AC_Super_PreviewMask

AC_Super_PreviewMask turns a mask into something you can actually look at. Masks are single-channel tensors of ones and zeros, and ComfyUI's preview widget doesn't show them the way it shows images. This node renders your mask as a grayscale PNG, displays it in the UI, and saves it to your output folder. If you've ever wired up an inpaint or detail pass and had to trust that your mask was right, this is the node that lets you check.

How it works

Under the hood it reshapes the single-channel mask into a three-channel image, expands it so every channel carries the mask value, and saves it as a PNG with the standard ComfyUI metadata (the workflow gets embedded, so you can drag the file back and see which mask made it). The filename_prefix is a string defaulting to ComfyUI, and like the core Save nodes it supports formatting tokens such as %date:yyyy-MM-dd% in the prefix.

The inputs

• mask - the MASK to visualize. The author's own tooltip describes it as "the images to save," which is a small hint that this node treats the mask as pixels.
• filename_prefix - the filename prefix; supports the same date/value formatting tokens as the save nodes.

There are no outputs - it's an output node, the end of a branch. That's the point: you drop it on a mask wire purely to inspect the mask at that point in the graph.

Installing it

Part of ComfyUI_AC_FUNV8Beta1 by Cc啊程 (Bilibili ComfyUI teacher; README is Chinese). ComfyUI Manager (search "AC_FUN") or:

cd ComfyUI/custom_nodes
git clone https://github.com/A719689614/ComfyUI_AC_FUNV8Beta1

Restart ComfyUI. No downloads.

Where people get burned

Two things catch people. First, this node saves - unlike AC_Super_PreviewImage, it writes real PNGs to your ComfyUI/output folder, so leaving it in a workflow while iterating will pile up mask files. Rename the prefix to something meaningful or remove it when you're done. Second, a grayscale mask shows you where it covers but not the size mismatch problem: if the mask's resolution doesn't match the image it's meant to affect, it will still preview fine and then misalign downstream. Use this node to confirm coverage, and pair it with AC_Super_MaskScale when the sizes don't line up.