Methods and measurement notes

Why one-step error and not video PSNR?

H3's sampler is chaotic: computing only the AdaLN in fp32 instead of bf16 already moves a 20-step clip to ~17 dB PSNR against the shipped model. End-of-run pixel metrics therefore cannot rank recipes. Each recipe is instead fed the exact recorded inputs of every step and compared with the BF16 output of that step.

Measurement notes

Noise floor ~1–2%. Replays are bitwise exact within one container but differ by 1–2% across GPU hosts, and any bf16-level change lands at that level. Compare recipes within a run; each run reports its own floor.
Size is the approximate transformer checkpoint size with the rank-12 AdaLN (NVFP4 counted as 4.5 bits per weight including block scales).
Simulation vs ComfyUI. NVFP4 activation rounding is implementation-defined: ComfyUI's CUDA and Triton kernels agree with each other but differ from ModelOpt's simulation by ~2% on a quantized activation. Rankings should carry over; final quality is judged in ComfyUI itself (below).