UltraPixel Load

If your first instinct looking at seven model dropdowns is "what did I sign up for," relax. UltraPixel Load is the setup node for the UltraPixel pack, and its whole job is to assemble one research model that writes images at 2K–4K natively. It is not a checkpoint in the sense you already know - you won't find it on a normal Load Checkpoint menu - because UltraPixel doesn't squeeze through a VAE into latent space the way SD or Flux do. It's a pixel-space generator built on Stable Cascade's three-stage Würstchen backbone, and Load is how you gather every piece of that stack before the actual generation.

What it actually does

This pack is a straight wrapper around the original UltraPixel research code (arXiv 2407.02158), and the author, 2kpr (u/gto2kpr on Reddit), says as much in the README: it's a "modified wrapper," with a proper native ComfyUI rewrite planned once life lets him get to it. So think of Load as the research pipeline's front door, not a polished native node. It works - the July 2024 announcement on r/StableDiffusion showed off 2K/4K images that were explicitly not upscales - but it's held together with a little duct tape.

When you queue it, Load's main event is downloading. Seven files from two Hugging Face repos: ultrapixel_t2i.safetensors (the trained UltraPixel weights) from 2kpr/UltraPixel, and the other six - stage_a, stage_b_lite_bf16, stage_c_bf16, effnet_encoder, previewer, and a canny controlnet - from stabilityai/stable-cascade. Everything lands in ComfyUI/models/ultrapixel by default. First run is roughly 10 GB of downloading before anything generates, so don't mistake it for a hang.

The inputs that matter

The seven dropdowns are pre-filled with the exact filenames the pack expects, and the node scans your models/ultrapixel folder, so any other .safetensors you drop there shows up as a swap option. In practice you won't touch them. The two you might actually set are the STRING fields:

• ultrapixel_directory - where the UltraPixel t2i weights go. Default "default" means ComfyUI/models/ultrapixel.
• stablecascade_directory - same idea, for the Stable Cascade files. Useful if you already have a Cascade install elsewhere and don't want a second copy.

One footgun: if you type a path that doesn't exist, the node silently falls back to the default directory and prints a console note. No error, just a quietly ignored setting, so double-check your path.

The single output is model (type ULTRAPIXELMODEL), and it plugs into exactly one place: UltraPixel Process's model input. The whole graph is Load → Process → your preview/save node.

Installing it

cd ComfyUI/custom_nodes
git clone https://github.com/2kpr/ComfyUI-UltraPixel
pip install -r requirements.txt

ComfyUI Manager can grab it too - search "UltraPixel" in the install screen. Fair warning: that requirements.txt is a research repo's idea of a dependency list - insightface, onnxruntime, kornia, webdataset, plus two packages installed straight from git URLs. insightface in particular is known to demand build tools on Windows. If the install fails, that file is the first suspect.

Troubleshooting

The one historical landmine is the stage_b_lite_bf16.safetensors download, which shipped broken and was fixed in a specific commit. If your first run stalls halfway through the downloads, update the pack to the latest commit and try again. And remember the VRAM envelope this pack is built around: roughly 10 GB cards handle 2048×2048, 12 GB cards 3072×3072, 16 GB cards 4096×4096. Load itself is cheap - Process is where that budget gets spent.