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 convert 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, COLOR_BGR2GRAY);

cv::cvtColor

void cvtColor(InputArray src, OutputArray dst, int code, int dstCn=0)

Converts an image from one color space to another.

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 .


Original author	Charu Hans
Compatibility	OpenCV >= 3.0

Table of Contents

Goal

Introduction

Output