Image Hash/deduplication. Scaling the image down, and converting to a predefined color palette will remove most scaling and compression artefacts, and will make images with similar content compareable/findable, independent of original scaling and file formats.

imgdedupe.php

<?php 
public function generateImageHash(bool $getRaw = false)
    {
        $imgHandle = new \Imagick();
        try {
            $imgHandle->readImageBlob($this->getData());

            $imgHandle = $imgHandle->coalesceImages();

            $imgHandle->setImageFormat("png");

            #$imgHandle->gaussianBlurImage(0.5, 1);

            $imgHandle->thumbnailImage(50, 50);
            #$imgHandle->blurImage(3, 3);
            $imgHandle->posterizeImage(3, false);
            foreach ($imgHandle->getImageProperties() as $key => $value) {
                $imgHandle->deleteImageProperty($key);
            }
            if ($getRaw) {
                return $imgHandle->getImageBlob();
            }
            $this->imghash = base64_encode(hex2bin(md5($imgHandle->getImageBlob())));
        } catch (\ImagickException $e) {
            echo "error creating imgHash.\n";
            echo '>>>>>' . $e->getMessage() . "\n";
            $this->imghash = $this->md5;
        }
        return $this->imghash;
    }