Sort array in filterContours

[pblankenbaker]

When sorting objects with the Collections.sort() method, you will typically want to supply the "comparator" parameter that lets you control the comparison between each object (decide which should come before the other).

For example, if you want to sort by area, I think you can use a static helper method something like the following:

  public static void sortContoursByArea(List<MatOfPoint> contours) {
    Collections.sort(contours, new Comparator<MatOfPoint>() {

      @Override
      public int compare(MatOfPoint a, MatOfPoint b) {
        // Not sure how expensive this will be computationally as the
        // area is computed for each comparison
        double areaA = Imgproc.contourArea(a);
        double areaB = Imgproc.contourArea(b);

        // Change sign depending on whether your want sorted small to big
        // or big to small
        if (areaA < areaB) {
          return -1;
        } else if (areaA > areaB) {
          return 1;
        }
        return 0;
      }
    });
  }

Hope that helps

[SamCarlberg]

FWIW you could also write that as

public static void sortContoursByArea(List<MatOfPoint> contours) {
  Collections.sort(contours, Comparator.comparingDouble(Imgproc::contourArea));
}

[PhilBot]

Originally Posted by SamCarlberg

FWIW you could also write that as

public static void sortContoursByArea(List<MatOfPoint> contours) {
  Collections.sort(contours, Comparator.comparingDouble(Imgproc::contourArea));
}

That's a really interesting syntax.

Is there a name for what is hapening with "Imgproc::contourArea" ?
I'd like to look up the explanation of how that works...

Clearly it's getting expanded internally into Imgprog.contourArea(contours[n]) for the comparison.... but I don't know how/why.....

[SamCarlberg]

That's called a method reference.

This article explains it quite well

Also, you don't even need a special method for sorting a list. The List interface defines a method "sort(Comparator)" that you can use inline like

contours.sort(Comparator.comparingDouble(Imgproc::contourArea));

[PhilBot]

Originally Posted by SamCarlberg

That's called a method reference.

This article explains it quite well

Also, you don't even need a special method for sorting a list. The List interface defines a method "sort(Comparator)" that you can use inline like

contours.sort(Comparator.comparingDouble(Imgproc::contourArea));

Dang... As an "vintage" programmer, it's sometimes scary when I realize how much I don't know about the "newer" languages. That's pretty darn powerful.

[PhilBot]

Originally Posted by pblankenbaker

When sorting objects with the Collections.sort() method, you will typically want to supply the "comparator" parameter that lets you control the comparison between each object (decide which should come before the other).

For example, if you want to sort by area, I think you can use a static helper method something like the following:
Hope that helps

Terrific. We were heading in that direction but couldn't get the eclipse to accept our code.

We were just trying to call the Collections.sort in-line.

Your static method went in with no problems.

I'll have to investigate further to understand why the static method worked, but the inline code didn't.

Thanks again for the detailed response.