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Indexing beyond size of Mat still gives a result

asked 2014-07-24 03:36:22 -0500

I have loaded a jpg image that is 480 columns by 640 rows into a Mat and I want to return the pixel value at a certain position. I load a colour image and then I convert it to greyscale. Obviously I want to return pixel values between columns 0 to 479 and rows 0 to 639 but if I use an index of say (500,500) then I still get an answer without any crash or error. In fact an index up to (0,622575) works or even (1,622095), basically 622575 minus the length of 1 row (480). How is is it possible to index beyond the size of the Mat ? I am running this in Xcode 5.5.1 (debug mode) and I am using OpenCV 2.4.9 installed via MacPorts.

Mat colourImage = imread("/Path_to/a.jpg");

Mat greyImage;

int rowpos = 500;

int colpos = 500;

cvtColor(colourImage,greyImage,CV_BGR2GRAY);

uchar gg = greyImage.at< uchar>(rowpos,colpos);

cout << "pixel value " << (int)gg << endl;

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if you run a debug build, you should get an exception.

berak gravatar imageberak ( 2014-07-24 03:38:29 -0500 )edit

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answered 2014-07-24 04:02:36 -0500

Your a lucky guy (or not… because it could crash at any time…). As @berak says, in Debug, you should get an exception as bounds are checked. It's your job, as a developer to checks the bounds of images you are scanning. The bounds are not checked in release, and if you want to be sure that there is not out-of-bound access, and obviously don't want to use the rows and cols members, have a look at the iterator (begin and end) of the cv::Mat class. They are similar to STL iterator.

If you are interested in the many ways you could use to scan your image, have a look at that tutorial that will show you most of the common way to scan an image, and highlight the performance you could expect for each of them.

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Thanks berak and Mathieu. I am in debug build and it was checking the bounds of my arrays that lead me to this short piece of test code. By doing a bit more googling I found that my version of OpenCV from MacPorts may have been compiled without debug symbols and by luck and luck alone it does not crash!

http://stackoverflow.com/questions/20134498/debugging-a-dylib-in-xcode-that-was-built-without-debug-symbols

I will try my own build using the source code form the OpenCV website

eugclark gravatar imageeugclark ( 2014-07-24 05:25:23 -0500 )edit
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Asked: 2014-07-24 03:36:22 -0500

Seen: 94 times

Last updated: Jul 24 '14