GLKit Object not rotating properly

Question

İ have a 3d object.

i am rotating it with touches like this :

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInView:self.view];
CGPoint lastLoc = [touch previousLocationInView:self.view];
CGPoint diff = CGPointMake(lastLoc.x - location.x, lastLoc.y - location.y);

float rotX = -1 * GLKMathDegreesToRadians( diff.y / 2.0 );
float rotY =  GLKMathDegreesToRadians( diff.x / 3.0 );


GLKVector3 yAxis = GLKMatrix4MultiplyAndProjectVector3(GLKMatrix4Invert(_rotMatrix, &isInvertible), GLKVector3Make(0, 0, 1) );
_rotMatrix = GLKMatrix4Rotate(_rotMatrix, rotY, yAxis.x, yAxis.y, yAxis.z);

GLKVector3 xAxis = GLKVector3Make(1, 0, 0);
_rotMatrix = GLKMatrix4Rotate(_rotMatrix, rotX, xAxis.x, xAxis.y, xAxis.z);

}

and setting matrices like this :

_modelViewMatrix = GLKMatrix4Identity;

_modelViewMatrix = GLKMatrix4Translate(_modelViewMatrix, 0.0f, 0.0f, -60.0f);
_modelViewMatrix = GLKMatrix4Translate(_modelViewMatrix, 0.0f, 5.5f, -4.0f);
// i know i can to this by one code

//çevirme işlemleri ilki klimanın kameraya doğru bakması için
//ikincisi parmak hareketlerinden gelen transform matrisi
// 90 derece döndermeyi kapatıyorum
_modelViewMatrix = GLKMatrix4RotateX(_modelViewMatrix, GLKMathDegreesToRadians(90.0f));
_modelViewMatrix = GLKMatrix4Multiply(_modelViewMatrix, _rotMatrix);

_modelViewMatrix = GLKMatrix4Translate(_modelViewMatrix, 0.0f, -5.5f, +4.0f);

self.reflectionMapEffect.transform.modelviewMatrix = _modelViewMatrix;

i am translating modelViewMatrix to objects centre. rotating it. than translating back. than translating -65 on z. but everytime i tried to do it. it's rotates like on the same vector. i think object has it's own centre. and rotating with it's own center and scene's center.

how can i change object's centre with code or how can i rotate this object properly?

Answer 1

The way the matrix multiplication works is considering the object base vectors. You can imagine it as looking form a first person perspective (from the object/model that is). If you first move the object (translate) and then rotate the object will still be at the same position but facing a different rotation, that means it will not simulate the orbiting. If you change the operations to rotate first and then move it will simulate orbiting (but rotating as well). For instance if you rotate the model to face your right and than translate forward it will seem as if translated to your right. So a true orbiting consists of first rotating by some angle, then translating by radius and then rotating by same negative angle. Again, try looking as from the model perspective.

I hope this helps as you did not explain what exactly is it you want/need to accomplish.