Question: I have a UIImage which is a pic captured from an iPhone Camera now I want the UIImage to be converted to cv::Mat (OpenCV). I am using the following lines of code to accomplish this :

```
-(cv::Mat)CVMat
{

CGColorSpaceRef colorSpace = CGImageGetColorSpace(self.CGImage);
CGFloat cols = self.size.width;
CGFloat rows = self.size.height;

cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels

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
                                                kCGImageAlphaNoneSkipLast |
                                                kCGBitmapByteOrderDefault); // Bitmap info flags

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

return cvMat;
}

```

This Code works fine for the UIImage in Landscape mode but when I use the same code with Images taken from Portrait mode the images gets rotated by 90 degrees towards the right.

I am a newbie in iOS and Objective C and hence I am not able to figure out what is wrong.

Can somebody tell me what is wrong with the code or am I missing out something.

Answer: 
This will be because the `UIImage` is not actually portrait. All photos taken with the iPhone camera are landscape in their raw bitmap state, eg 3264 wide x 2488 high. A "portrait" photo is displayed as such by the orientation EXIF flag set in the image, which is honoured, for example, by the photo library app which swivels images according to this flag and the viewing orientation of the camera.

The flag also affects how `UIImage` reports its width and height properties, transposing them from their bitmap values for images flagged as portrait. 

`cv::Mat` doesn't bother with any of that. This means that (i) when translating to `cv::Mat` a portrait image will have its `size.width` and `size.height` values transposed, and (ii) when translating back from `cv::Mat` you will have lost the orientation flag. 

The simplest way to handle this when going from `UIImage` to `cv::Mat` is to swap width and height values if the image is flagged as portrait:

```
if  (self.imageOrientation == UIImageOrientationLeft
      || self.imageOrientation == UIImageOrientationRight) {
         cols = self.size.height;
         rows = self.size.width;
    }

```

When translating back from `cv::Mat` to `UIImage`, you will want to reinstate the orientation flag. Assuming your `cv::Mat -> UIImage` code contains this:

```
self = [self initWithCGImage:imageRef];

```

you can use this method instead, and reset the orientation as per the original.

```
self = [self initWithCGImage:imageRef scale:1 orientation:orientation];

```
You should consider using native OpenCV functions to convert forth and back :

```
#import <opencv2/imgcodecs/ios.h>
...
UIImage* MatToUIImage(const cv::Mat& image);
void UIImageToMat(const UIImage* image,
                         cv::Mat& m, bool alphaExist = false);

```

Note: if your UIImage comes from the camera, you should 'normalize' it ( iOS UIImagePickerController result image orientation after upload) before converting to `cv::Mat` since OpenCV does not take into account Exif data. If you don't do that the result should be misoriented.