ComparisonGate

Overview

ComparisonGate keeps comparison viewers tidy by only emitting images when a complete pair is available. It listens to a designated source_image input together with up to three comparison sockets and mirrors ComfyUI's unplugged behaviour (None, None) until two populated tensors are ready to forward.

Inputs

source_image (IMAGE, optional): Preferred reference image for the secondary output. Leave disconnected when you want the first populated comparison to feed both sockets.
image_a (IMAGE, optional): Primary comparison feed. Becomes the final_image output whenever populated.
image_b (IMAGE, optional): Secondary comparison feed used either as a fallback final_image or to backfill the source_image output when the dedicated source socket is empty.
image_c (IMAGE, optional): Tertiary comparison feed that participates in the fallback cascade.

All inputs accept None; the node filters out empty lists/tuples as well as bare None values so placeholder widgets will not trigger accidental forwards.

Outputs

final_image (IMAGE): The first populated comparison input (image_a › image_b › image_c). Returns None until at least one comparison produces data.
source_image (IMAGE): Mirrors the dedicated source_image socket, or falls back to the next populated comparison input so the viewer always receives a pair. Returns None alongside final_image when fewer than two valid images are present.

Forwarding Rules

The node inspects every input on each execution and counts how many look like real image tensors. If fewer than two are found it emits (None, None) to keep downstream nodes blank.
When only comparison sockets are populated, the first comparison feeds final_image and the second feeds source_image, ensuring A/B layouts remain synchronised.
When source_image is connected, it always feeds the source_image output unless it is empty—in that case the node automatically falls back to the second non-empty comparison input.
Containers such as one-item tuples or lists are unwrapped only when they contain real data; sequences of None values are ignored.

Tips

Place the gate directly before two-input comparison viewers to prevent half-populated renders during workflow warm-up.
Combine with PairedImageLoader for workflow QA: let the loader drive comparisons and feed its outputs through the gate so the viewer only refreshes when both sockets advance.
Because the node simply relays its inputs, it adds no meaningful processing cost; you can safely sprinkle it anywhere you need to guarantee paired images.*** End Patch

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