);
break;
case GLUT_KEY_PAGE_UP:
ObjRotate(&object[1],0,0,5);
break;
case GLUT_KEY_PAGE_DOWN:
ObjRotate(&object[1],0,0,-5);
break;
}
}

Answer 1

(Broad generalization) In OpenGL all transformations are done by manipulating the modelview matrix. This means that the matrix always contains a transformation between the model and the view (it is, in fact a combined transformation of model to world, then world to viewer coordinates).

In your case, you are applying a rotation to the modelview matrix. It is nothing more than a matrix multiplication with a proper rotation matrix (which is done for you by ObjRotate). Obviously, after the modelview matrix contains the rotation, any further translation operations (which are, again, multiplied onto the matrix) relate to the rotation. This is, because the rotation is now part of the coordinate transformation.

The easiest solution to your problem is to apply the transformations in the right logical order. The logical order is that the tank is moving and the turret is moving with it. The rotation of the turret however is a rotation relative to the tank. So first both tank and turret need to be in the right position, then the rotation needs to be applied.

In your case it means that you cannot just manipulate further the modelview matrix whenever the user presses a button. Instead, you have to remember the position/rotation of the tank and the rotation of the turret yourself, so that you can then apply both transformations in the correct order.

One way to do is to keep two matrices: One transformation matrix for the tank positioning, another one for the turret rotation. You can then just alter these transformations and then apply them on the modelview. Another way is to keep the parameters of translation and rotation and call ObjRotate, ObjTranslate etc. (in the right order!) every time something changes.

OpenGL manages a stack of its transformation matrices. That means, before you apply these transformations, you should push to the stack with glPush. Then you can apply them and draw the object. Then you can pull from the stack again. This means, that the original state is preserved so you are able to re-apply transformations without corruption.