node-opencv/examples/detect-shapes.js

// Detects triangles and quadrilaterals
var cv = require('../lib/opencv');

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

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/shapes.jpg', function(err, im) {
  if (err) throw err;

  width = im.width()
  height = im.height()
  if (width < 1 || height < 1) throw new Error('Image has no size');

  var out = new cv.Matrix(height, width);
  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++) {

    if (contours.area(i) < minArea) continue;

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

    switch(contours.cornerCount(i)) {
      case 3:
        out.drawContour(contours, i, GREEN);
        break;
      case 4:
        out.drawContour(contours, i, RED);
        break;
      default:
        out.drawContour(contours, i, WHITE);
    }
  }

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