Removing selected rows from a Mat ?

asked May 24 '13

yes123
1412 ●12 ●28 ●52

updated Nov 15 '0

sturkmen

6772 ●3 ●48 ●79 https://github.com/stu...

For some reasons I need to filter out rows of a Mat. (I need to filter out some descriptors of ORB)

Which way do you suggest me? I haven't find any method to remove a single row from a Mat. So I was thinking I could iteratively inserting the good rows in a new Mat.

C++ pseudocode:

Mat desc;
ORB(myImage,cv::noArray,kp,desc);
matcher->match( myPreviousDesc, desc, matches );

for(auto i=0;i<matches.size();i++) {
   if (some conditions) {
      Remove the ith row of desc:
      erase( desc.row( matches[i].queryIdx ) );
   }
}

? How would you erase a single row from a Mat after checking for some conditions (or adding only the selected row in a new Mat) ?

answered May 24 '13

berak
32993 ●7 ●81 ●312

updated May 24 '13

desc.row() is a copy. no cigar erasing that
erase messes with the size(),so break out after that, or face doom.

you can't do it "inplace", so copy anything but the offending row(s) to a new Mat

Comments

how would you add iteratively the good rows?

yes123 (May 24 '13)edit

mat.push_back(orig.row(i));

later:

mat.reshape(0,orig.cols);

berak (May 24 '13)edit

to istanciate correctly the new Mat (for the first row) do i need to pass some params?

yes123 (May 24 '13)edit

no, don't think so ;)

type will get inferred with 1st push_back(), rows/cols will get rearranged with reshape()

berak (May 24 '13)edit

thanks a lot

yes123 (May 24 '13)edit

hehe, you removed the suspicious comment, and i know, it sound pretty counterintuitive, but yes, look at reshape

move your ass ( cols ) and the rest will follow ..

berak (May 24 '13)edit

@berak: berak, I am testing my code without .reshape and stuff seems to working properly even without the reshape, maybe its not needed?

yes123 (May 25 '13)edit

oh,maybe not, depends on how you instanciated your mat.

berak (May 25 '13)edit

@berak: I instanciated it with a simple: Mat myNewMat;

yes123 (May 25 '13)edit

Maybe it's because when you insert the first row with push_back(firstRow) Mat infers the number of cols too?

yes123 (May 25 '13)edit

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