Images being rotated when converted from Matrix

Question

I am working on a app and making use of the opencv library.

The problem I am having happens only to certain images (usually if made with the phone's camera) and I pinpointed as being just a conversion problem. When I convert a (problematic) Image to a cv::Mat object and then back it just rotates 90 degrees.

Here is the call that causes the problem:

cv::Mat tmpMat = [sentImage CVMat];

UIImage * tmpImage = [[UIImage alloc] initWithCVMat:tmpMat];

[imageHolder setImage: tmpImage];

And here are the functions that do the conversion from image to matrix and vice-versa.

-(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;
}


- (id)initWithCVMat:(const cv::Mat&)cvMat
{
NSData *data = [NSData dataWithBytes:cvMat.data length:cvMat.elemSize() * cvMat.total()];

CGColorSpaceRef colorSpace;

if (cvMat.elemSize() == 1)
{
    colorSpace = CGColorSpaceCreateDeviceGray();
}
else
{
    colorSpace = CGColorSpaceCreateDeviceRGB();
}

CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)data);

CGImageRef imageRef = CGImageCreate(cvMat.cols,                                     // Width
                                    cvMat.rows,                                     // Height
                                    8,                                              // Bits per component
                                    8 * cvMat.elemSize(),                           // Bits per pixel
                                    cvMat.step[0],                                  // Bytes per row
                                    colorSpace,                                     // Colorspace
                                    kCGImageAlphaNone | kCGBitmapByteOrderDefault,  // Bitmap info flags
                                    provider,                                       // CGDataProviderRef
                                    NULL,                                           // Decode
                                    false,                                          // Should interpolate
                                    kCGRenderingIntentDefault);                     // Intent

self = [self initWithCGImage:imageRef];
CGImageRelease(imageRef);
CGDataProviderRelease(provider);
CGColorSpaceRelease(colorSpace);

return self;
}

Now I am using a "Aspect Fill" property in my imageHolder (a UIImageView) and tried changing it without success. I also tried seeing if it was a problem of a matrix being transposed on the conversion and tried to change without success and it also would not be logical since it does not turn every picture.

I do not understand why it works with some pictures but other not (all photos taken with the phone's camera don't work).

If anyone can shed a light on the matter I would appreciate.

My suggestion would be to check tmpMat straight after populating it with image data. See if it has the same orientation for both normal and phone camera images using imshow(). Also check the enumeration types of your matrix for any differences using tmpMat.type(). If that doesn't reveal anything, my guess is that the way phone stores image data is probably column-major rather than row-major — DevGoldm, Sep 06 '13 at 16:01

Olotiar · Accepted Answer · 2013-09-08T12:36:22.010

Images from the camera that are taken with different orientations (Portrait / Landscape) are saved in the same resolution (same number of rows and columns) by the iPhone camera. The difference is that the JPEG contains a flag (to be precise, the Exif.Image.Orientation flag) to tell the displaying software how the image needs to be rotated to be displayed correctly.

My guess is that OpenCV looses that information (that is stored in the UIImage in the imageOrientation property) when converting, so when the image is converted back to UIImage this piece of information is set to default (UIImageOrientationUp), explaining why certain images appear to be rotated.

score 1 · Answer 2 · edited May 23 '17 at 10:29

I was having the same issue. This solves the problem of the image rotating when converting from UIImage to cvMat. Add the method at the bottom, call it after you dismiss the picker controller. It is the 'second answer' located here: Rotating a CGImage

Also, there are two methods in the ios.h for UIImage to cvMat and vice versa, that you can just include. highgui/ios.h. Then add the rotation method and you are good to go.

Images being rotated when converted from Matrix

2 Answers2