Question

Using Android opengl I need to move an object from point A to point B and rotate it around its local Z axis in the same time. I have been reading tutorials for the past 3 days, everybody gives you bits of informations and hints, but nobody is capable of explaining this from top to bottom for beginners.

I know how to only translate the object from point A to point B.
I also know how to rotate the object in point A around its local axis (translate it to origin, rotate it, translate it back)
I DON’T know how to rotate and translate in the same time.

I’ve tried to translate to origin, rotate, translate back, then translate to point B. It doesn’t work, and I think I know why (the rotation is messing the object axis, so the translation to point B is incorrect)

A(-x1, y1 , -z1)
B(-x1 + deltaX, y1 + deltaY, -z1 + deltaZ)
_gl.glTranslatef(x1, -y1 , z1);         
_gl.glRotatef(degrees, x1, -y1 , z1);                               
_gl.glTranslatef(-x1, y1 , -z1);
_gl.glTranslatef(deltaX, deltaY, deltaZ);

I need to take into consideration the way the rotation is chaning the axes. Some say I can do that with quaterninons, or with rotation matrixes, etc.
But I don’t have enough opengl knowledge to use apis to resolve this.

Can someone explain this to me? With somecode also?

Thank you in advance.

Answer 1

If you have the following code:

glTranslate(x, y, z);
glRotatef(angle, 0, 0, 1);
drawObject();

The object will first be rotated around it’s local z-axis and then translated with (x, y, z). The transform call that is closest to the draw call is the one that happens first.

From your code it seems like you actually don’t want to rotate the object around it’s own origin but some other point, in this case you should do the following:

glTranslate(x, y, z);                         //Transform 4
glTranslate(origin.x, origin.y, origin.z);    //Transfrom 3
glRotatef(angle, 0, 0, 1);                    // Transform 2 
glTranslate(-origin.x, -origin.y, -origin.z); // Transform 1
drawObject();