How to find nonzero pixels in specific territory in Mat - opencv

Question

I want to find nonzero pixels with findNonZero function in specific place, not the whole Mat.
In the picture posted below, I find the teritory of white patches with findContours function. Later, I invert the image posted below with bitwise_not function and I need to find the places of pixels of those black patterns seperately for each white patch. How can I do that? For each white patch there should be a Mat or Array with black pixel coordinates.
My current approach is to find the contours of the white patches and draw them all each into seperate Mats. Then, find the coordinates of white patch white pixels with findNonZero, mix the image with black patterns and with for loop check whether the pixel, which was white, now is black. Put the coordinates of those pixels in a List and later do other things... But this method is neither smart and simple, nor efficient.
Is there a possibility to do it much simplier and more efficiently? Like being able to find nonZero pixels inside the contours?

Answer 1

Hi Here is a sample implementation! Please use this!
void findNonZero( InputArray _src, InputArray _Mask, OutputArray _idx )
{
Mat src = _src.getMat();
Mat msk = _Mask.getMat();
CV_Assert( src.type() == CV_8UC1 );
CV_Assert( src.size() == msk.size());
int n = countNonZero(src);
if( n == 0 )
{
_idx.release();
return;
}
if( _idx.kind() == _InputArray::MAT && !_idx.getMatRef().isContinuous() )
_idx.release();
_idx.create(n, 1, CV_32SC2);
Mat idx = _idx.getMat();
CV_Assert(idx.isContinuous());
Point* idx_ptr = idx.ptr<Point>();
for( int i = 0; i < src.rows; i++ )
{
const uchar* bin_ptr = src.ptr(i);
const uchar* msk_ptr = msk .ptr(i);
for( int j = 0; j < src.cols; j++ )
if( bin_ptr[j] && msk_ptr[j])
*idx_ptr++ = Point(j, i);
}
}
int _tmain(int argc, _TCHAR* argv[])
{
string sFileNameEx="F:\\Balaji\\Image Samples\\Test.jpg";
size_t lastindex = sFileNameEx.find_last_of(".");
String sFileName = sFileNameEx.substr(0, lastindex);
bool bSaveImages=true;
Mat mSrc_Gray,mSrc_Mask,mResult_Bgr;
mSrc_Gray= imread(sFileNameEx,0);
mSrc_Mask= Mat(mSrc_Gray.size(),CV_8UC1,Scalar(0));
cvtColor(mSrc_Gray,mResult_Bgr,COLOR_GRAY2BGR);
if(mSrc_Gray.empty())
{
cout<<"[Error]! Invalid Input Image";
return 0;
}
threshold(mSrc_Gray,mSrc_Gray,10,255,THRESH_BINARY);
imshow("mSrc_Gray",mSrc_Gray);
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
/// Find contours
findContours( mSrc_Gray.clone(), contours, hierarchy, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE, Point(0, 0) );
for( size_t i = 0; i < contours.size(); i++ )
{
if(contourArea(contours[i])>500)
{
drawContours(mSrc_Mask,contours,i,Scalar(255),-1);
}
}
//May be further Filtering like erode needed?
imshow("mSrc_Mask",mSrc_Mask);
vector<Point> locations; // output, locations of non-zero pixels
findNonZero(mSrc_Gray,mSrc_Mask,locations);
for( size_t i = 0; i < locations.size(); i++ )
{
circle(mResult_Bgr,locations[i],1,Scalar(0,255,0),1);
}
imshow("mResult_Bgr",mResult_Bgr);
waitKey(0);
return 0;
}