OpenCV: IplImage versus Mat, which to use?

Question

Hi All,

I'm pretty new to OpenCV (about 2 months now). I have the book Learning OpenCV by Bradski and Kaehler. My question is, if I want to do everything in a 2.0+ manner, when should I use Matrices (Mat) and when should I use IplImage?

Bradky's book states upfront (Preface) that it's written based on OpenCV 2.0, and it mostly uses IplImage in its sample code, but the more recent online documentation makes it sound like Mat is now a catch-all data type for images, masks, etc, kind of like a basic matrix in Matlab. This leaves me wondering if IplImage should be considered obsolete.

So, should I be totally avoiding IplImages when writing new code? Or is there important stuff IplImages allow me to do that Mats do not?

Thanks.

Answer 1

I would highly recommend using Mat. I've been using it for a while, and it's great. The member functions and the matrix expressions make things much simpler than dealing with IplImage and as you said it's a catch-all data type.

Go for Mat!

Answer 2

IplImage has been in OpenCV since the very beginning. It is a part of the C interface for OpenCV. You need to allocate and deallocate memory for IplImage structures yourself. (remember the cvReleaseImage commands?)

The new Mat structure is a part of the C++ structure. So obviously it is object oriented. Also, it manages all memory for you! It keeps a track of references to it. Then the number of references goes to zero, it deallocates automatically. This is one superb feature!

Go for Mat. It should be easy to translate code from the IplImage thingy to the Mat thingy if you are using some IDE that has Intellisense (it drops down a list of possible functions, variables, etc as you type)

Answer 3

I would say this would actually depend on the platform that you are going to run your application on. If you are developing an application for an embedded system you would be more likely to use C. In that case you would have to use IplImage

Answer 4

Hi all, thanks for the help.

I also discovered since posting this question that a function with a Mat as a argument can take an IplImage directly in the place of that Mat argument, which makes it pretty easy to update your code in chunks if it's already broken into convenient functions. Just change the function arguments from IplImage* to Mat, then modify the function to work on a Mat. Other code calling that function should still work fine (it has in my experience).