OpenCV pointer to bitmap processing - c++

Question

I've created a shared library for contour detection that is loaded from a Delphi/Lazarus application. The main app, passes a pointer to a bitmap to be processed by a function inside the library.
Here's the function inside the library. The parameter "img" is the pointer to my bitmap.
extern "C" {
void detect_contour(int imgWidth, int imgHeight, unsigned char * img, int &x, int &y, int &w, int &h)
{
Mat threshold_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
Mat src_gray;
int thresh = 100;
int max_thresh = 255;
RNG rng(12345);
/// Load source image and convert it to gray
Mat src(imgHeight, imgWidth, CV_8UC4);
int idx;
src.data = img;
/// Convert image to gray and blur it
cvtColor( src, src_gray, CV_BGRA2GRAY );
blur( src_gray, src_gray, Size(10,10) );
/// Detect edges using Threshold
threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
/// Find contours
findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
/// Approximate contours to polygons + get bounding rects and circles
vector<vector<Point> > contours_poly( contours.size() );
vector<Rect> boundRect( contours.size() );
vector<Point2f>center( contours.size() );
vector<float>radius( contours.size() );
int lArea = 0;
int lBigger = -1;
for( int i = 0; i < contours.size(); i++ )
{
approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
boundRect[i] = boundingRect( Mat(contours_poly[i]) );
if(lArea < boundRect[i].width * boundRect[i].height)
{
lArea = boundRect[i].width * boundRect[i].height;
lBigger = i;
}
}
if(lBigger > -1)
{
x = boundRect[lBigger].x;
y = boundRect[lBigger].y;
w = boundRect[lBigger].width;
h = boundRect[lBigger].height;
}
}
}
From the Delphi side, I'm passing a pointer to an array of this structure:
TBGRAPixel = packed record
blue, green, red, alpha: byte;
end;
I need to process the bitmap in-memory, that's why I'm not loading the file from inside the library.
The question is: Is this the right way to assign a bitmap to a cv::Mat ?
I ask this because the code works without problems in Linux, but fails on Windows compiled with Mingw.
Note: it fails with a SIGSEGV on this line:
blur( src_gray, src_gray, Size(10,10) );
EDIT: The SIGSEGV is raised only if I compile OpenCV in Release mode, in Debug mode it works ok.
Thanks in advance,
Leonardo.

Answer 1

So you are creating an image this way:
Mat src(imgHeight, imgWidth, CV_8UC4);
int idx;
src.data = img;
The first declaration and instantiation
Mat src(imgHeight, imgWidth, CV_8UC4) will allocate memory for a new image and a reference counter that automatically keeps track of the number of references to the allocated memory.
Then you mutate an instance variable through
src.data = img;
When the the instance src goes out of scope, the destructor is called and most likely tries to deallocate the memory at src.data, which you assigned and this might cause a segmentation fault. The right way to do it is to not change instance variable of an object, but to simply use the right constructor when you instantiate src:
Mat src(imgHeight, imgWidth, CV_8UC4, img);
This way, you just create a matrix header and no reference counter or deallocation will be performed by the destructor of src.
Good luck!
EDIT: I am not sure that the segfault is actually caused by an attempt to deallocate memory incorrectly, but it is a good practice not to break data abstraction by assigning directly to instance variables.