OpenCV C++, getting Region Of Interest (ROI) using cv::Mat

Question

I'm very new to OpenCV (started using it two days ago), I'm trying to cut a hand image from a depth image got from Kinect, I need the hand image for gesture recognition. I have the image as a cv::Mat type. My questions are:

1. Is there a way to convert cv::Mat to cvMat so that I can use cvGetSubRect method to get the Region of interest?
2. Are there any methods in cv::Mat that I can use for getting the part of the image?

I wanted to use IplImage but I read somewhere that cv::Mat is the preferred way now.

Solution 1

You can use the overloaded function call operator on the cv::Mat:

cv::Mat img = ...;
cv::Mat subImg = img(cv::Range(0, 100), cv::Range(0, 100));

Check the OpenCV documentation for more information and for the overloaded function that takes a cv::Rect. Note that using this form of slicing creates a new matrix header, but does not copy the data.

Solution 2

Maybe an other approach could be:

//Create the rectangle
cv::Rect roi(10, 20, 100, 50);
//Create the cv::Mat with the ROI you need, where "image" is the cv::Mat you want to extract the ROI from
cv::Mat image_roi = image(roi)

I hope this can help.

Comments

[vprasad]

Thanks for the answer! I tried the Range but it gave me a runtime error but the cv::Rect() worked just fine!

[Michael Koval]

Could you edit your question with the cv::Range code that failed? Also, please accept my answer if it was helpful.

[Froyo]

cv:Range gave me runtime error, but cv::Rect worked like a charm! thanks!

[Fanglin]

Yes it seems to be using Rect. Check jayrambhia.wordpress.com/2012/09/20/… for an example.

[Zhengfang Xin]

Rect_(x, y, w, h), (x, y) left corner coordinate., and w, h are width and height of the rectangle.

[Kai Wang]

Does this work for 16 bit data?