Select Nth Mask (Inspire)

Plenty of nodes hand you a batch of masks when you really wanted one of them. A detector finds several regions; Inspire's Color Map To Masks pulls out the top N colors as separate masks; a segmentation step returns a stack. Select Nth Mask is the little scalpel that grabs a single mask out of that batch by its index. Nothing clever - but exactly the node you go hunting for the moment a mask input refuses your batch and you just need mask number two.

How it works

A mask batch in ComfyUI is several masks stacked together. This node takes that stack, an index, and returns the one mask at that position. Zero-based, so index 0 is the first mask, 1 the second, and so on. Whatever produced the batch usually has some order to it - Color Map To Masks, for instance, extracts colors by pixel count, so index 0 tends to be the largest region - and Select Nth Mask lets you reach into that ordering and pick.

From there the single mask goes wherever a MASK input wants it: an inpaint node, a Set Area conditioning, Regional CFG, a detailer. It's the bridge between "a node gave me a batch" and "the next node wants one."

The inputs and outputs

• masks (MASK) - the mask batch to index into.
• idx (INT, default 0) - which one to grab, zero-based.
• Output is a single MASK.

That's the whole node. Set the index, get the mask.

How to install it

ComfyUI Manager: Install Custom Nodes → search ComfyUI Inspire Pack → install → restart. Or:

cd ComfyUI/custom_nodes
git clone https://github.com/ltdrdata/ComfyUI-Inspire-Pack

Then restart. Inspire Pack is Dr.Lt.Data's - the ComfyUI Manager and Impact Pack author - so it's dependable and installs with no model downloads.

Common issues

The one real footgun is the index: it's zero-based, and asking for an index past the end of the batch is an error, not a silent empty mask. So if your batch has three masks, valid indices are 0, 1, 2 - request 3 and it'll complain. When the batch size varies run to run (a detector that finds a different number of regions each time), a hardcoded index can fall off the end; either clamp it or drive it from a counter that respects the batch size. Second, don't assume the ordering is what you'd eyeball - it's whatever the upstream node decided (often by region size or color), so if you're grabbing "the wrong" mask, the fix is understanding that source node's order, not this one.