In my previous post I tried to break an OBJ down into a few basic sections:
- Global properties of the OBJ.
- Raw Mesh Data
- Commands, which in turn set per-batch state and then draw the batches.
The performance cost of an OBJ feature often has a lot to do with where in the OBJ the command shows up, e.g. is it global or per batch.
Global properties tend to affect OBJ performance on a one-time basis. For example, if you use cockpit regions, you pay a fairly large penalty for having the panel texture be set up even if you only apply that panel texture to a single texture. Sure enough, COCKPIT_REGION is in the global properties section of an OBJ.
Per-batch properties affect the OBJ in two ways:
- Every command you see in the commands section is going to involve some CPU intervention. A very long commands section is more work for an OBJ.
- Every time there are attributes between TRIS commands, it defines a new “batch” – that is, a separate instruction to the graphics card to draw a new and distinct setup. Think of this as shutting down the factory to reconfigure the assembly line.
Generally batch count is more important than total commands. In other words, in evaluating this:
TRIS 0 300
ATTR_light_level 0 1 some_dataref
TRIS 300 12
the fact that there are two attributes is less interesting than the fact that there are two batches (the two TRIS commands run with different state). Even if you got rid of the no-blend attribute, you’d still have two batches because of the light-level change.
The most powerful aspect of the OBJ format is bulk data handling – that is, you have to add a huge number of triangles before the number of triangles becomes a performance problem.
For this reason, you should never use an attribute to reduce geometry count. A few examples:
- Don’t use ATTR_no_cull to reduce triangle count – simply issue the indices of the triangle twice.
- Don’t use ATTR_flat_shade to reduce vertex count – simply use more vertices with correct per-vertex normals to simulate flat shading.
- Prefer texturing to materials whenever possible.
Finally a note on weighting: for airplanes, where the total number of objects is low (a few dozen) global object properties often matter most. For example, on an airplane, choosing to use huge panel regions, or huge textures can make a big difference in performance. By comparison, batches aren’t that expensive unless you do something really crazy.
By comparison, for scenery, batches matter more; X-Plane will share the global properties of objects across hundreds or thousands of objects, but each batch hurts framerate. So when making autogen-style scenery, batches are most important.