OpenCV (JavaCV) vs OpenCV (C/C++ interfaces)

Question

I am just wondering whether there would be a significant speed performance advantage relatively on a given set of machines when using JavaCV as opposed to the C/C++ implementation of OpenCV.

Please correct me if I am wrong, but my understanding is that the c/c++ implementation of opencv is closer to the machine where as the Java implementation of OpenCV, JavaC, would have a slight speed performance disadvantage (in milliseconds) as there would be a virtual machine converting your source code to bytecode which then gets converted to machine code. Whereas, with c/c++, it gets converted straight to machine code and thus doesn't carry that intermediary step of the virtual machine overhead.

Please don't kill me here if I made mistakes; I am just learning and would welcome constructive criticism.

Thank you

Answer 1

I'd like to add a couple of things to @ejbs's answer.

First of all, you concerned 2 separate issues:

1. Java vs. C++ performance
2. OpenCV vs JavaCV

Java vs. C++ performance is a long, long story. On one hand, C++ programs are compiled to a highly optimized native code. They start quickly and run fast all the time without pausing for garbage collection or other VM duties (as Java do). On other hand, once compiled, program in C++ can't change, no matter on what machine they are run, while Java bytecode is compiled "just-in-time" and is always optimized for processor architecture they run on. In modern world, with so many different devices (and processor architectures) this may be really significant. Moreover, some JVMs (e.g. Oracle Hotspot) can optimize even the code that is already compiled to native code! VM collect data about program execution and from time to time tries to rewrite code in such a way that it is optimized for this specific execution. So in such complicated circumstances the only real way to compare performance of implementations in different programming languages is to just run them and see the result.

OpenCV vs. JavaCV is another story. First you need to understand stack of technologies behind these libraries.

OpenCV was originally created in 1999 in Intel research labs and was written in C. Since that time, it changed the maintainer several times, became open source and reached 3rd version (upcoming release). At the moment, core of the library is written in C++ with popular interface in Python and a number of wrappers in other programming languages.

JavaCV is one of such wrappers. So in most cases when you run program with JavaCV you actually use OpenCV too, just call it via another interface. But JavaCV provides more than just one-to-one wrapper around OpenCV. In fact, it bundles the whole number of image processing libraries, including FFmpeg, OpenKinect and others. (Note, that in C++ you can bind these libraries too).

So, in general it doesn't matter what you are using - OpenCV or JavaCV, you will get just about same performance. It more depends on your main task - is it Java or C++ which is better suited for your needs.

There's one more important point about performance. Using OpenCV (directly or via wrapper) you will sometimes find that OpenCV functions overcome other implementations by several orders. This is because of heavy use of low-level optimizations in its core. For example, OpenCV's filter2D function is SIMD-accelerated and thus can process several sets of data in parallel. And when it comes to computer vision, such optimizations of common functions may easily lead to significant speedup.