how to convert from cvMat to UIImage in objective-c?

Question

I'm using the OpenCV framework with XCode and want to convert from cvMat or IplImage to UIImage, how to do that? Thanks.

Answer 1

Note: most implementations don't correctly handle an alpha channel or convert from OpenCV's BGR pixel format to iOS's RGB.

This will correctly convert from cv::Mat to UIImage:

+(UIImage *)UIImageFromCVMat:(cv::Mat)cvMat {
    NSData *data = [NSData dataWithBytes:cvMat.data length:image.step.p[0]*image.rows];

    CGColorSpaceRef colorSpace;
    CGBitmapInfo bitmapInfo;

    if (cvMat.elemSize() == 1) {
        colorSpace = CGColorSpaceCreateDeviceGray();
        bitmapInfo = kCGImageAlphaNone | kCGBitmapByteOrderDefault;
    } else {
        colorSpace = CGColorSpaceCreateDeviceRGB();
        bitmapInfo = kCGBitmapByteOrder32Little | (
            cvMat.elemSize() == 3? kCGImageAlphaNone : kCGImageAlphaNoneSkipFirst
        );
    }

    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
        bitmapInfo,                 // 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; 
}

And to convert from UIImage to cv::Mat:

+ (cv::Mat)cvMatWithImage:(UIImage *)image
{
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
    size_t numberOfComponents = CGColorSpaceGetNumberOfComponents(colorSpace);
    CGFloat cols = image.size.width;
    CGFloat rows = image.size.height;

    cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels
    CGBitmapInfo bitmapInfo = kCGImageAlphaNoneSkipLast | kCGBitmapByteOrderDefault;

    // check whether the UIImage is greyscale already
    if (numberOfComponents == 1){
        cvMat = cv::Mat(rows, cols, CV_8UC1); // 8 bits per component, 1 channels
        bitmapInfo = kCGImageAlphaNone | kCGBitmapByteOrderDefault;
    } 

    CGContextRef contextRef = CGBitmapContextCreate(cvMat.data,             // Pointer to backing data
                                                cols,                       // Width of bitmap
                                                rows,                       // Height of bitmap
                                                8,                          // Bits per component
                                                cvMat.step[0],              // Bytes per row
                                                colorSpace,                 // Colorspace
                                                bitmapInfo);              // Bitmap info flags

    CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
    CGContextRelease(contextRef);

    return cvMat;
}

Answer 2

From opencv 2.4.6 on this functionality is already included. Just include

opencv2/highgui/ios.h

In OpenCV 3 this include has changed to:

opencv2/imgcodecs/ios.h

And you can use these functions:

UIImage* MatToUIImage(const cv::Mat& image);
void UIImageToMat(const UIImage* image, cv::Mat& m, bool alphaExist = false);

Answer 3

Here is the correct method to convert a cv::Mat to a UIImage.

Every other implementation I've seen — including OpenCV's documentation — is incorrect: they do not correctly convert from OpenCV's BGR to iOS's RGB, and they do not consider the alpha channel (if one exists). See comments above bitmapInfo = ….

+(UIImage *)UIImageFromCVMat:(cv::Mat)cvMat {
    NSData *data = [NSData dataWithBytes:cvMat.data length:cvMat.elemSize()*cvMat.total()];

    CGColorSpaceRef colorSpace;
    CGBitmapInfo bitmapInfo;

    if (cvMat.elemSize() == 1) {
        colorSpace = CGColorSpaceCreateDeviceGray();
        bitmapInfo = kCGImageAlphaNone | kCGBitmapByteOrderDefault;
    } else {
        colorSpace = CGColorSpaceCreateDeviceRGB();
        // OpenCV defaults to either BGR or ABGR. In CoreGraphics land,
        // this means using the "32Little" byte order, and potentially
        // skipping the first pixel. These may need to be adjusted if the
        // input matrix uses a different pixel format.
        bitmapInfo = kCGBitmapByteOrder32Little | (
            cvMat.elemSize() == 3? kCGImageAlphaNone : kCGImageAlphaNoneSkipFirst
        );
    }

    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
        bitmapInfo,                 // 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; 
}