Preview Mask ♾️Mixlab

Masks are invisible until you look at them, and half of debugging a mask-heavy workflow is just seeing what a mask actually contains. Preview Mask does that one thing: wire a MASK into it and it renders it as a black-and-white image on the node so you can confirm the region is where you think it is. It's the mask counterpart to PreviewImage, and it's one of those nodes you don't think you need until a graph goes wrong and you're squinting trying to figure out which mask is empty.

Every mask pipeline benefits from a couple of these dropped in. Built a mask from a detector? Preview it. Inverted it? Preview it. Grew, blurred, combined, outlined it? Preview at each step and you'll catch the "oh, that's inverted" or "that's all black" mistakes in seconds instead of chasing a weird final result.

How it works

It takes the mask tensor and visualizes it - white where the mask is on, black where it's off - and displays it in the UI. It's a terminal output node, so it also saves the visualization to your ComfyUI output directory (its own description says as much: it saves the input to the output folder). No settings, no transformation. Inspect and move on.

Inputs and outputs

• mask (MASK, required) - the mask you want to look at. That's the only input.

No data outputs - it's an endpoint. The value is the on-node preview (and the saved image). If you need to keep using the mask, it's already flowing from wherever you tapped it; Preview Mask is a side branch for looking, not a pass-through in the chain.

How to install it

Ships with the Mixlab pack.

• ComfyUI Manager: search comfyui-mixlab-nodes, install, restart.
• Manual: cd ComfyUI/custom_nodes && git clone https://github.com/shadowcz007/comfyui-mixlab-nodes.git, install requirements (install.bat on Windows or ../../../python_embeded/python.exe -s -m pip install -r requirements.txt), restart.

Common issues

The preview is all black: that's not the node failing - that's your mask genuinely being empty. Which is exactly the bug this node exists to catch. Trace back: a detector that missed, a threshold that zeroed everything out, an invert that flipped a full mask to nothing.

The preview is all white: the whole frame is masked. Often a mistaken invert, or a "select everything" default. Again, the node did its job by showing you.

It's just a preview: every pack ships a version of this (PreviewImage, mask previews in RMBG, Impact, etc.), so there's nothing special about Mixlab's beyond it being here. Use whichever you've already got loaded - no reason to install the whole Mixlab pack just for this if you have another mask preview handy. If you're already in Mixlab for the app/real-time stuff, though, it's right there.