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I'm a new in OpenCV. I have some issues with image converting. I create a new project. It has 2 more files Wrapper.h/mm and UIImage+OpenCV.h/.mm. The following code is here:

//UIImage+OpenCV.h
#import <UIKit/UIKit.h>

@interface UIImage (OpenCV)

//cv::Mat to UIImage
+ (UIImage *)imageWithCVMat:(const cv::Mat&)cvMat;
+ (UIImage *)imageWithCVMat:(const cv::Mat&)cvMat
                orientation:(UIImageOrientation)orientation;

- (id)initWithCVMat:(const cv::Mat&)cvMat
        orientation:(UIImageOrientation)orientation;

//UIImage to cv::Mat
- (cv::Mat)CVMat;
- (cv::Mat)CVMat3;  // no alpha channel
- (cv::Mat)CVGrayscaleMat;

@end


//UIImage+OpenCV.mm
#import "UIImage+OpenCV.h"

@implementation UIImage (OpenCV)

-(cv::Mat)CVMat
{
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(self.CGImage);
    CGFloat cols = self.size.width;
    CGFloat rows = self.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), self.CGImage);
    CGContextRelease(contextRef);



    return cvMat;
}

- (cv::Mat)CVMat3
{
    cv::Mat result = [self CVMat];
    cv::cvtColor(result , result , CV_RGBA2RGB);
    return result;

}

-(cv::Mat)CVGrayscaleMat
{
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    CGFloat cols = self.size.width;
    CGFloat rows = self.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
                                                    kCGImageAlphaNone |
                                                    kCGBitmapByteOrderDefault); // Bitmap info flags

    CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), self.CGImage);
    CGContextRelease(contextRef);
    CGColorSpaceRelease(colorSpace);

    return cvMat;
}

+ (UIImage *)imageWithCVMat:(const cv::Mat&)cvMat
{
    return [[UIImage alloc] initWithCVMat:cvMat];
}

- (id)initWithCVMat:(const 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
    self = [self initWithCGImage:imageRef];
    CGImageRelease(imageRef);
    CGDataProviderRelease(provider);
    CGColorSpaceRelease(colorSpace);

    return self;
}

@end

Above code I took here

//Wrapper.h
#import <Foundation/Foundation.h>

@interface CVWrapper : NSObject
+(UIImage*) returnPic: (UIImage*)image;
@end

//Wrapper.mm
#import "CVWrapper.h"
#import "UIImage+OpenCV.h"

@implementation CVWrapper

+(UIImage*)returnPic:(UIImage *)image
{
    UIImage *result = nil;
    if (image)
    {
        cv::Mat matImage = [image CVMat];
        cv::Mat greyMat;                      
        cv::cvtColor(matImage, greyMat, CV_RGBA2GRAY);  //It doesn't work
        result = [UIImage imageWithCVMat:greyMat];

    }

    return result;
}

@end

In ViewController I call the function that displays an image. I try to understand a basic concepts. How can i convert a picture in a grey color? Where i have to do it?