UltraFlux a High Quality VAE Decoder for Flux.1

All this is a newly trained VAE for Flux.1 which has been trained on high resolution 4k images. This means that when you generate large images of 4k resolution using Flux.1 this model VAE decoder will produce a higher quality results that the native VAE provided by Black Forrest Labs.

It’s details as provided by the creator are “UltraFlux is a Flux-based diffusion transformer for native 4K text-to-image generation with consistent quality across diverse aspect ratios (square, portrait, ultra-wide, etc.).
It couples a 4K-optimized DiT backbone with a curated MultiAspect-4K-1M dataset, VAE post-training, and 4K-aware objectives so that data, architecture, and loss are co-designed rather than tuned in isolation.“

All I have to say is that its definitely an improvement on the images produced by Flux.1 off the bat. This is without using any LoRA or detailers to begin with. The results are crisp and have lot of fine details when the Flux generated Latent is decoded using this VAE model.

You can download the VAE from their Hugging Face page

Place the download file in the ComfyUI\models\vae folder, give it a make like UltraFlux-VAE.safetensors so you know that this is the different file.

The VAE simply decodes the same latent image as generated by Sampler and decodes it using this custom VAE which produces a higher quality image that is sharper and has more details in it than the others. The workflow used in ComfyUI is the original workflow that was released for Flux.1 Dev and I have simply added an additional decoder node, loaded the custom vae and added the preview image.

To make the comparison I use the Image Compare node from Rgthree and also to animate the result I use my WWAA Before After Animator which is then combined using Video Combine node to save it as video.

Sample results as shown in the video above.

The image details are clear and the typical Flux softness is not there at all. All you have is a clean and crisp image to begin with, now if you want to take it further just add some detailer LoRA for Flux.1 and you are on your way to a winner!

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