Fast Cross-Platform C/C++ Image Processing Libraries

imagemagick is quite popular.

I don't think I've seen anything better in features and performance than HALCON from MVTec. It provides all sort computer vision and image processing algorithms out-of-the-box and plenty of real life examples. The library uses multithreading as much as algorithms could possibly allow and GPU when available. It's very cross-platform and provides a fantastic IDE that will allow you to export your prototype code (algorithm) to many languages including C, C++, C# and more.

One of the best features of this library is how they treat region objects. It is just incredibly smart and efficient both for storage and mask processing. Unfortunately OpenCV has a lot to learn from it.

The main problem with this package is the price (stupidly high) but if you are working on a project where you don't need to deploy runtime licenses (e.g. SaaS) then this is the way to go, look no further if you require serious image processing and computer vision.

Don't forget to look at CxImage - I've used it professionally in globally deployed graphics intensive mobile phone applications, where it performed perfectly and it's so full of features. Do check it out!

OpenCV has quite good performance. It should be sufficient for most cases.

To improve performance, you can also use OpenCV together with Intel IPP, which is however a non-free commercial product. If OpenCV detects that IPP is installed it will use it where possible.

As a third option you can use IPP directly. IPP was designed with high performance (on Intel architectures) as a goal. It is optimized to use the intel SIMD instructions.