Hello

I'm currently investigating optimizations for a game project. The vertex processing is fairly expensive. One thing that struck me is that the average vertex-per-face count seems rather high. It's currently 1.2. Theoretically a mesh average could be around 0.5 if each vertex is shared between 6 as each triangle has 3 vertices (1/6 * 3 = 0.5). Of course this isn't achievable in real life due to many factors such as hard edges, uv-mapping etc.

I tried to search for this online and on this forum but didn't find any good resources. So I ask anyone with an opinion in this subject. As a rule of thumb:

What would you think is a good scene average?

What is a good value for a character mesh?

What is a good value for a props mesh?

As an artist besides keeping the vertex count down in low poly modeling. Are you applying any particular techniques to keep the vertex-per-face count down, e.g. uv map in certain ways etc?

Maybe someone know some webpage concerning this issue as well?

Thanks a lot!

Yeah, like said above, vertices per face is not an interesting metric. The total number of vertices and the total number of triangles are more important. If an artist needs to do e.g. a hard edge on a seam to deliver some effect, the artist will need to do a hard edge on a seam and there's no way around it. Perhaps the opportunity in improving vertices per face might occur if for some odd reason two faces that share a vertex should have the same normal direction at the vertex, but somehow ended up having normals that are almost identical but not quite, and as a result the vertex got (unintentionally) duplicated for the two adjacent faces. Although I imagine such mistakes would be quite rare in practice.

On the GPU there's this thing called post transform vertex cache. Before the vertex shader process a new vertex it first check if it's in the cache already. It's much faster to fetch a vertex from the cache than to process the vertex shader. So if there's a lot of sharing of the vertices between the faces this means a lot more vertices are fetched from the cache and the GPU works faster. So I disagree!

I don't think anyone is saying that the number of shared vertices doesn't have an impact, I think they're saying that its not a factor you can control for -- you can't go to your artists and say something like "I need you guys to make sure you share more vertices" and expect anything other than an incredulous stare -- the sharing factor is essentially a function of the subject matter and the level of detail, assuming the artists (or model processing tools) aren't doing anything incompetent.

As a thought experiment, consider a simple cube: 12 faces, 8 vertices is 0.66... verts/face -- you can get a much lower ratio by adding a vertex to the center of each side of the cube: 24 faces, 14 vertices is 0.583... verts/face. So, you can achieve more sharing of vertices in a cube, but only by adding vertices which is bad. Going the other direction, if you take away even one vertex from the original cube, its no longer a cube. Thus the conclusion: more sharing is just more sharing, it does not produce an optimal model; 0.66... is the optimal sharing ratio for a cube -- other kinds of shapes have different optimal ratios.

My gut instinct tells me that for any given subject matter (person, machine, etc) and given level-of-detail (that is, the number of total vertices) the law-of-averages/scales-of-economy says that the model will converge towards the ratio that's optimal, given those parameters. Again, assuming that the artist/processing tools aren't incompetent.

As to the cache, yes you want to make effective use of it, but you need to understand how it works -- it doesn't remember every vertex that's been transformed anywhere in the model, its been a long time since I've investigated, but last I remember the cache was only 16 verts deep. You don't benefit from the cache at all if you come back to that vertex 17 indices later. While it is the case that more shared vertices are likely to exercise the cache more, that doesn't necessarily translate to better overall performance, because you might be sharing more vertices at the cost of simply having more vertices, each of which has to be processed at least once. That's why the general approach to optimizing a mesh is to reduce vertex count instead of trying to increase sharing.

Sure I'm fully aware of the limited size of the cache and you can also run methods in the code during asset loading/conversion that attempt to optimize the mesh in relation to cache hits. We currently miss the cache 10% of the time by the way which doesn't seems too bad.

Of course only concentrating on the vertex-per-face ratio would be stupid, I do not disagree with that! Well demonstrated in your example. However I don't see a problem in consider it as well. I fully understand that different kind of meshes will have different average. On the other hand if it turns out that all or many of the objects in a scene has unusually high ratio this would indicate that things can get optimized.

With this thread I was mainly interested in hearing peoples opinion in this matter. If people have rough ideas of where the numbers should be at, e.g. you usually have a budget for how many verts a main character should have and in case the real number diverts considerably from this the asset will get iterated. For example when I sorted our meshes according to ratio some high poly meshes had over 2.5, to me that number is way higher than what you should usually expect. From what I gathered from people replying so far, it has not been of their concern. If you're modeling in a way were the ratio is naturally pretty low that is not an issue of course