Flex Mask Math ⚡🅡🅞🅣🅘

Sometimes one mask isn't enough - you've got a subject mask from segmentation and a hand-painted region you want to intersect it with, or two reactive masks from different sources you want combined into one. Flex Mask Math is the node for that: it takes two masks and combines them with a plain mathematical operation, then runs the result through the same reactive post-processing every other Flex mask node in the pack shares.

How it works

combination_method picks the operation: add brightens the overlap (unions get brighter, up to full white), subtract removes mask_b's coverage from masks, multiply keeps only where both masks agree (a true intersection - anywhere either mask is black, the result is black), minimum takes the darker value at each pixel, and maximum takes the lighter one (effectively a union, similar to add but without values stacking past white). max_blend caps how much of the combination actually comes through - at 0 you'd get back close to the original masks input regardless of the operation, at 1 the full combined result. feature_param can target max_blend, so the strength of the combination itself can pulse with an audio or motion signal rather than sitting fixed.

Inputs and outputs that matter

• masks / mask_b (required, MASK) - the two masks being combined.
• combination_method - add, subtract, multiply, minimum, or maximum.
• max_blend (default 1, 0–1) - how much of the combined result comes through.
• opt_feature (optional, FEATURE) - make the blend strength reactive.
• Output - a single MASK.

Installing it

Via ComfyUI Manager: search "RyanOnTheInside," install, restart. Manually:

cd ComfyUI/custom_nodes
git clone https://github.com/ryanontheinside/ComfyUI_RyanOnTheInside.git
cd ComfyUI_RyanOnTheInside
pip install -r requirements.txt

then restart. No model downloads - it's per-pixel arithmetic between two masks you already have.

Common issues & troubleshooting

Result doesn't match the mask size you expected. Both masks and mask_b need to be the same resolution and frame count - a mismatch here behaves like any other dual-mask operation in ComfyUI and will either error or produce a broadcast result you didn't intend. Resize upstream first.

multiply gives you an almost-black mask. That's expected if the two masks don't overlap much - multiply is a true intersection, so two masks covering different parts of the frame will multiply down to close to nothing. If you actually wanted a union (either region counted), use maximum or add instead.

Picked the wrong operation for the goal. It's easy to reach for subtract when you actually want multiply, or vice versa - subtract removes one mask's coverage from the other regardless of overlap, while multiply only keeps the shared region. If the result looks inverted from what you expected, double-check which of those two you actually needed.