Using the GD PHP Function ImageCopy

PHP’s GD library supports various image formats, including JPG and PNG. The GD libraries also support several different functions for manipulating images. These functions can be used to create a number of different effects, from creating a simple thumbnail to cropping an image, resizing it, and more. However, many of these functions use a very simplistic algorithm to perform their task. Specifically, they use the nearest-neighbor and bicubic resampling methods, which only account for the pixel values themselves and do not take into consideration any other information contained in the original image file. This information, particularly critical chunks such as the PLTE chunk, is often disregarded when an image is resized, and this can potentially leave room for attackers to smuggle in arbitrary PHP payloads into the resulting resized images.

The snippet below takes the rectangular portion of the image that is to be copied, and copies it into a new image at full size. The result of this is an image that has been resized, but that has still retained a great deal of clarity. The original image is also drawn using anti-aliasing, to further improve the quality of the resulting image.

While the resampling function used here is not perfect, it does produce a better quality result than Tim’s simpler code. The resampling method is used to create a smoother and more natural-looking image, and this is especially important when dealing with smaller images that will be displayed in a small size.