OpenGL ES: Rotating 3d model around itself

Question

I'm playing with OpenGL ES on iPhone and I'm trying to rotate a model by panning with the finger. I discovered the open source app Molecules that let's you do that and I'm looking at that code, but when it comes to rotate a model of mine I'm able to rotate it only around a point distant in the space (like it was in orbit as a satellite and I am the fixed planet). Any suggestion on what can be wrong? I can post the code later , maybe on demand (many lines) For the most part refer to Molecules you can find it here MOLECULES

Answer 1

If my memory serves me correctly, I think you need to translate the model to the origin, rotate, and then translate back to starting position to get the effect you are after.

I think there is a glTranslate() function, Say the object is at 1,0,0. You should then translate by -1,0,0 to go to origin. That is translate by a vector going from the center of the object to the origin.

Answer 2

The draw code probably looks roughly like this:

glLoadIdentity();
glTranslate(0, 0, -10);
glRotate(...);
drawMolecule();

Now it's important to realize that these transformations are applied in reverse order. If, in drawMolecule, we specify a vertex, then this vertex will first be rotated about the axis given to glRotate (which by definition passes through the local origin of the molecule), and then be translated 10 units in the −z direction.

This makes sense, because glTranslate essentially means: "translate everything that comes after this". This includes the glRotate call itself, so the result of the rotation also gets translated. Had the calls been reversed, then the result of the translation would have been rotated, which results in a rotation about an axis that does not pass through the origin anymore.

Bottom line: to rotate an object about its local origin, put the glRotate call last.