iOS - Issues cropping an image to a detected face - ios

Question

I am trying to crop a UIImage to a face that has been detected using the built-in CoreImage face detection functionality. I seem to be able to detect the face properly, but when I attempt to crop my UIImage to the bounds of the face, it is nowhere near correct. My face detection code looks like this:
-(NSArray *)facesForImage:(UIImage *)image {
CIImage *ciImage = [CIImage imageWithCGImage:image.CGImage];
CIContext *context = [CIContext contextWithOptions:nil];
NSDictionary *opts = #{CIDetectorAccuracy : CIDetectorAccuracyHigh};
CIDetector *detector = [CIDetector detectorOfType:CIDetectorTypeFace context:context options:opts];
NSArray *features = [detector featuresInImage:ciImage];
return features;
}
...and the code to crop the image looks like this:
-(UIImage *)imageCroppedToFaceAtIndex:(NSInteger)index forImage:(UIImage *)image {
NSArray *faces = [self facesForImage:image];
if((index < 0) || (index >= faces.count)) {
DDLogError(#"Invalid face index provided");
return nil;
}
CIFaceFeature *face = [faces objectAtIndex:index];
CGRect faceBounds = face.bounds;
CGImageRef imageRef = CGImageCreateWithImageInRect(image.CGImage, faceBounds);
UIImage *croppedImage = [UIImage imageWithCGImage:imageRef];
return croppedImage;
}
I have an image with only 1 face in it I'm using for testing, and it appears to detect it with no problem. But the crop is way off. Any idea what could be the problem with this code?

Answer 1

For anyone else having a similar issue -- transforming CGImage coordinates to UIImage coordinates -- I found this great article explaining how to use CGAffineTransform to accomplish exactly what I was looking for.

Answer 2

The code to convert the face geometry from Core Image to UIImage coordinates is fussy. I haven't messed with it in quite a while, but I remember it giving me fits, especially when dealing with images that are rotated.
I suggest looking at the demo app "SquareCam", which you can find with a search of the Xcode docs. It draws red squares around faces, which is a good start.
Note that the rectangle you get from Core Image is always a square, and sometimes crops a little too closely. You may have to make your cropping rectangles taller and wider.

Answer 3

This class does the trick! A quite flexible and handy override of UIImage. https://github.com/kylestew/KSMagicalCrop

Answer 4

Do It with this code It is worked for me.
CIImage* image = [CIImage imageWithCGImage:facePicture.image.CGImage];
//Container for the face attributes
UIView* faceContainer = [[UIView alloc] initWithFrame:facePicture.frame];
// flip faceContainer on y-axis to match coordinate system used by core image
[faceContainer setTransform:CGAffineTransformMakeScale(1, -1)];
// create a face detector - since speed is not an issue we'll use a high accuracy
// detector
CIDetector* detector = [CIDetector detectorOfType:CIDetectorTypeFace
context:nil options:[NSDictionary dictionaryWithObject:CIDetectorAccuracyHigh forKey:CIDetectorAccuracy]];
// create an array containing all the detected faces from the detector
NSArray* features = [detector featuresInImage:image];
for(CIFaceFeature* faceFeature in features)
{
// get the width of the face
CGFloat faceWidth = faceFeature.bounds.size.width;
// create a UIView using the bounds of the face
UIView* faceView = [[UIView alloc] initWithFrame:faceFeature.bounds];
if(faceFeature.hasLeftEyePosition)
{
// create a UIView with a size based on the width of the face
leftEyeView = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.leftEyePosition.x-faceWidth*0.15, faceFeature.leftEyePosition.y-faceWidth*0.15, faceWidth*0.3, faceWidth*0.3)];
}
if(faceFeature.hasRightEyePosition)
{
// create a UIView with a size based on the width of the face
RightEyeView = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.rightEyePosition.x-faceWidth*0.15, faceFeature.rightEyePosition.y-faceWidth*0.15, faceWidth*0.3, faceWidth*0.3)];
}
if(faceFeature.hasMouthPosition)
{
// create a UIView with a size based on the width of the face
mouth = [[UIView alloc] initWithFrame:CGRectMake(faceFeature.mouthPosition.x-faceWidth*0.2, faceFeature.mouthPosition.y-faceWidth*0.2, faceWidth*0.4, faceWidth*0.4)];
}
[view addSubview:faceContainer];
CGFloat y = view.frame.size.height - (faceView.frame.origin.y+faceView.frame.size.height);
CGRect rect = CGRectMake(faceView.frame.origin.x, y, faceView.frame.size.width, faceView.frame.size.height);
CGImageRef imageRef = CGImageCreateWithImageInRect([<Original Image> CGImage],rect);
croppedImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
//----cropped Image-------//
UIImageView *img = [[UIImageView alloc]initWithFrame:CGRectMake(faceView.frame.origin.x, y, faceView.frame.size.width, faceView.frame.size.height)];
img.image = croppedImage;