iOS Face detection transformation - iphone

Question

I have followed a tutorial to detect a face within an image, it works. It creates a red rectangle around the face by making a UIView *faceView. Now i am trying to obtain the coordinates of the face detected however the results returned are off slightly in the y-axis. How can i fix this? where am i going wrong.
This is what i have attempted :
CGRect newBounds = CGRectMake(faceFeature.bounds.origin.x,
imageView.bounds.size.height - faceFeature.bounds.origin.y - faceFeature.bounds.size.height,
faceFeature.bounds.size.width,
faceFeature.bounds.size.height);
This is the source code for the detection :
-
(void)markFaces:(UIImageView *)facePicture
{
// draw a CI image with the previously loaded face detection picture
CIImage* image = [CIImage imageWithCGImage:facePicture.image.CGImage];
// 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];
// we'll iterate through every detected face. CIFaceFeature provides us
// with the width for the entire face, and the coordinates of each eye
// and the mouth if detected. Also provided are BOOL's for the eye's and
// mouth so we can check if they already exist.
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];
// add a border around the newly created UIView
faceView.layer.borderWidth = 1;
faceView.layer.borderColor = [[UIColor redColor] CGColor];
CGRect newBounds = CGRectMake(faceFeature.bounds.origin.x,
imageView.bounds.size.height - faceFeature.bounds.origin.y - faceFeature.bounds.size.height,
faceFeature.bounds.size.width,
faceFeature.bounds.size.height);
NSLog(#"My view frame: %#", NSStringFromCGRect(newBounds));
[self.view addSubview:faceView];
if(faceFeature.hasLeftEyePosition)
{
}
if(faceFeature.hasRightEyePosition)
{
}
if(faceFeature.hasMouthPosition)
{
}
}
}
-(void)faceDetector
{
// Load the picture for face detection
UIImageView* image = [[UIImageView alloc] initWithImage:[UIImage imageNamed:#"jolie.jpg"]];
// Draw the face detection image
[self.view addSubview:image];
// flip image on y-axis to match coordinate system used by core image
[image setTransform:CGAffineTransformMakeScale(1, -1)];
// flip the entire window to make everything right side up
[self.view setTransform:CGAffineTransformMakeScale(1, -1)];
// Execute the method used to markFaces in background
[self performSelectorInBackground:#selector(markFaces:) withObject:image];
}

Answer 1

CoreImage Coordination system and UIKit coordination system are quite different. CIFaceFeature provides coordinates in coreimage coordination system and for your work you need to convert them into uikit coordinate system:
// CoreImage coordinate system origin is at the bottom left corner and UIKit is at the top left corner
// So we need to translate features positions before drawing them to screen
// In order to do so we make an affine transform
// **Note**
// Its better to convert CoreImage coordinates to UIKit coordinates and
// not the other way around because doing so could affect other drawings
// i.e. In the original sample project you see the image and the bottom, Isn't weird?
CGAffineTransform transform = CGAffineTransformMakeScale(1, -1);
transform = CGAffineTransformTranslate(transform, 0, -_pickerImageView.bounds.size.height);
for(CIFaceFeature* faceFeature in features)
{
// Translate CoreImage coordinates to UIKit coordinates
const CGRect faceRect = CGRectApplyAffineTransform(faceFeature.bounds, transform);
// create a UIView using the bounds of the face
UIView* faceView = [[UIView alloc] initWithFrame:faceRect];
faceView.layer.borderWidth = 1;
faceView.layer.borderColor = [[UIColor redColor] CGColor];
// get the width of the face
CGFloat faceWidth = faceFeature.bounds.size.width;
// add the new view to create a box around the face
[_pickerImageView addSubview:faceView];
if(faceFeature.hasLeftEyePosition)
{
// Get the left eye position: Translate CoreImage coordinates to UIKit coordinates
const CGPoint leftEyePos = CGPointApplyAffineTransform(faceFeature.leftEyePosition, transform);
// Note1:
// If you want to add this to the the faceView instead of the imageView we need to translate its
// coordinates a bit more {-x, -y} in other words: {-faceFeature.bounds.origin.x, -faceFeature.bounds.origin.y}
// You could do the same for the other eye and the mouth too.
// Create an UIView to represent the left eye, its size depend on the width of the face.
UIView* leftEyeView = [[UIView alloc] initWithFrame:CGRectMake(leftEyePos.x - faceWidth*EYE_SIZE_RATE*0.5f /*- faceFeature.bounds.origin.x*/, // See Note1
leftEyePos.y - faceWidth*EYE_SIZE_RATE*0.5f /*- faceFeature.bounds.origin.y*/, // See Note1
faceWidth*EYE_SIZE_RATE,
faceWidth*EYE_SIZE_RATE)];
leftEyeView.backgroundColor = [[UIColor magentaColor] colorWithAlphaComponent:0.3];
leftEyeView.layer.cornerRadius = faceWidth*EYE_SIZE_RATE*0.5;
//[faceView addSubview:leftEyeView]; // See Note1
[_pickerImageView addSubview:leftEyeView];
}
}