🎭 Preview Mask

Masks are the most invisible thing in ComfyUI. You generate one, feed it into an inpaint or a control pass, and only find out it was inverted, shifted, or full of holes when the result comes back wrong. The 🎭 Preview Mask node is the "show me what I'm actually working with" step: it composites your mask over your image and previews the result so you can see exactly what a sampler is going to see.

How it works

It's built on ComfyUI's SaveImage, so the preview lands in your temp directory and shows up in the normal image preview area - no custom viewer needed. You feed it an image and a mask, pick how you want the preview drawn, and it saves a composited PNG. It's an output node: no outputs, it exists purely to render something for your eyes.

The interesting input is Preview, a dropdown with nine modes:

• none - the image with the mask as alpha (transparent where the mask is).
• image - just the image, unmodified.
• mask - the mask itself, rendered as a grayscale image.
• Black / White / Gray / Red / Green / Blue - the image shown through the mask, with the unmasked area filled with the chosen solid color. Red is the one you'll reach for: mask-as-red-overlay is the classic "show me where this is going to act" view.

There's also invert_mask, whose tooltip says it plainly: "Invert mask values (0->1, 1->0)". It does the inversion before compositing, so you can check what the inverted interpretation of your mask looks like without changing anything upstream - very handy for diagnosing the classic inverted-alpha mishap.

Honest limitations

The node normalizes masks (scales 0–1, crops/pads to the image size), which is convenient, but it silently truncates a mask that's larger than the image rather than erroring. And the tooltip attached to invert_mask is the most useful documentation in the whole node - believe it, not any assumption about which side of a mask is "the part that gets edited," because that convention varies between workflows.

When you'd use it

Any time a mask matters: inpaint workflows, background-removal passes, ControlNet area conditioning, or before you run that expensive re-denoise and wish you'd checked the mask alignment first. It's a debugging node, and it's exactly the kind of cheap sanity check that saves you a wasted render.

Install

Part of Light-x02 Nodes - ComfyUI Manager → search "ComfyUI-Lightx02-Nodes", or:

cd ComfyUI/custom_nodes
git clone https://github.com/Light-x02/ComfyUI-Lightx02-Nodes

Restart ComfyUI, find it under 💡Lightx02/utilities. No extra dependencies beyond torch and torchvision, which ComfyUI already requires.