OpenCV iOS - Image Processing {#tutorial_image_manipulation}
=============================

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.
@code{.m}
- (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;
}
@endcode
@code{.m}
- (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;
 }
@endcode
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).
@code{.m}
cv::Mat greyMat;
cv::cvtColor(inputMat, greyMat, COLOR_BGR2GRAY);
@endcode
After the processing we need to convert it back to UIImage.
@code{.m}
-(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;
 }
@endcode

Output
--------

![](images/output.jpg)

Check out an instance of running code with more Image Effects on
[YouTube](http://www.youtube.com/watch?v=Ko3K_xdhJ1I) .

@youtube{Ko3K_xdhJ1I}
