Re: Roi-Contour is smaller as expected

Posted by Rasband, Wayne (NIH/NIMH) [E] on Dec 15, 2012; 6:20pm
URL: http://imagej.273.s1.nabble.com/Roi-Contour-is-smaller-as-expected-tp5001160p5001178.html

On Dec 14, 2012, at 6:08 AM, Thorsten Wagner wrote:

> Hi
> i've used a contour tracing algorithm to extracted the contour of the following object:
> https://dl.dropbox.com/u/560426/imagej/object.png
>
> The extracted polygon is described by the following points:
> int[] xpoints = {3,4,5,6,7,8,9,10,11,11,11,10,9,8,7,6,5,4,3,2,2,2,3};
> int[] ypoints = {1,1,2,2,2,1,1,2,2,3,4,4,5,5,4,4,4,5,5,4,3,2,1};
> These Points seems to be correct.
>
> I've now converted this polygon to a PolygonRoi and add it to the RoiManager:
> Roi roi = new PolygonRoi(mypolygon, Roi.POLYGON);
> roiManager.add(imp, roi, n);
>
> If I click on the added Roi in the ROI-Manager the overlayed contour is smaller than expected:
> https://dl.dropbox.com/u/560426/imagej/contour_imagej.png

The overlaid contour will look better if you place the polygon vertices at pixel centers by adding 0.5 to the x and y coordinates of the contour pixels.

-wayne

  float[] x = {3,4,5,6,7,8,9,10,11,11,11,10,9,8,7,6,5,4,3,2,2,2,3};
  float[] y = {1,1,2,2,2,1,1,2,2,3,4,4,5,5,4,4,4,5,5,4,3,2,1};
  n = x.length;
  for (i=0; i<n; i++) {
     x[i] += 0.5f;
     y[i] += 0.5f;
  }
  roi = new PolygonRoi(x, y, n, Roi.POLYGON);
  imp = IJ.getImage();
  imp.setRoi(roi);

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