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

问题

I will need to identify, rotate and crop a rectangle(business card) from a photo took from an iphone. I believe this can be done by using OpenCV, but I have not used it before. Could anyone give some tips on this?

回答1:

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

回答2:

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

回答3:

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

回答4:

If you know the approximate height and width of the rectangle then you can do the following steps:

• Object detection: component labeling using contour tracing.
• After all the objects have been labeled then filter the detected components by calculating the perimeter of the rectangle. P=2*(H+W) Any component having the size of less than or greater than P is ignored, only the components closer to P in size is retained.
• Find corner points of the rectangle from the contour points.
• Extract the rectangle from the original image.

All of the above steps can be done using either OpenCV or Aforge.Net, I have personally done it using Aforge.Net.

回答5:

Depending on the viewpoint, the card may end up not being a rectangle but more of a trapezoid. You can use HoughLines2 in OpenCV to identify the edges in the image and try to identify the 4 edges that are most likely to be the edges of the business card.

回答6:

I think goodFeaturesToTrack() is easier to use for Harris-Corner detection. CornerHarris() needs to set outout image to be specific type.

来源：https://stackoverflow.com/questions/6134109/how-do-i-find-a-rectangle-in-a-photo-using-iphone