Cost sweep

Where a frame goes, measured on the device it runs on. The sweep switches one thing off at a time (a group of meshes, the render targets, the shadows), or turns a resolution lever (hardware scaling flat; in a headset it puts the viewport scale and the foveation back to full, so the row is what each is saving), holds it for a couple of seconds, and records the frame time. What comes back is a ranked list: the things whose absence bought the most.

It exists because a headset has no GPU timer and no profiler, and guesses about its fill cost made on a desktop are wrong in both directions. It is a button in the Perf Stats panel (Cost sweep), so it runs inside the session.

const rows = await document.querySelector('tosi-b3d').costSweep()
// [{ name: 'baseline', ms: 71.4, p95: 83.3, saved: 0 }, { name: '- water', ms: 55.6, saved: 15.8 }, …]

Reading it

• ms is the median interval between frames; saved is the baseline minus that.
• A headset's frame time moves in steps. The compositor shows a frame on a refresh or the one after, so at 72 Hz the readings are 13.9, 27.8, 41.7, 55.6 ms and so on. A saving smaller than a step can read as zero, and two small savings can add up to a step that neither shows alone.
• - everything is the floor: what the session costs with nothing drawn. If it is close to the baseline, the scene is not the problem.
• baseline again is the first measurement repeated at the end. If it differs from the first, the device drifted during the sweep (heat, a background task) and the rows are that much in doubt: run it again.
• Groups are meshes sharing a material, named after it. The sweep takes the twelve that could cover most of the view and lumps the rest as other.

Each experiment is restored before the next. A sweep changes nothing for good.

Shader sweep, the button beside it, measures the ground shader's other forms (costSweep(2, 'shaders')). It is separate because each row recompiles every ground and decoration shader twice, which is the heaviest thing either sweep does.

If it hangs the device

The first run on a Quest did. The sweep writes what it has measured, and the name of the experiment it is about to start, to localStorage before each one, once as it switches the experiment on and once as it starts measuring. Reload the page and Perf Stats shows the rows it got and STOPPED with the experiment that never finished. Runs are numbered and the last four are kept: a run that hangs at once does not replace the rows of the one before it. The two headset levers run last for that reason: they are the part no emulator can exercise.