Label Images in OpenCV - opencv

Question

Given the same binary input image (up to inversion), is there a guarantee that the labels in the images returned by all the following functions are all consistent?
cv::distanceTransform() (version with labels)
cv::connectedComponents()
cv::connectedComponentsWithStats()
Does this appear in the docs anywhere?

Answer 1

If you pass to distanceTransform a binary image inverted with respect to connectedComponents or connectedComponentsWithStats, the labels are computed the same way and will be consistent.
I was not able to find any reference in the doc, but the labels will be computed by the same algorithm (connectedComponents_sub1) in all cases.
connectedComponents[WithStats]
int cv::connectedComponents(InputArray _img, OutputArray _labels, int connectivity, int ltype){
const cv::Mat img = _img.getMat();
_labels.create(img.size(), CV_MAT_DEPTH(ltype));
cv::Mat labels = _labels.getMat();
connectedcomponents::NoOp sop;
if(ltype == CV_16U){
return connectedComponents_sub1(img, labels, connectivity, sop);
}else if(ltype == CV_32S){
return connectedComponents_sub1(img, labels, connectivity, sop);
}else{
CV_Error(CV_StsUnsupportedFormat, "the type of labels must be 16u or 32s");
return 0;
}
}
int cv::connectedComponentsWithStats(InputArray _img, OutputArray _labels, OutputArray statsv,
OutputArray centroids, int connectivity, int ltype)
{
const cv::Mat img = _img.getMat();
_labels.create(img.size(), CV_MAT_DEPTH(ltype));
cv::Mat labels = _labels.getMat();
connectedcomponents::CCStatsOp sop(statsv, centroids);
if(ltype == CV_16U){
return connectedComponents_sub1(img, labels, connectivity, sop);
}else if(ltype == CV_32S){
return connectedComponents_sub1(img, labels, connectivity, sop);
}else{
CV_Error(CV_StsUnsupportedFormat, "the type of labels must be 16u or 32s");
return 0;
}
}
As you can see, the labeling part is performed by the connectedComponents_sub1 function in both cases. The only difference between the two is the statistic computation: connectedcomponents::NoOp versus connectedcomponents::CCStatsOp, not relevant for label computation.
distanceTransform
void cv::distanceTransform( InputArray _src, OutputArray _dst, OutputArray _labels,
int distType, int maskSize, int labelType )
{
...
if( labelType == CV_DIST_LABEL_CCOMP )
{
Mat zpix = src == 0;
connectedComponents(zpix, labels, 8, CV_32S);
}
...
}
The labels are computed internally by the function connectedComponents.