The most common alternative to libgd is ImageMagick but I don’t consider it to be an option anymore as the library has been plagued with security vulnerabilities in recent years. The patch was never picked up by libgd, however. This would should have reduced the impact of the issue but at a possibly hefty performance cost. The resulting color values may vary greatly depending on the processor’s handling of high-precision floating numbers.Ī patch was suggested in 2010 that changed the behavior from cast to rounding the number. The core issue seems to be an optimistic cast from floating numbers representing the red-green-blue ( RGB) colors to an integer. This is after both images being compressed with the state-of-the-art lossless Zopfli compression method. The two downscaled example images above take up almost twice the number of bytes compared to the original despite having 46 % fewer pixels. The complex patterns, despite being repeating, don’t compress well.

The second image has a more complex background and shows a more varied noise pattern. You can see the noise patterns shift in the different encoding tiles. The first image has a solid background that should be the same color for most of the image. Click the thumbnails for larger versions. The rightmost image highlights the differences between the original and downscaled images. The below image groups demonstrates an example of the type of noise-artifact patterns that can be added to downscaled images when resizing with PHP+libgd. The artifacts introduced additional complexity in the images which would yield poor compression. On many photos it was almost impossible to see the effect, but on some simple illustrations or images with slight gradients it would be very obvious.

GD introduces some complex repeating and alternating patterns on downscaled images. I finally found a good solution that doesn’t involve switching to ImageMagick. The resulting images haven’t always been pretty and they’ve ended up larger than the source images. Until now, I’ve been using WordPress with the PHP GD graphics library (libgd) to downscale images for thumbnails and responsive images.

Wordpress plugin resize images