how to convert images from one color-space to another in IOS using OpenCV !

In OpenCV all the image processing operations are usually carried out on the Mat structure. In iOS however, to render an image on screen it have to be an instance of the UIImage class. To convert an OpenCV Mat to an UIImage we use the Core Graphics framework available in iOS

how to convert images from one color-space to another, like BGR \leftrightarrow Gray, BGR \leftrightarrow HSV etc

 

Changing Color-space

There are more than 150 color-space conversion methods available in OpenCV. But we will look into only two which are most widely used ones, BGR \leftrightarrow Gray and BGR \leftrightarrow HSV.

For color conversion, we use the function cv2.cvtColor(input_image, flag) where flag determines the type of conversion.

For BGR \rightarrow Gray conversion we use the flags cv2.COLOR_BGR2GRAY. Similarly for BGR \rightarrow HSV, we use the flag cv2.COLOR_BGR2HSV.

 

- (cv::Mat)cvMatFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols = image.size.width;
CGFloat rows = image.size.height;

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

CGContextRef contextRef = CGBitmapContextCreate(cvMat.data, // Pointer to 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), image.CGImage);
CGContextRelease(contextRef);

return cvMat;
}

 

- (cv::Mat)cvMatGrayFromUIImage:(UIImage *)image
{
CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);
CGFloat cols = image.size.width;
CGFloat rows = image.size.height;

cv::Mat cvMat(rows, cols, CV_8UC1); // 8 bits per component, 1 channels

CGContextRef contextRef = CGBitmapContextCreate(cvMat.data, // Pointer to 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), image.CGImage);
CGContextRelease(contextRef);

return cvMat;
}

 

After the processing we need to convert it back to UIImage. The code below can handle both gray-scale and color image conversions

cv::Mat greyMat;
cv::cvtColor(inputMat, greyMat, CV_BGR2GRAY);

 

After the processing we need to convert it back to UIImage.

-(UIImage *)UIImageFromCVMat:(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);

// Creating CGImage from cv::Mat
CGImageRef imageRef = CGImageCreate(cvMat.cols, //width
cvMat.rows, //height
8, //bits per component
8 * cvMat.elemSize(), //bits per pixel
cvMat.step[0], //bytesPerRow
colorSpace, //colorspace
kCGImageAlphaNone|kCGBitmapByteOrderDefault,// bitmap info
provider, //CGDataProviderRef
NULL, //decode
false, //should interpolate
kCGRenderingIntentDefault //intent
);



// Getting UIImage from CGImage
UIImage *finalImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
CGDataProviderRelease(provider);
CGColorSpaceRelease(colorSpace);

return finalImage;
}

Source: OpenCV