drawMatches bug

asked 2013-04-21 10:26:41 -0500

yes123 gravatar image

I think there is a bug in drawMatches.cpp

OpenCV/modules/features2d/src/draw.cpp line: 208

There are two assert:

int i1 = matches1to2[m].queryIdx;
int i2 = matches1to2[m].trainIdx;
CV_Assert(i1 >= 0 && i1 < static_cast<int>(keypoints1.size()));
CV_Assert(i2 >= 0 && i2 < static_cast<int>(keypoints2.size()));

The problem is that keypoints1 refers to the train image, while i1 referers to the queryIdx.

I believe it should be inverted:

int i1 = matches1to2[m].trainIdx;
int i2 = matches1to2[m].queryIdx;

I believe this is a commont mistake in OpenCV because almost every samples use the wrong order to call DescriptorMatcher::match in fact there is always this error in the documentation:

obj.push_back( kp_object[ good_matches[i].queryIdx ].pt );  //> It should be trainIdx
scene.push_back( kp_image[ good_matches[i].trainIdx ].pt );
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Comments

Hi! I have the same problem. Im making a real time object detector and for unknown reason, the trainIdx gets higher values than the train keypoints vector size (randomly), throwing an OpenCv Exception... I could not find any explanation about this...

aratzs gravatar imagearatzs ( 2013-06-07 02:59:23 -0500 )edit
2

I solved this by switching the train and query images in the call to drawMatches (use query first and train on the right)

yes123 gravatar imageyes123 ( 2013-06-07 12:24:01 -0500 )edit

@yes123, could you please explain more

kzbr93 gravatar imagekzbr93 ( 2017-12-07 17:47:55 -0500 )edit