Can frequent state changes decrease rendering performance?

Question

Can frequent texture and shader binding decrease rendering performance?

"Frequent" binding example:

for object
   for material in object
      render part of object using that material

"Low count" binding example:

for material
   for object in material
      render part of object using that material

I'm planning to use an octree later and with this "low count" method of rendering it can drastically increase memory consumption. So is it good idea?

Answer 1

State changes are, relatively speaking, an expensive operation and texture and shader state changes in particular can have the largest overhead. So yes, if you structure your render loop in such a way as to constantly change the shader and textures bound to the pipeline, that will have a negative impact on render performance. That impact may not be noticeable depending on the overall number of binds, however, it's still good to be aware of it.

Your second example will result in fewer state changes than your first example, and it is generally the way you want to go, especially for solid objects: essentially, sort what your going to render by shader and texture so that you can bind the appropriate combination and then render several objects using those resources. Transparent objects also need additional depth-based sorting or require you to use an order-independent transparency technique such as depth peeling.

An octree can be a useful optimization but it is intended as a means to cull out entire objects that are not visible or otherwise contributing to the scene. You should only have one octree, maintaining an octree-per-material is wasteful and doesn't provide you any real benefits.

Answer 2

Are your examples the right way round? If you sort by material, you bind once per material. If you sort by object, you might end up binding [numObjects * numMaterials] times.

To avoid those subloops, forget about the objects. You're not really rendering objects, you're rendering parts, so when you instantiate objects, add their parts to a render list. Then your two example loops would look like this:

for material
    bind material & render all parts

and

for part
    bind material & render

You would probably have more parts than materials, so you can see the first option would result in fewer bind calls.