OpenCV findContours - 2.4.5 Heap Corruption - c++

Question

I have a heap corruption happening with 100% certainty on the findContours function. When I do not use it, everything works fine.
unsigned char* UCFromMatUC(cv::Mat& input)
{
int size = input.size.p[0] * input.size.p[1];
unsigned char* result = new unsigned char[size];
memcpy(result, input.data, size);
return result;
}
unsigned char* CannyEdgeCV(unsigned char* input, int width, int height)
{
std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;
cv::RNG rng(12345);
cv::Mat inp(cv::Size(width, height), CV_8UC1, input);
cv::Mat canny_output;
cv::Mat outp;
cv::blur(inp, outp, cv::Size(3,3));
cv::Canny(outp, canny_output, 4.0, 8.0);
if(canny_output.type()!=CV_8UC1){
return NULL;
}
cv::findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );
cv::Mat drawing = cv::Mat::zeros( canny_output.size(), CV_8UC3 );
for( int i = 0; i< contours.size(); i++ )
{
cv::Scalar color = cv::Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, cv::Point() );
}
cv::imwrite( "contours.jpg", drawing );
unsigned char* result = UCFromMatUC(canny_output);
return result;
}
Originally I was only using the canny edge map, but later on I wanted to test the results of the contour functionality.
The Canny Edge works fine, and I get an image as expected, but the findContours (both the one in code and the commented version) fail with a heap corruption error. What causes this?
The entry point for this is the CannyEdgeCV(), and is called with a 640x480 8bit grey image.
Edit: updated code.
Edit2: when I tried to create a minimal example to reproduce this, my code failed at even imread("imagename.bmp"); which was really odd, so I began investigating what might cause this. Now someone else wrote in a corresponding SO question that you cannot mix debug / release mode libraries, so if you are in debug, you have to use debug DLLs, and that worked for me now, I get the expected results.

Answer 1

The main culprit is your use of cv::Mat* and new. This is a Bad Idea. It is unnecessary and often problematic (as you have discovered) to dynamically allocate cv::Mat objects. A better solution is to pass them by value or const reference, since the underlying image data is refcounted, and cv::Mat are shallow copied.
The first specific problem is that you manually assign the data member in CVMatFromUC():
resultMat->data = input
You should not do this. cv::Mat has other members which also reference the data location, and you are asking for trouble. If you need to create a cv::Mat header for external data, you should create a cv::Mat like so:
cv::Mat inp(cv::Size(width, height), CV_8UC1, input); //Create cv::Mat header, no memory copied
Also, your type check for canny_output is incorrect. !canny_output.type() is evaluated first, and implicitly converts to true, as does CV_8UC1. So the expression is always true. The condition you want is: canny_output.type() != CV_8UC1
Given this, it turns out that the CVMatFromUC() function is unnecessary. An improved version of your function follows:
uchar* CannyEdgeCV(uchar* input, int width, int height)
{
std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;
cv::Mat inp(cv::Size(width, height), CV_8UC1, input); //Create cv::Mat header, no memory copied
cv::Mat canny_output;
cv::Mat outp;
cv::blur(inp, outp, cv::Size(3,3));
cv::Canny(outp, canny_output, 10.0, 15.0);
if(canny_output.type()!=CV_8UC1){
return NULL;
}
cv::findContours(canny_output, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE );
//cv::findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, cv::Point(0, 0) );
unsigned char* result = UCFromMatUC(&inp);
return result;
}
I should note that it appears that CannyEdgeCV returns the same data that it receives, so it may be possible to remove the call to UCFromMatUC() and the associated data copy entirely. However, I tried and got memory errors, so there might be other problems lurking elsewhere.

Answer 2

-Change findContours as follows
findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);
If it doesn't work again, your problem is probabely uninitialized memory. You can try to create new Mat outP2 and clone the original one into this.
Then use outP2 from the following step:
cv::Canny(*outP2, canny_output, 10.0, 15.0);
...
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