What is the correct way to map textures onto SCNSphere

Question

I want to use this image as a material for a SCKSphere:

I can do this as follows:

SCNSphere *sphereGeometry = [SCNSphere sphereWithRadius:5];
sphereGeometry.firstMaterial.diffuse.contents = [UIImage imageNamed:@"navball.png"];

However, when I run the application, the image has been distorted as it is mapped onto the sphere, and the lines that run from the top to the bottom of the sphere are no longer straight:

Is there any way to ensure that the material is mapped onto the sphere correctly and the lines remain straight?

This image on the left below shows what I was expecting, and the image on the right shows what I'm experiencing - wavy lines when I would expect them to be straight:

Answer 1

I solved the problem by increasing the sphere geometry's segmentCount value, which increases the detail of the polygon mesh used to render the shape, making it appear "smoother".

For more details see the SCNSphere documentation.

Answer 2

Real-time texture mapping is generally done with 'UV' texture coordinates that define which pixel in the image is pinned to which vertex in the mesh. Between these pins, the image is just stretched linearly. For a curved surface, this approximation will result in visible artifacts if the flat polygons approximating the surface are too coarse. That's why increasing the segments (and thus smoothness) in the sphere made the image look better.