Question

There’s an article describing how to do this here, that seems to have worked for other people but it does not compile for me.

Here’s a copy of the .h file that was used:

//
//  NSImage+OpenCV.h
//

#import <AppKit/AppKit.h>

@interface NSImage (NSImage_OpenCV) {

}

+(NSImage*)imageWithCVMat:(const cv::Mat&)cvMat;
-(id)initWithCVMat:(const cv::Mat&)cvMat;

@property(nonatomic, readonly) cv::Mat CVMat;
@property(nonatomic, readonly) cv::Mat CVGrayscaleMat;

@end

I’m on Xcode 4.4, using openCV 2.4.2. The compiler errors I’m getting for the header file are 4x of the following:

Semantic issue: Use of undeclared identifier 'cv'

This error seems rather obvious…the header file does not define what a cv::Mat is.
So I took a guess from looking at the OpenCV 2.4 tutorial, that I needed to add

#include <opencv2/core/core.hpp>

This generated 20 other errors, where the compiler was complaining about the core.hpp file.
The first of which says:

Semantic issue: Non-const static data member must be initialized out of line

So my question is what am I doing wrong? How do I get this code to work?

Answer 1

Another stackoverflow Q&A (link: How to include OpenCV in Cocoa Application?) had the missing piece on the undefined symbols.

To summarize:

1. The OpenCV headers have to be included before the Cocoa headers, due to a macro conflict.
2. Xcode automatically includes the cocoa headers at the top of every objective-C file (*.m, *.mm), which prevents you from adding the OpenCV headers first, as stated in point 1.
3. Xcode has a “-Prefix.pch” file that defines any code which is automatically prepended to any code.

Normally, it looks like:

#ifdef __OBJC__
    #import <Cocoa/Cocoa.h>
#endif

Instead, the file should be changed to look like:

#ifdef __cplusplus
#import <opencv2/opencv.hpp>
#endif

#ifdef __OBJC__
    #import <Cocoa/Cocoa.h>
#endif

So in the case of any *.cpp file, the opencv header gets added automatically. In the case of any *.m file, the Cocoa headers are added. And in the special case of *.mm files, they BOTH get added, and in the correct order.

Kudos to Ian Charnas and user671435 for figuring this out.