Sol-H3-VAE

A fast video decoder for MiniMax-H3. It replaces the 36-layer H3 VAE decoder with a 24-layer noncausal decoder (1.62B parameters) distilled from it: whole frames with windowed attention instead of spatial tiles, and each native 5-latent block decoded bidirectionally. It takes the same normalized latents and returns the same frames as the H3 VAE decoder. The audio decoder is unchanged.

It is the video decoder of one stage in Sol-H3 in ComfyUI.

One 5-second clip	H3 VAE	Sol-H3-VAE
768p, RTX 5090	12.5 s	5.0 s
720p, GB200	6.16 s	0.34 s
2K, GB200	14.5 s	0.89 s

Against the H3 VAE decoder, PSNR is 45.4 dB on held-out latents (720p to 2K) and 38.2 dB on a generated 768p clip. Timings on GB200 use a captured CUDA graph; the RTX 5090 timing is eager, as Sol-H3 runs it.

Usage

Requirements: PyTorch with CUDA, Triton, Diffusers with MiniMax-H3 support, FlashAttention-4 (flash_attn.cute) and QuACK. Tested on RTX 5090 with PyTorch 2.12 (CUDA 13), Triton 3.7, Diffusers 0.40, FlashAttention-4 4.0.0b19 and QuACK 0.6.3.

import sys
from huggingface_hub import hf_hub_download, snapshot_download

repo = snapshot_download("Efficient-Large-Model/Sol-H3-VAE")
sys.path.insert(0, f"{repo}/code")
from spark_decoder import SparkFastH3VAEDecoder

vae_config = hf_hub_download("MiniMaxAI/MiniMax-H3", "vae/config.json")
decoder = SparkFastH3VAEDecoder(vae_config, f"{repo}/fast_h3_vae_noncausal_24l.safetensors")

# z: normalized H3 video latents, (1, T, 24, H/16, W/16); T = 0 starts a native 5-latent block
pixels = decoder.decode(z)          # (1, 3, F, H, W), the H3 VAE decoder's normalized pixel space
rgb = decoder.decode_rgb_uint8(z)   # (F, H, W, 3) uint8 on the GPU

The decoder needs only the H3 VAE config.json, not the H3 VAE weights. The first call for each latent shape compiles kernels and captures a CUDA graph (about 25 s on RTX 5090); later calls reuse it. Pass engine="ref" for the plain PyTorch decoder (slower).

Limitations

• Noncausal: it decodes a whole clip at once, so it cannot stream latent by latent.
• Memory grows with clip length: about 24 GiB at 720p and 48 GiB at 2K for a 5-second clip with the CUDA graph. Sol-H3 runs the eager engine and moves the weights to host RAM between requests to fit a 32 GB card.
• It is distilled, so it is close to the H3 VAE decoder but not identical.

License

The code is released under Apache-2.0; code/_spark_flash_fwd_guarded.py is adapted from FlashAttention (BSD-3-Clause). The weights are distilled from the MiniMax-H3 VAE and are subject to the MiniMax-H3 license.