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 theMat structure. In iOS however, to render an image on screen it have to be an instance of theUIImage class. To convert an OpenCV Mat to an UIImage we use theCore 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 theif 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 .