Image Fallback

Image Fallback is the boring, useful cousin of Image Exists: pass the input image through, and if nothing is connected, output a black image at the size you specify. That's the entire job, and it's the right tool for a genuinely annoying class of ComfyUI problem - nodes that require an IMAGE input but only need it conditionally. Instead of fighting the graph editor over whether an input is mandatory, you stick Image Fallback in front and it guarantees a valid tensor flows through.

Think of it as a default value for an image slot. Where Image Exists gives you a boolean so you can branch on presence, Image Fallback gives you a stand-in so you don't have to branch at all. Same family, different philosophy.

How it works

At run time it checks whether the image input was connected. Connected → it's a straight pass-through, untouched. Not connected → it returns a single black frame of height × width pixels. Unlike some fallbacks that output a degenerate 1x1 tensor and let the shape error propagate downstream, this one builds a real, full-size image, so anything downstream - a latent encode, a control-net apply, a concat - sees a normal image batch and behaves normally.

The width/height pair is what makes it useful for iterative video: the black frame can match your canvas exactly, which matters when the fallback feeds a VAE or a control processor that's sensitive to dimensions. Default is 512×512; a black frame isn't a "no signal" - it's a "here's a blank canvas of the right size."

The inputs and outputs

• image (IMAGE, optional) - the real image to pass through when present.
• width (default 512) and height (default 512) - the size of the generated fallback frame, 1 to 16384.

The single output, image, is either your input or the black fallback. Wire it to whatever needed a guaranteed IMAGE.

Installing it

Part of the Mickmumpitz pack. ComfyUI Manager → search "Mickmumpitz" → install → restart:

cd ComfyUI/custom_nodes
git clone https://github.com/mickmumpitz/ComfyUI-Mickmumpitz-Nodes.git

No models, no extra deps.

Common issues

Set the fallback size to something plausible for your pipeline. A 512×512 black frame into a workflow that expects 1280×736 (the pack's own LTX presets, say) will get resized or rejected depending on the next node - it doesn't auto-match. And remember the semantics: a fallback means "valid but blank," so if your downstream logic treats blank images as real content (or feeds them into a sampler expecting detail), you'll get artifacts. If you need to know whether the fallback kicked in, reach for Image Exists instead - this node deliberately doesn't tell you.