Question

I currently can rotate around a pivot point by first translating to the pivot point then performing the rotation and finally translating back to the origin. I do that easily enough for the shoulder in my example. However I cannot figure out how to also add in a rotation around the elbow for the forearm.

I’ve tried the following for the forearm rotation around the elbow:

1. translate to shoulder, rotate, translate to origin, translate to forearm, rotate, translate to origin
2. translate to shoulder, rotate, translate to forearm, rotate, translate to shoulder, translate to origin

Neither work for me. Any suggestions? I’m really stuck on this one.

Answer 1

I ran into a similar problem when I was doing some skeletal animation. It’s very helpful to use recursion for this. Also, structure your bones hierarchically (e.g. shoulder is a parent of forearm which is a parent of hand, etc.). By doing that you can write your code as follows:

void drawBone(Bone *root) {  
    if (!root) return;  
    glPushMatrix();  
    glTranslatef(root->x,root->y,0);  
    glRotatef(root->a,0,0,1);  
    // insert code to actually draw bone here  
    int i;  
    glTranslatef(root->l,0,0);  
    for (i=0; i<root->childCount; i++)   
        drawBone(root->child[i]);  
    glPopMatrix();  
}  

My bone struct looked like this:

typedef struct _Bone {  
    float x,y,a,l;    // position, angle, length of bone  
    int childCound;   // number of children for this bone  
    struct _Bone *child[MAX_CHILD_COUNT], *parent;  
} Bone;  

This website is a great resource for skeletal animation.