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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 done on Mat. iOS uses UIImage object to display image. One of the thing is to convert UIImage object to Mat object. Below is the code to convert UIImage to Mat.

    - (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

      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);
      CGColorSpaceRelease(colorSpace);

      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);
      CGColorSpaceRelease(colorSpace);

      return cvMat;
     }

Once we obtain the Mat Object. We can do all our processing on Mat object, similar to cpp. For example if we want to convert image to gray, we can do it via below code.

    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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