The convex hull indices are not monotonous, which can be in the case when the input contour contains self-intersections in function 'convexityDefects'

[Nv7-GitHub]

I am working on a modified version of the gesture detector (the gesture detector doesn't work, at least for me). You can find it at https://github.com/Nv7-Github/gestures. When using convexity defects, it mostly works, but occasionally it crashes the program with the error

libc++abi.dylib: terminating with uncaught exception of type cv::Exception: OpenCV(4.5.1) /tmp/opencv-20210228-9361-r4z33j/opencv-4.5.1/modules/imgproc/src/convhull.cpp:360: error: (-5:Bad argument) The convex hull indices are not monotonous, which can be in the case when the input contour contains self-intersections in function 'convexityDefects'

Steps to Reproduce

1. go get github.com/Nv7-Github/gestures
2. gestures
3. Wait for a bit, move your hand around and make various figures with your hand
4. It crashes.

Your Environment

• Operating System and version: MacOS Big Sur
• OpenCV version used: 4.5.1
• How did you install OpenCV? brew install opencv
• GoCV version used: v0.27.0
• Go version: go version go1.16.3 darwin/amd64
• Did you run the env.sh or env.cmd script before trying to go run or go build? No.

[deadprogram]

That looks like it is OpenCV itself see https://github.com/opencv/opencv/issues/4539 which is a very long running thread. Does not look specific to GoCV.

[Nv7-GitHub]

Is there a way to sort the indices using GoCV?

[McApe]

maybe this could work?

...
gocv.ConvexHull(cnt, &hull, false, false)
monoHull := gocv.NewMat()
gocv.Sort(hull, &monoHull , gocv.SortEveryColumn)
gocv.ConvexityDefects(cnt, monoHull , &defects)
...