Do you want to cut (collapse to black) dark regions or remove (restore image) shadows ?
Shadow removal is a complex task. Use "shadow" in the search box here to read about this subject .
I don't know if @thienlamnhan 's code for detect shadow works. In addiction read this:
Use histEqualization to enhance bright vs dark region.
To collapse dark regions to full black you could use same principle of brightness and contrast normalization
cvtColor(src, gray, CV_BGR2GRAY);
// get full available range
double minGray,maxGray;
cv::minMaxLoc(gray, &minGray, &maxGray);
//suppose current range is 30...220
bool useRelative = true;
if(useRelative)
{
// Relative clipping dark range to black
double clipPercent = 10;
minGray = cvRound(minGray * (1 + clipPercent/100.0) );
//all below minGray+10% will become black
}
else
{
//absolute clipping. Use a fixed lower bound for dark regions
double minGrayWanted = 50;
minGray = minGrayWanted;
//all below 50 will become black
}
// current range
float inputRange = maxGray - minGray;
alpha = 255.0 / inputRange; // alpha expands current range. MaxGray will be 255
beta = -minGray * alpha; // beta shifts current range so that minGray will go to 0
src.convertTo(dst, -1, alpha, beta);
Result will be:
src[0...minGray] -> dst[0]
src[minGray...maxGray] -> src[0...255]
[EDIT] from user comment It seems that needs is to correct nonuniform illumination. In this case MORPH_TOPHAT can do the job. See my answer on this subject.