FFT Add

FFT Add is a two-wire glue node: take two FREQUENCY inputs, add them elementwise, hand back one FREQUENCY. That's the whole job. It looks like nothing, but it's what makes this pack composable - it's the node you reach for when you've split a spectrum apart, edited the pieces, and now want to put them back together.

The most common shape is the low/high split. FFTCrossover hands you a low_pass and a high_pass spectrum that are exact complements - add them back together and you recover the original, bit for bit. That's the point: you can do frequency separation (soften the low band, sharpen the high band, whatever) and then use FFTAdd to recombine the edited halves into a final image. It's the frequency-domain version of "work on separate layers, then merge," which is exactly how the retouching crowd thinks about it.

How it works

Elementwise addition of two complex spectra. Because both inputs follow the pack's DC-at-center convention, alignment is automatic. If the two spectra have different heights or widths, the second is center-cropped or zero-padded to match the first rather than raising an error - the same lenient sizing rule as the other math nodes. So you can add a mask-sized spectrum to a full image spectrum without thinking about dimensions.

Inputs are frequency_a and frequency_b; the output is frequency. There's nothing to tune, which is either refreshing or a reminder that the interesting work happens in the filter nodes upstream.

Where it bites

Not much to trip over here, honestly. The one thing worth knowing: addition is not the inverse of everything. If you multiplied a spectrum by something and want to undo it, that's FFTDivide, not this node. And because addition preserves whatever's already in the spectrum, adding two unfiltered spectra just doubles the signal - there's no magic. If your recombined image looks brighter than expected, remember you added two energy sources and the IFFT's clip at [0, 1] will quietly crush anything over white.

How to install it

Same pack, same story. ComfyUI Manager → search ComfyUI-FFT-2D → install, or:

cd ComfyUI/custom_nodes
git clone https://github.com/NobutakaKuroki/ComfyUI-FFT-2D

Restart ComfyUI and you'll find it under fft alongside its siblings FFTSubtract, FFTMultiply, and FFTDivide. No pip extras, no models to fetch - just numpy under the hood. It's part of Dr. Nobutaka Kuroki's educational pack, and the lesson_workflows/4_LowHighSplit.json file shows the split-and-recombine pattern this node exists to serve.