3D graphics eBook - Course Materials Repository

Level of detail 73
A discrete LOD example
As a simple example, consider the following sphere. A discrete LOD approach would cache a certain number of
models to be used at different distances. Because the model can trivially be procedurally generated by its
mathematical formulation, using a different amount of sample points distributed on the surface is sufficient to
generate the various models required. This pass is not a LOD-ing algorithm.
Image
Visual impact comparisons and measurements
Vertices ~5500 ~2880 ~1580 ~670 140
Notes Maximum
detail,
for closeups.
Minimum
detail,
very far objects.
To simulate a realistic transform bound scenario, we'll use an ad-hoc written application. We'll make sure we're not
CPU bound by using simple algorithms and minimum fragment operations. Each frame, the program will compute
each sphere's distance and choose a model from a pool according to this information. To easily show the concept, the
distance at which each model is used is hard coded in the source. A more involved method would compute adequate
models according to the usage distance chosen.
We use OpenGL for rendering because its high efficiency in managing small batches, storing each model in a display
list thus avoiding communication overheads. Additional vertex load is given by applying two directional light
sources ideally located infinitely far away.
The following table compares the performance of LoD aware rendering and a full detail (brute force) method.
Rendered
images
Visual impact comparisons and measurements
Brute DLOD Comparison
Render time 27.27 ms 1.29 ms 21 × reduction
Scene
vertices
(thousands)
2328.48 109.44 21 × reduction