Is sharing vertices between faces worth it?

Question

I am currently working on a WebGL project, although I imagine this question is generic across many graphics APIs.

Let me use the example of a simple cube to demonstrate what I am asking.

A cube has 6 faces with 4 vertices per face so in total we have 24 vertices that make up the cube. However, we could reduce the total number of vertices to only 8 if we share vertices between faces. As I have been reading this can save a lot of precious GPU memory especially when working with complex models and scenes.

On the other hand though, I have experienced first-hand some of the drawbacks with sharing vertices between faces. These include:

• Complex vertex normal calculations as we must find the 'average' normal for each vertex, taking into account the face normals of each face that said vertex is a part of.

• Some vertices must be duplicated anyway to 'match up' with their corresponding UV coordinates.

• As a vertex may be shared by many faces, we are not able to specify a different colour per face using per vertex colouring.

The book I have been reading really stresses the importance of vertex sharing to minimise memory usage, so when I came across some of the disadvantages of vertex sharing I was unsure as to how viable/helpful vertex shading really is, and as the author did not mention any of the downsides of vertex sharing I would like to get the opinions of you guys. So is the memory saving produced from vertex shading really that important?

Answer 1

The disadvantages you named are indeed very real, especially for shapes with lots of sharp edges or different textures. A cube is the worst possible example for vertex sharing, each vertex has 3 different normals and possibly texture coordinates. It is essentially impossible to share the vertices.

However think of some organic shape. Like a ball, the body of some animal, cars, trees or even something as simple as a desert or something. These shapes probably need a high amount of vertices to look like anything decent, but a lot of those vertices are shared between faces. They need the exact same normals, texture coordinates and whatevers in order to look smooth.

Furthermore, the first disadvantage is not really that important. Calculating the vertices can be done in preprocessing, in most cases even by the modeller. This is basically never done realtime, instead you simply already have it in this format. However if it does need to be done realtime you can imagine this becoming an actual issue, and you need to start thinking about the trade offs and profile. But even then it can probably be dealt with using geometry shaders, if the visual fidelity is needed this can be a preferable solution.

In conclusion it heavily depends on what you're doing. In some cases vertex sharing isn't really viable because of the reasons you mentioned. Regardless, in many many cases it can potentially save a lot of memory.