Using OpenCV in Swift iOS

After adding the OpenCV 2 framework in my xcode project, I tried searching for samlpes or tutorials for integration with swift.

Are there any good tutorials for the same?

OpenCV is a framework written in C++. Apple's reference tell us that

> You cannot import C++ code directly into Swift. Instead, create an Objective-C or C wrapper for C++ code.

so you cannot directly import and use OpenCV in a swift project, but this is actually not bad at all because you (need) continue to use the C++ syntax of the framework which is pretty well documented all over the net.

So how do you proceed?

1. Create a new Objective-C++ class (.h, .mm) for calling C++ OpenCV

OpenCVWrapper.h

#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>

@interface OpenCVWrapper : NSObject

    + (UIImage *)processImageWithOpenCV:(UIImage*)inputImage;

@end

OpenCVWrapper.mm (use the File -> New... Wizard for Objective-C and rename the .m file to .mm)

#include "OpenCVWrapper.h"
#import "UIImage+OpenCV.h" // See below to create this

#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

@implementation OpenCVWrapper : NSObject

    + (UIImage *)processImageWithOpenCV:(UIImage*)inputImage {
        Mat mat = [inputImage CVMat];
        
        // do your processing here
        ...
        
        return [UIImage imageWithCVMat:mat];
    }

@end

As an alternative to creating new classes such as the example OpenCVWrapper.h/mm you can use Objective-C categories to extend existing Objective-C classes with OpenCV functionality. For instance UIImage+OpenCV category:

UIImage+OpenCV.h

#import <UIKit/UIKit.h>
#import <opencv2/opencv.hpp>

@interface UIImage (OpenCV)

    //cv::Mat to UIImage
+ (UIImage *)imageWithCVMat:(const cv::Mat&)cvMat;
- (id)initWithCVMat:(const cv::Mat&)cvMat;

    //UIImage to cv::Mat
- (cv::Mat)CVMat;
- (cv::Mat)CVMat3;  // no alpha channel
- (cv::Mat)CVGrayscaleMat;

@end

UIImage+OpenCV.mm

See https://github.com/foundry/OpenCVSwiftStitch/blob/master/SwiftStitch/UIImage%2BOpenCV.mm

2. Update the Bridging-Header to make all Objective-C++ classes you created available to Swift by importing our newly created wrappers (#import "OpenCVWrapper.h")

3. Use your wrapper in your Swift files:

   let image = UIImage(named: "image.jpeg")
   let processedImage = OpenCVWrapper.processImageWithOpenCV(image)
   

All Objective-C++ classes included in the bridge header are available directly from Swift.

I have a demo on github:

https://github.com/foundry/OpenCVSwiftStitch

It shows how to use the OpenCV stitcher from a Swift-based project. There is not a lot to the openCV side, it's really just a demo of how to put the various pieces together.

As Swift won't talk C++ directly, it uses a thin Objective-C++ wrapper to mediate between the iOS side and the openCV side.

Made in answer to these questions:
https://stackoverflow.com/questions/24042774/can-i-mix-swift-with-c-like-the-objective-c-mm-files/24045562#24045562
https://stackoverflow.com/questions/14062932/libraries-to-capture-panorama-in-ios-6/14064788#14064788