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I have been playing around with this now for a long time and I can't get the following code going for some reason.

+(UIImage *)processInvertedImage:(UIImage *)image 

 cv::Mat mat;
 UIImageToMat(image, mat); ///this is CV_8U from iPhone camera

 cv::Mat output;
 mat.convertTo(output, CV_32F); ///cv::invert() requires CV_32F!! (apparently)

 cv::Mat invert;
 cv::invert(output, invert); /// app crashes here, error message below

 cv::Mat gray;
 cv::cvtColor(invert, gray, CV_RGB2GRAY); 

 UIImage *binImg = MatToUIImage(gray);

 return binImg;

I am getting the following error code:

libc++abi.dylib: terminating with uncaught exception of type cv::Exception: OpenCV(3.4.2) /Volumes/build-storage/build/3_4_iOS-mac/opencv/modules/core/src/lapack.cpp:839: error: (-215:Assertion failed) type == 5 || type == 6 in function 'invert'

what does this type == 5 || type == 6 mean?

Let me try to answer this one for good. type == 5 || type == 6 means you need to work with CV_32F or CV_64F. However, UIImage usually has 4 channels. I bet your UIImageToMat yields a mat of type CV_8UC3 . So mat has type CV_8UC3, and output will most likely has type CV_32FC3, not just CV_32F. And again, when you use cv::invert(), you are looking at mathematical inverse, that is output * mat = identity_matrix.

+(UIImage *)processInvertedImage:(UIImage *)image 

 cv::Mat mat;
 UIImageToMat(image, mat); ///this is CV_8U from iPhone camera

 if (mat.type() == CV_8UC3) 
 NSLog(@"mat type is CV_8UC3"); // mostlikely this
 else if (mat.type() == CV_8UC4)
 NSLog(@"mat type is CV_8UC4");
 

 cv::Mat output;
 mat.convertTo(output, CV_32F); ///cv::invert() requires CV_32F!! (apparently)
 if (output.type() == CV_32FC3)
 NSLog(@"output type is CV_32FC3"); // most likely this 
 

 // this is really bad naming, since you seem to use namespace cv somewhere, 
 // as CV_32F doesn't have cv:: prefix
 cv::Mat invert; 
 cv::invert(output, invert); /// app crashes here, error message below

 cv::Mat gray;
 cv::cvtColor(invert, gray, CV_RGB2GRAY); 

 UIImage *binImg = MatToUIImage(gray);

 return binImg;

Edit: If you need image processing inverse, then you can do something similar to what you had in last question:

+(UIImage *)processInvertedImage:(UIImage *)image 
 cv::Mat mat;
 UIImageToMat(image, mat);

 // convert to RGB
 cv::Mat rgb
 cv::cvtColor(mat, rgb, COLOR_RGBA2RGB);

 // create mat of same size and type with values 255
 cv::Mat dummy(rgb.size(), rgb.type(), cv::Scalar(255,255,255));

 // three channels
 cv::Mat rgb_invert = dummy ^ rgb;

 cv::Mat gray;
 cv::cvtColor(rgb, gray, COLOR_RGB2GRAY);

 cv::Mat gray_invert = 255 ^ gray;

 // depending what invert you would like
 UIImage* binImg = MatToUIImage(rgb_invert);

 return binImg;