stepfun-ai / NextStep-1-f8ch16-Tokenizer

Improved Image Tokenizer

This is an improved image tokenizer of NextStep-1, featuring a fine-tuned decoder with a frozen encoder. The decoder refinement improves performance while preserving robust reconstruction quality. We recommend using this Image Tokenizer for optimal results with NextStep-1 models.

Usage

import torch
from PIL import Image
import numpy as np
import torchvision.transforms as transforms

from modeling_flux_vae import AutoencoderKL

device = "cuda"
dtype = torch.bfloat16

model_path = "/path/to/vae_dir"
vae = AutoencoderKL.from_pretrained(model_path).to(device=device, dtype=dtype)

pil2tensor = transforms.Compose(
    [
        transforms.ToTensor(),
        transforms.Normalize([0.5, 0.5, 0.5], [0.5, 0.5, 0.5]),
    ]
)

image = Image.open("/path/to/image.jpg")
pixel_values = pil2tensor(image).unsqueeze(0).to(device=device, dtype=dtype)

# encode
latents = vae.encode(pixel_values).latent_dist.sample()

# decode
sampled_images = vae.decode(latents).sample
sampled_images = sampled_images.detach().cpu().to(torch.float32)

def tensor_to_pil(tensor):
    image = tensor.detach().cpu().to(torch.float32)
    image = (image / 2 + 0.5).clamp(0, 1)
    image = image.mul(255).round().to(dtype=torch.uint8)
    image = image.permute(1, 2, 0).numpy()
    return Image.fromarray(image, mode="RGB")

rec_image = tensor_to_pil(sampled_images[0])
rec_image.save("/path/to/output.jpg")

Evaluation

Reconstruction Performance on ImageNet-1K 256×256

Robustness of NextStep-1-f8ch16-Tokenizer

Impact of Noise Perturbation on Image Tokenizer Performance. The top panel displays quantitative metrics (rFID↓, PSNR↑, and SSIM↑) versus noise intensity. The bottom panel presents qualitative reconstruction examples at noise standard deviations of 0.2 and 0.5.