iPhone Racer, Part 6

Making the wheels spin is actually a bit more complicated than one might think. The 3D model of the car is divided in five different geometric objects. Four of these objects represent the wheels and the fifth object is the rest of the car.

Each object is defined by a number of vertices (points) in 3D space. If you want to spin a wheel, you have to rotate its vertices around its center axis. The problem is that when you rotate vertices using OpenGL, they are rotated around the origin of the coordinate system. Consequently, if you apply a rotation transform to a wheel, it will not spin around its center axis. So what you have to do is move the wheel to the origin, rotate it, and then move it back to its original position.

I extended my OBJ converter script to calculate the center point for each object in the 3D model and then save it along with the geometric data. Then in the car rendering code, I added a translation transform to move the wheel to the origin by subtracting the center point, followed by a rotation, and another translation to move it back by adding the center point again.

The order of the OpenGL transformation calls may look like they're in the wrong order, but this is due to the order in which the transformation matrices are multiplied.

Currently, it takes one separate call to the drawing function for each object in the 3D model, because different transforms have to be applied to the objects. This can be optimized by using the matrix palette OpenGL extension.

Instead of pushing matrices on the OpenGL matrix stack, you precalculate the entire transformation matrix for each object and place them in a matrix palette, which is really an array of matrices. Then each vertex struct is extended with a field that contains an index into the matrix palette, indicating which matrix should be used to transform that particular vertex.

This way you only need one call to the OpenGL draw function to draw the entire model, because no state changes need to occur in between the rendering of the different objects in the model.

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