Q:

Why do we need merge geometries ourselves?

I'm finally understanding the performance impact of merged geometries. I've noticed the GPU has draw calls for all shared geometries combined with materials (or perhaps its just the material count). My question is why are developers forced to figure out how to merge geometries when its clear that the three.js will just divide everything out of your merged geometry anyways?
I'm probably missing something.

A:

It is difficult to make general statements about WebGLRenderer performance, because each use case is different.
But typically, performance will improve if the number of draw calls is reduced. If you have multiple meshes, and they all share the same material, then by merging the mesh geometries into a single geometry, you reduce the number of draw calls to one.
Remember, this approach makes sense only if your multiple meshes are static relative to each other, because once merged, they cannot be transformed independently.
And yes, it is true, that if your mesh has multiple materials, the the renderer will break the geometry up into smaller geometries -- one for each material -- prior to rendering.
Here is a useful tip: In the console, type renderer.info and inspect the resulting object returned. This will give you insight into what is going on behind the scenes.
You can also display renderer.info live on your screen by using this utility.
three.js r.68

