Bounding box in opengl c programming - opengl

Question

How can I create a bounding box around a 3d object that I've created in 3DS MAX, and then used it in an openGL application? I need the bounding box for collision detection, but I don't understand how to link a bounding box to an object. The function that creates the object is:
void drawR2D2(){
if (xrot <= 5.0) xrot = 5.0;
if (xrot >= 70.0) xrot = 70.0;
if (ztransKit2 >= 0.7) ztransKit2 = 0.7;
if (xtransS1 >= 0.2) xtransS1 = 0.2;
if (ztransKit2 <= 0.0) ztransKit2 = 0.0;
if (xtransS1 <= 0.0) xtransS1 = 0.0;
glPushMatrix();
drawBodyR2();
glPopMatrix();
glPushMatrix();
glTranslatef(0.0, 1.2, 0.0);
glScalef(2.85, 2.85, 2.85);
glRotatef(yrotHead, 0.0, 1.0, 0.0);
drawHeadR2();
glPopMatrix();
glPushMatrix();
glScalef(3.5, 3.5, 3.5);
drawLegsR2();
glPopMatrix();
glPushMatrix();
glTranslatef(-0.41, 0.45, 0.26);
glScalef(1.4, 1.4, 1.4);
glRotatef(xrotKit1, 1.0, 0.0, 0.0);
drawKit1R2();
glPopMatrix();
glPushMatrix();
glTranslatef(-0.2, 0.5, 0.0 + ztransKit2);
glScalef(2.0, 2.0, 2.0);
glRotatef(zrotKit2, 0.0, 0.0, 1.0);
drawKit2R2();
glPopMatrix();
glPushMatrix();
glTranslatef(-0.25 + xtransS1, 0.48, 0.45);
glScalef(1.55, 1.55, 1.55);
drawShield1R2();
glPopMatrix();
glPushMatrix();
glTranslatef(-0.44 + xtransS2, 0.48, 0.37);
glScalef(1.12, 1.12, 1.12);
drawShield2R2();
glPopMatrix();
}
The other functions: drawBodyR2(),drawHeadR2, etc are the components of the entire object.
The code for the drawBodyR2, for example, is:
void drawBodyR2(){
InitModel("r2d2/body.obj", &body);//function to initialize the object
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, bodyId);
glmDraw(body, GLM_SMOOTH | GLM_TEXTURE);
glDisable(GL_TEXTURE_2D);
}
Can anyone show me how to create a bounding box around the object resulted from the drawR2D2 function?

Answer 1

The "best" box you can build for collision detection is an oriented bounding box (OBB), that is the smallest possible box that fits the object. In order to do that you need to learn some maths (what eigenvectors are mainly) and use them for the OBB (check this answer for a detailed explanation about OBB computation).
Note that:
OBB has nothing to do with OpenGL. You can compute the OBB/ABB having a model and never drawing it. Separate the tasks!
An OBB should never be computed twice for the same model, it is computational nonsensical. The ideal thing would be to precompute the coordinates of the 8 points forming the OBB of each of the objects you are going to draw and store them. Remember that this implies that whatever geometric transformation you apply to your object, you need to also apply it to the OBB.