node.py

from nodes import PreviewImage
class SimpleCompare(PreviewImage):
    """Shows two images in one preview; click it to switch between them."""

    CATEGORY = "image"
    FUNCTION = "compare_images"

    RETURN_TYPES = ("IMAGE",)
    RETURN_NAMES = ("image",)
    OUTPUT_NODE = True

    @classmethod
    def INPUT_TYPES(cls):
        return {
            "required": {},
            "optional": {
                "image_a": ("IMAGE",),
                "image_b": ("IMAGE",),
            },
            "hidden": {
                "prompt": "PROMPT",
                "extra_pnginfo": "EXTRA_PNGINFO",
                "id": "UNIQUE_ID",
            },
        }

    @classmethod
    def IS_CHANGED(cls, **kwargs):
        return float("NaN")

    def compare_images(
        self,
        image_a=None,
        image_b=None,
        filename_prefix="comparer.",
        prompt=None,
        extra_pnginfo=None,
        id=None,
    ):
        ui_a = []
        ui_b = []

        if image_a is not None and len(image_a) > 0:
            ui_a = self.save_images(image_a, filename_prefix, prompt, extra_pnginfo)["ui"]["images"]

        if image_b is not None and len(image_b) > 0:
            ui_b = self.save_images(image_b, filename_prefix, prompt, extra_pnginfo)["ui"]["images"]

        output_image = image_a if image_a is not None else image_b

        return {
            "ui": {
                "a_images": ui_a,
                "b_images": ui_b,
            },
            "result": (output_image,),
        }