Question

I need to implement SURF algorithm in objc iOS.
I have searched on openCV and also tried to implement following examples

jonmarimba and ishai jaffe

The examples are not working and I need to make any one of them work so atleast I can get relieved that yes SURF can work on iOS as well. I have tried to build from scratch but I am totally FUSED with SHORT CIRCUIT.

I am trying to use openCV 2.4.2 in jonmarimba’s example.

And also trying to use iOS5.1.1 with Xcode 4.3

Answer 1

First of all: Go with OpenCVs C++-interface. Objective-C is a strict super set of C, so you can just use it.

To get a grip on the topic take a look at OpenCVs official docs and the example code about Feature Description.

The next step is to grab a copy of the current OpenCV version for iOS. As of version 2.4.2 OpenCV has official iOS-support and you just need the opencv2.framework.

To convert an UIImage to a cv::Mat use this function:

static UIImage* MatToUIImage(const cv::Mat& m) {
    CV_Assert(m.depth() == CV_8U);
    NSData *data = [NSData dataWithBytes:m.data length:m.elemSize()*m.total()];
    CGColorSpaceRef colorSpace = m.channels() == 1 ?
        CGColorSpaceCreateDeviceGray() : CGColorSpaceCreateDeviceRGB();
    CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)data);
    // Creating CGImage from cv::Mat
    CGImageRef imageRef = CGImageCreate(m.cols, m.cols, m.elemSize1()*8, m.elemSize()*8,
        m.step[0], colorSpace, kCGImageAlphaNoneSkipLast|kCGBitmapByteOrderDefault,
        provider, NULL, false, kCGRenderingIntentDefault);
    UIImage *finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef); CGDataProviderRelease(provider);
    CGColorSpaceRelease(colorSpace); return finalImage;
}

… and vice-versa:

static void UIImageToMat(const UIImage* image, cv::Mat& m) {
    CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
    CGFloat cols = image.size.width, rows = image.size.height;
    m.create(rows, cols, CV_8UC4); // 8 bits per component, 4 channels
    CGContextRef contextRef = CGBitmapContextCreate(m.data, m.cols, m.rows, 8,
        m.step[0], colorSpace, kCGImageAlphaNoneSkipLast | kCGBitmapByteOrderDefault);
    CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);
    CGContextRelease(contextRef); CGColorSpaceRelease(colorSpace);
}

The rest of the work you have to do is plain OpenCV Stuff. So grab you a coffee and start working.

If you need some “inspiration” take a look at this repo gsoc2012 – /ios/trunk It’s dedicated to OpenCV + iOS.