How do I find a rectangle in a photo using iphone?

From iOS 8, you can now use the new detector CIDetectorTypeRectangle that returns CIRectangleFeature. You can add options :

• Accuracy : low/hight

• Aspect ratio : 1.0 if you want to find only squares for example

  CIImage *image = // your image
  NSDictionary *options = @{CIDetectorAccuracy: CIDetectorAccuracyHigh, CIDetectorAspectRatio: @(1.0)};
  CIDetector *rectangleDetector = [CIDetector detectorOfType:CIDetectorTypeRectangle context:nil options:options];
  
  NSArray *rectangleFeatures = [rectangleDetector featuresInImage:image];
  
  for (CIRectangleFeature *rectangleFeature in rectangleFeatures) {
      CGPoint topLeft = rectangleFeature.topLeft;
      CGPoint topRight = rectangleFeature.topRight;
      CGPoint bottomLeft = rectangleFeature.bottomLeft;
      CGPoint bottomRight = rectangleFeature.bottomRight;
  }
  

More information in WWDC 2014 Advances in Core Image, Session 514.

An example from shinobicontrols.com

See opencv sample code OpenCV2.2\samples\cpp\squares.cpp. They do the following:

• Detect edges using Canny

• Retrieve contours by findContours

• For each contour

  • approximate contour using approxPolyDP to decrease number of vertices

  • if contour has 4 vertices and each angle is ~90 degrees then yield a rectangle

To get you started, you should look at the feature detection api of OpenCV. Especially

• cv::Canny (for edge detection),
• maybe cv::cornerHarris (for corner detection),
• and cv::HoughLines (for finding straight lines in the edge image).

HTH