CODEWITHSUNDEEP
c#opencvimage-processingedge-detection
dan
dan

Reputation: 893

c# detect points of polygon in black/white image

I have to detect all the points of a white polygon on a black background in c#. Here is an image of several examples. I wouldn't think it is too difficult, but I am unable to detect this properly with all the variations. My code is too much to post here, but basically I went through each side and look for when it changes from black and white. Should I use Open CV? I was hoping for a simple algorithm I could implement in C#. Any suggestions? Thank you.

enter image description here

Upvotes: 0

Views: 1574

Answers (1)

Spektre
Spektre

Reputation: 51845

In your case I would do this:

  1. pre process image

    so remove noise in color if present (like JPG distortion etc) and binarize image.

  2. select circumference pixels

    simply loop through all pixels and set each white pixel that has at least one black neighbor to distinct color that will represent your circumference ROI mask or add the pixel position to some list of points instead.

  3. apply connected components analysis

    so you need to find out the order of the points (how are connected together). The easiest way to do this is use flood filing of the ROI from first found pixel until all ROI is filled and remember the order of filled points (similar to A*). There should be 2 distinct paths at some point and both should join at last. So identify these 2 points and construct the circumference point order (by reversing one half and handling the shared part if present).

    connected components

  4. find vertexes

    if you compute the angle change between all consequent pixels then on straight lines the angle change should be near zero and near vertexes much bigger. So threshold that and you got your vertexes. To make this robust you need to compute slope angle from a bit more distant pixels then the closest pixels. Also thresholding this angle change against sliding average often provides more stable results.

    edge detect

    So find out how far the pixels should be to compute angle so you got not too big noise and vertexes has still big peaks and also find out the threshold value that is safe above any noise.

    This can be done also by hough transform and or find contours functions that are present in many CV libs. Another option is also regress/fit the lines in the point list directly and compute intersections which can provide sub pixel precision.

For more info see related QAs:

Upvotes: 1

Related Questions