Using SIFT with Mask (Bugfix #2892)

I'd like to report a bug in the SIFT detector implementation of version 2.4.4 (possibly others too). When using a mask, I get SegFaults. The reason is, that the check, if keypoints are inside the mask is done on the scaled keypoint variants, given by the ScaleSpace and the original mask. This check has to be done after "backscaling" the keypoints. The problem is given in the operator():

    if( !mask.empty() )
        KeyPointsFilter::runByPixelsMask( keypoints, mask );

    if( nfeatures > 0 )
        KeyPointsFilter::retainBest(keypoints, nfeatures);
    //t = (double)getTickCount() - t;
    //printf("keypoint detection time: %g\n", t*1000./tf);

    if( firstOctave < 0 )
        for( size_t i = 0; i < keypoints.size(); i++ )
        {
            KeyPoint& kpt = keypoints[i];
            float scale = 1.f/(float)(1 << -firstOctave);
            kpt.octave = (kpt.octave & ~255) | ((kpt.octave + firstOctave) & 255);
            kpt.pt *= scale;
            kpt.size *= scale;
        }

Simply doing the "is Mask empty" check after rescaling works for me.

    if( nfeatures > 0 )
        KeyPointsFilter::retainBest(keypoints, nfeatures);
    //t = (double)getTickCount() - t;
    //printf("keypoint detection time: %g\n", t*1000./tf);

    if( firstOctave < 0 )
        for( size_t i = 0; i < keypoints.size(); i++ )
        {
            KeyPoint& kpt = keypoints[i];
            float scale = 1.f/(float)(1 << -firstOctave);
            kpt.octave = (kpt.octave & ~255) | ((kpt.octave + firstOctave) & 255);
            kpt.pt *= scale;
            kpt.size *= scale;
        }

    if( !mask.empty() )
        KeyPointsFilter::runByPixelsMask( keypoints, mask );

It would be more efficent to just search in the given mask for regions of interest, though. Maybe this problem can be fixed in the next version? I'm new to the issue tracker and hopefully this post helps.