node-opencv/examples/quad-crosses.js

// Finds quadrilaterals and fills them with an X
var cv = require('../lib/opencv');

var lowThresh = 0;
var highThresh = 100;
var nIters = 2;
var minArea = 2000;
var maxArea = 100000;

var BLUE = [0, 255, 0]; //B, G, R
var RED   = [0, 0, 255]; //B, G, R
var GREEN = [0, 255, 0]; //B, G, R
var WHITE = [255, 255, 255]; //B, G, R

cv.readImage('./files/quads.jpg', function(err, im) {
  if (err) throw err;
  if (im.width() < 1 || im.height() < 1) throw new Error('Image has no size');

  var out = im.copy();

  im.convertGrayscale();
  im_canny = im.copy();

  im_canny.canny(lowThresh, highThresh);
  im_canny.dilate(nIters);

  contours = im_canny.findContours();

  for (i = 0; i < contours.size(); i++) {

    var area = contours.area(i);
    if (area < minArea || area > maxArea) continue;

    var arcLength = contours.arcLength(i, true);
    contours.approxPolyDP(i, 0.01 * arcLength, true);

    if (contours.cornerCount(i) != 4) continue;

    var points = [
      contours.point(i, 0),
      contours.point(i, 1),
      contours.point(i, 2),
      contours.point(i, 3)
    ]

    out.line([points[0].x,points[0].y], [points[2].x, points[2].y], RED);
    out.line([points[1].x,points[1].y], [points[3].x, points[3].y], RED);
  }

  out.save('./tmp/quad-crosses.png');
  console.log('Image saved to ./tmp/quad-crosses.png');
});