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

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; 
}