Cropping an image with OpenCV and C

Question

Hi am trying to debug a C++/C developers code, He wrote us a dll that we are using in an adobe native extension that basically takes pictures with a grid of web cams and after doing some face detection is suppose to crop the images and write them to disc.

But the app always hangs and eventually crashes on this line:

smallFrame = image(Rect(x, y, CROPPING_WIDTH, CROPPING_HEIGHT));

i have narrowed it down to this line by throwing a try/catch around it and the exception it spits out is not very helpful, it just says ???????n as the exception.

like so:

try

{

smallFrame = image(Rect(x, y, CROPPING_WIDTH, CROPPING_HEIGHT));

}

catch(exception ex)

{

wsprintf (str, L"Exception Occured during Face Found : %s", ex.what());

WriteLogFile(str);

smallFrame = frame;

}

here is the whole method:

Mat cropFaceFrame( Mat frame)

{

std::vector<Rect> faces;

Mat frame_gray, smallFrame;

int height = 0;

unsigned index, i;



cvtColor( frame, frame_gray, CV_BGR2GRAY );



equalizeHist( frame_gray, frame_gray );



face_cascade.detectMultiScale( frame_gray, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(60, 60));



index = faces.size();

wsprintf (str, L



for (i = 0; i < faces.size(); i++ )

{


if (height < faces[i].height)

{

height = faces[i].height;

index = i;



}

}

Mat image(frame);

int maxRight, maxDown;

maxRight = IMAGE_WIDTH-CROPPING_WIDTH -1;

// right margin

maxDown = IMAGE_HEIGHT-CROPPING_HEIGHT-1;

// down margin

if (index == faces.size())

{

// crop the center part if no face found



try

{

smallFrame = image(Rect(maxRight/2, maxDown/2, CROPPING_WIDTH, CROPPING_HEIGHT));

}

catch(exception ex)

{



smallFrame = frame;

}


}

else

{



int x, y;

x = faces[index].x - (CROPPING_WIDTH-faces[index].width)/2;



if (x < 0) x = 0;

else if (x > maxRight) x = maxRight;



y = faces[index].y - (CROPPING_HEIGHT-faces[index].height)/3;



if (y < 0) y = 0;

else if (y > maxDown) y = maxDown;


try

{

smallFrame = image(Rect(x, y, CROPPING_WIDTH, CROPPING_HEIGHT));

}

catch(exception ex)

{

 wsprintf (str, L

"Exception Occured during no Face Found : %s", ex.what());

WriteLogFile(str);

smallFrame = frame;

}



}



return smallFrame;

}

score 3 · Answer 1 · answered Aug 12 '13 at 14:30

3

smallFrame = image(Rect(x, y, CROPPING_WIDTH, CROPPING_HEIGHT));

* a fixed CROPPING_WIDTH or HEIGHT won't do. you've got to check, if your Rect did not end up partly outside the image, i.e if x+CROPPING_WIDTH < img.cols-1

answered Aug 12 '13 at 14:30

berak

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score 0 · Answer 2 · edited May 23 '17 at 12:11

berak explained you the probable cause of your error, but the reason for which you cannot debug is that you are using:

wsprintf (str, L"Exception Occured during Face Found : %s", ex.what());

in Visual Studio environment, with ex.what() returning a const char*. Unfortunately, %s behaviour is platform dependent, in this case it expects a wide character string (so, %s is wide string for wsprintf and byte string for sprintf). In Unix, you would have the correct behaviour. In Visual Studio, you have to use %S.

Check this: printf, wprintf, %s, %S, %ls, char* and wchar*: Errors not announced by a compiler warning? and in particular this.

Cropping an image with OpenCV and C

2 Answers2