I have a real-time image processing DLL written in C++ based on OpenCV v3 and a C# program with GUI based on that dll. I want to use that dll in Windows 10 IOT core, is that possible? The DLL is using an IP camera to get frames and process them.
Short answer is Yes, but with some extra work.
You won't be able to use the C++ unmanaged dll directly with Windows IoT Core. You need to create some "wrapper" classes around your native C++ APIs, using pInvoke.
Microsoft has an official github repository fork of OpenCV, https://github.com/Microsoft/opencv, follow the tutorial here to use it with Windows IoT Core https://developer.microsoft.com/en-us/windows/iot/samples/opencv
Also, notice that EMGU CV is introducing support for UWP in the latest revision, however, you'll need to get the commercial license for that.