Using OpenCV in Swift iOS

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.