RemapReverseBarrelDistortion

When you want to remove lens distortion from a real photo, there are two ways to go about it. RemapBarrelDistortion has a use_inverse_variant toggle that uses a reciprocal formula - quick and good enough for a lot of cases, but mathematically it's a different distortion than the true inverse. This node does it the rigorous way: it takes the same a/b/c/d polynomial model and, for every pixel, actually solves the polynomial backward to find the radius that maps to the target. Where the variant is a shortcut, this is the real undo.

The cost of that rigor is speed. Per-pixel root-finding on a cubic polynomial is expensive, and the source code shows the author reaching for joblib's parallel execution and numpy polynomial root solvers to make it tolerable. On a large image it's the slowest node in the Remap family, full stop. You use it when the shortcut's approximation is visibly not good enough.

It's a REMAP generator like all its siblings: RemapReverseBarrelDistortion → Remap → image.

How it works

Same lens model as its sibling - radius through the cubic r' = (a·r³ + b·r² + c·r + d) · r - but now you start from the distorted radius and need to find which original radius, when passed through the model, produces it. That means inverting a cubic, which has no clean closed form, so the node approximates the inverse function per pixel and then locates the correct root within a sane range (the code filters roots to real values in a plausible radius band and ignores the rest). The use_inverse_variant toggle behaves a bit differently here - when on, it approximates the inverse of the inverse-variant formula, because that polynomial doesn't have a direct solvable form either.

The inputs

• a, b, c (FLOAT, −10 to 10) - the polynomial coefficients describing the distortion you're undoing.
• d (FLOAT, optional) - derived as 1 - (a+b+c) if not connected.
• use_inverse_variant (BOOLEAN, default on).

Output: REMAP.

When it's the right tool

Correcting an actual fisheye or wide-angle photo where the barrel is strong enough that the shortcut leaves residual bowing at the edges - that's this node's lane. The workflow mirror of barrel: dial in coefficients that describe the lens's distortion, then the reverse node untwists it. Start small (a around 0.001–0.005), preview through the Remap node, and expect each tweak to take a moment on big images.

Installing it

cd ComfyUI/custom_nodes
git clone https://github.com/bmad4ever/comfyui_bmad_nodes

restart, or grab comfyui_bmad_nodes in ComfyUI Manager. OpenCV plus the pack's requirements; joblib arrives with scikit-image, so the parallel path comes along. No model downloads.

The honest bottom line: for mild distortion the sibling's use_inverse_variant is indistinguishable and much faster. This one earns its keep on strong, real-world lens warp where you can see the shortcut's approximation breaking down. If your image is mostly fine and you're just fussing over a subtle bow, don't pay the speed tax.