Determining Image Similarity with PHP's GD Extension

Comparing images by their MD5 hash is a simple and efficient way to determine their equality. However, there may be situations where you want to compare images visually instead. PHP's GD extension provides a range of tools for image manipulation, including the ability to calculate the difference between two images.

Calculating Image Differences

To calculate the difference between two images, use the following steps:

1. Load both images using the imagecreatefrompng(), imagecreatefromjpeg(), or other appropriate functions.
2. Use the imagecompare(). It takes two images and returns the value of the difference as a percentage. A value close to 0 indicates high similarity, while a larger value indicates greater difference.
3. Optionally, use the imagesavealpha() function to preserve transparency if needed.

Creating an Onion Skin Effect

To create an onion skin effect by overlaying two images with 50% transparency, follow these steps:

1. Load both images as described above.
2. Create a new image with the same dimensions as the originals.
3. Copy the first image onto the new image, with a transparency of 50% using imagecopymerge().
4. Copy the second image onto the new image, again with 50% transparency, positioned over the first image.
5. Save the new image using imagepng(), imagejpeg(), or another suitable function.

Note: The imagecompare() function compares the visual content of images, not their file metadata. This provides a more precise comparison than hashing, particularly for images with minor visual differences that may be imperceptible to the human eye.