Error setting ROI OpenCV Android

Question

I am trying to figure out how to set the ROI for an Image in OpenCV on Android. I have done this on other operating systems, so I think HOW I am doing it is semantically correct, but there is an error somewhere.

So far I have tried this

Rect roi = new Rect(0, 0, inputFrame.cols(), inputFrame.rows());
Mat cropped = new Mat(inputFrame, roi);

However I get an error somewhere in the OpenCV classes that looks like this

Utils.matToBitmap() throws an exception:/home/reports/ci/slave/opencv/modules/java/generator/src/cpp/utils.cpp:107: 
error: (-215) src.dims == 2 && info.height == (uint32_t)src.rows && info.width ==
(uint32_t)src.cols in function void Java_org_opencv_android_Utils_nMatToBitmap2
(JNIEnv*, jclass, jlong, jobject, jboolean)

I am calling this in the onCameraFrame callback provided by opencv wrapper class

Not sure how to go about this, has anyone successfully done this?

Answer 1

OpenCV for Android (and the new Java API) has its own way to create ROI's.

All you have to do is to call the submat method of your Mat object. If I am not mistaken, calling submat does not create a copy of that image region, if you want a copy you can use copyTo on the submat for that purpose.

Mat roi = inputFrame.submat(rowStart, rowEnd, colStart, colEnd);

You can call submat in 3 different ways, check the links for more details:

• submat(int rowStart, int rowEnd, int colStart, int colEnd)

submat 1

• submat(Range rowRange, Range colRange)

submat 2

• submat(Rect roi)

submat 3

Answer 2

The error is stating that you are trying to set a ROI with the same dimensions of the image, so there's no need for a ROI in this case.

To make sure this is the problem, the following should work:

Rect roi = new Rect(0, 0, inputFrame.cols() - 1, inputFrame.rows() - 1);
Mat cropped = new Mat(inputFrame, roi);