It’s not that rare. The surfaces can have any orientation, they just can’t be deformed easily. But the idea here is that it’s easier to understand. If you show

It’s not that rare. The surfaces can have any orientation, they just can’t be deformed easily. But the idea here is that it’s easier to understand. If you show an object space normal map, and then show the rendered model, you can say, “aha, it’s really obvious how that works”. And then you can introduce the tangent space normal map, and describe how you use transformation matrices to convert tangent space to object space. Showing the object-space normal map gives you insight into how tangent space normal maps work.