Get all the points in triangular ROI in a xy plane

Question

Input: I have some 50000 points in a xy plane as shown in the below picture.

Now, I need to get all the possible points in a triangular ROI. How to get it. It can can be opencv or Matlab.

The below is the sample where I need to get the possible points of the triangular areas.

enter image description here

Do you select these interactively or just have the triangles? — bla, Feb 27 '13 at 05:31
Interactively.. I have the (x y) of the vertices of the triangular ROI. — 2vision2, Feb 27 '13 at 05:44

Kavka · Accepted Answer · 2013-02-27T05:51:38.587

5

MATLAB has an inpolygon command: inpolygon.

For example, this code

 xv = [0.1 0.4 0.15 0.1]; yv = [0 0.4 0.8 0];
 x = rand(250,1); y = rand(250,1);
 in = inpolygon(x,y,xv,yv);
 plot(xv,yv,x(in),y(in),'r+',x(~in),y(~in),'bo')

generates a picture like this:

enter image description here

edited Feb 27 '13 at 05:51

answered Feb 27 '13 at 05:40

Kavka

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Is there a similar function in OpenCV...?? – 2vision2 Feb 27 '13 at 05:46
@ Kavka :: Thanks for the example... Can you let me know OpenCV equivalent of inpolygon... I can accept the answer... – 2vision2 Feb 27 '13 at 05:53
Sorry, I don't know OpenCV. – Kavka Feb 27 '13 at 05:53
2

@2vision2 didn't you ask "It can can be opencv or Matlab." :) – iiro Feb 27 '13 at 05:55

iiro · Answer 2 · 2013-02-27T07:39:31.277

3

Something like this, in c++?

Mat plane = imread("plane.png"); // this is the 50 000 pixel plane image

// I assume your triangles are here. E.e. findContours function would save them here, but I don't know your situation.
vector<vector<Point> > triangles;

// this size is something like 1000, so here are only points that should be checkedd if they are inside some triangle
    vector<Point> points; 

// lets loop all rois and check if point is inside one of them
for (int j = 0; j < triangles.size(); j++) {
    for (int i = 0; i < points.size(); i++) {
     double test = pointPolygonTest(trigangles[j], points[i] false);
     if (test < 0) {
      cout << " point " << points[i] << " is outside of triangle " << j << endl;
     } else if (test > 0) {
      cout << " point " << points[i] << " is inside of triangle " << j << endl;
     } else if (test == 0) {
      cout << " point " << points[i] << " is on edge of triangle " << j << endl;
     }
    }
}

for more info: http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=pointpolygontest#pointpolygontest

here is OpenCV's example: http://docs.opencv.org/trunk/doc/tutorials/imgproc/shapedescriptors/point_polygon_test/point_polygon_test.html

edited Feb 27 '13 at 07:39

answered Feb 27 '13 at 06:00

iiro

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Can we do it without a for loop?? I have 50,000 points in the plane – 2vision2 Feb 27 '13 at 06:07
Is amount of points _inside_ triangels 50 000 or in whole image? – iiro Feb 27 '13 at 06:09
no..50,000 points in the plane and may be some 1000 points in each triangle – 2vision2 Feb 27 '13 at 06:09
Ya..I got the pointPolygonTest..can you comment on -- if I can avoid the for loop for the 50k points ....and Thanks again – 2vision2 Feb 27 '13 at 06:13
you can't change the fact, that you'll have to look at all 50000 points, to see if they're "in" or "out". so having a function that hides the loop or not does not make a difference. (ok, cleaner code. but..) – berak Feb 27 '13 at 08:01
That's true. Well i don't know if asker knows triangle positions or not. If he know them he doesn't need to loop 50 000 points. Only points he want to check if one point is inside a triangle or not. – iiro Feb 27 '13 at 08:04
@break :: Correct ..But like in Matlab there might be a possibility to deal in one line for all the points... like in the answer given by kavka – 2vision2 Feb 27 '13 at 08:04
@iiro :: I know the position (x y) of the triangles..Now for each triangle...I have to check which of the 50k points lie inside the triangle... – 2vision2 Feb 27 '13 at 08:07

score 1 · Answer 3 · answered Feb 27 '13 at 16:39

In OpenCV, you can quickly filter out the points that are not in the minimal bounding rectangle for each triangle. This rectangle can be previously computed by hand or with cv::boundingRect(). The hit test is done with Rect::contains(). This is a fast operation (at least much faster than cv::pointPolygonTest)and this will filter out the points that are obviously not in any triangle. Afterward, you test the points that pass the filter with cv::pointPolygonTest().

That is:

std::vector<cv::Point> pointList;
std::vector<cv::Rect> rectList;
std::vector<std::vector<Point>> triangleList;

for (int pointIndex = 0; pointIndex < pointList.size(); pointIndex++)
{
  for (int rectIndex = 0; rectIndex < rectList.size(); rectIndex++)
  {
    if (!rectList[rectIndex].contains(pointList[pointIndex]))
      continue;

    if (cv::pointPolygonTest(triangleList[rectIndex], pointList[pointIndex], false) < 0)
      continue;

    /* TODO Mark the point for future use */
  }
}

Get all the points in triangular ROI in a xy plane

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