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# OpenCV iOS - Image Processing

## Goal

In this tutorial we will learn how to do basic image processing using OpenCV in iOS.

## Introduction

In OpenCV all the image processing operations are usually carried out on the Mat structure. In iOS however, to render an image on screen it have to be an instance of the UIImage class. To convert an OpenCV Mat to an UIImage we use the Core Graphics framework available in iOS. Below is the code needed to covert back and forth between Mat’s and UIImage’s.

    - (cv::Mat)cvMatFromUIImage:(UIImage *)image
    {
      CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
      CGFloat cols = image.size.width;
      CGFloat rows = image.size.height;

      cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels (color channels + alpha)

      CGContextRef contextRef = CGBitmapContextCreate(cvMat.data,                 // Pointer to  data
                                                     cols,                       // Width of bitmap
                                                     rows,                       // Height of bitmap
                                                     8,                          // Bits per component
                                                     cvMat.step[0],              // Bytes per row
                                                     colorSpace,                 // Colorspace
                                                     kCGImageAlphaNoneSkipLast |
                                                     kCGBitmapByteOrderDefault); // Bitmap info flags

      CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
      CGContextRelease(contextRef);

      return cvMat;
    }

    - (cv::Mat)cvMatGrayFromUIImage:(UIImage *)image
    {
      CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
      CGFloat cols = image.size.width;
      CGFloat rows = image.size.height;

      cv::Mat cvMat(rows, cols, CV_8UC1); // 8 bits per component, 1 channels

      CGContextRef contextRef = CGBitmapContextCreate(cvMat.data,                 // Pointer to data
                                                     cols,                       // Width of bitmap
                                                     rows,                       // Height of bitmap
                                                     8,                          // Bits per component
                                                     cvMat.step[0],              // Bytes per row
                                                     colorSpace,                 // Colorspace
                                                     kCGImageAlphaNoneSkipLast |
                                                     kCGBitmapByteOrderDefault); // Bitmap info flags

      CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
      CGContextRelease(contextRef);

      return cvMat;
     }

After the processing we need to convert it back to UIImage. The code below can handle both gray-scale and color image conversions (determined by the number of channels in the if statement).

    cv::Mat greyMat;
    cv::cvtColor(inputMat, greyMat, CV_BGR2GRAY);

After the processing we need to convert it back to UIImage.

    -(UIImage *)UIImageFromCVMat:(cv::Mat)cvMat
    {
      NSData *data = [NSData dataWithBytes:cvMat.data length:cvMat.elemSize()*cvMat.total()];
      CGColorSpaceRef colorSpace;

      if (cvMat.elemSize() == 1) {
          colorSpace = CGColorSpaceCreateDeviceGray();
      } else {
          colorSpace = CGColorSpaceCreateDeviceRGB();
      }

      CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)data);

      // Creating CGImage from cv::Mat
      CGImageRef imageRef = CGImageCreate(cvMat.cols,                                 //width
                                         cvMat.rows,                                 //height
                                         8,                                          //bits per component
                                         8 * cvMat.elemSize(),                       //bits per pixel
                                         cvMat.step[0],                            //bytesPerRow
                                         colorSpace,                                 //colorspace
                                         kCGImageAlphaNone|kCGBitmapByteOrderDefault,// bitmap info
                                         provider,                                   //CGDataProviderRef
                                         NULL,                                       //decode
                                         false,                                      //should interpolate
                                         kCGRenderingIntentDefault                   //intent
                                         );

      // Getting UIImage from CGImage
      UIImage *finalImage = [UIImage imageWithCGImage:imageRef];
      CGImageRelease(imageRef);
      CGDataProviderRelease(provider);
      CGColorSpaceRelease(colorSpace);

      return finalImage;
     }

## Output

Check out an instance of running code with more Image Effects on YouTube .

## Help and Feedback

You did not find what you were looking for?

- Ask a question on the Q&A forum.
- If you think something is missing or wrong in the documentation, please file a bug report.

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