Rotating an object in OpenGL ES for iPhone [translate to origin --> rotate --> translate back is not working]

Question

I recently started working with OpenGL ES for the iPhone, and I am having a bit of trouble with it. I want to be able to rotate an object with your fingers. My problem is that I have my object placed at (0, 0, -3), and I would like to rotate it about its center. I know that I need to translate back to the origin, rotate, and then bring it back to the original place. I think I am facing a problem because I am using a matrix to keep track (?) of all of my rotations/translation/scaling etc, and I think it may be combining the operations in a way that order is not even considered (so the two translations would cancel each other). I just started learning OpenGL a day ago and am a complete newbie, so my assumption may be wrong.

Here is the part of the in drawView that I am having trouble with:

GLfloat matrix[16]; 
glGetFloatv(GL_MODELVIEW_MATRIX, matrix);
glLoadIdentity();
glTranslatef(0, 0, 3); // bring to origin
glRotatef(self.angle, self.dy, self.dx, 0); // rotate 
glTranslatef(0, 0, -3); // put it back in place 
glMultMatrixf(matrix); // save the transformations performed

Help would be much appreciated, thank you!

Answer 1

Retrieving the modelview matrix, and then multiplying it back on after your rotation seems fishy, but it depends on what your other transforms are and what coordinate space things are supposed to be. Your comment on the glMultMatrix line doesn't correspond with what you're doing.

Normally you would just do the translate+rotate+translate as the most local actions on the object, just before you render it. Also note that this would only apply if you're object is at (0, 0, -3) in object-space. If it's at that location in world space, then the rotation will already rotate the object around its own center if you have previously made a series of transform calls (translate, rotate, etc) to move the object to its intended position in the world.

Transform order is one of the tricky parts of learning OpenGL. As a general rule of thumb, your operations start with the outer-most and progress to the inner-most. So a typical simple set of transforms would be: the inverse of the camera transform to move the world to match up with the camera, then the object's translation to move it to its world-space position, then the rotation to set it's intended orientation. The PushMatrix/PopMatrix stack functions let you save and undo part of that series of transforms so that groups of objects can share portions of that chain.