Translating CIDetector (face detection) results into UIImageView coordinates - ios

Question

I've been struggling to translate the CIDetector (face detection) results into coordinates relative to the UIImageView displaying the image so I can draw the coordinates using CGPaths.
I've looked at all questions here and all the tutorials I could find and most of them use small images that are not scaled when displayed in a UIImageView (example). The problem I am having is with using large images which are scaled using aspectFit when displayed in a UIImageView and determining the correct scale + translation values.
I am getting inconsistent results when testing with images of different sizes/aspect ratios, so I think my routine is flawed. I'd been struggling with this for a while so if anyone has some tips or can x-ray what I am doing wrong, that would be a great help.
What I am doing:
get the face coordinates
use the frameForImage routine below (found here on SO) to get the scale and bounds of the UIImageView image
create transform for scale + translation
apply transform to the CIDetector result
// my routine for determining transform values
NSDictionary* data = [self frameForImage:self.imageView.image inImageViewAspectFit:self.imageView];
CGRect scaledImageBounds = CGRectFromString([data objectForKey:#"bounds"]);
float scale = [[data objectForKey:#"scale"] floatValue];
CGAffineTransform transform = CGAffineTransformMakeScale(scale, -scale);
transform = CGAffineTransformTranslate(transform,
scaledImageBounds.origin.x / scale,
-(scaledImageBounds.origin.y / scale + scaledImageBounds.size.height / scale));
CIDetector results transformed using:
mouthPosition = CGPointApplyAffineTransform(mouthPosition, transform);
// example of bad result: scale seems incorrect
// routine below found here on SO for determining bound for image scaled in UIImageView using 'aspectFit`
-(NSDictionary*)frameForImage:(UIImage*)image inImageViewAspectFit:(UIImageView*)myImageView
{
float imageRatio = image.size.width / image.size.height;
float viewRatio = myImageView.frame.size.width / myImageView.frame.size.height;
float scale;
CGRect boundingRect;
if(imageRatio < viewRatio)
{
scale = myImageView.frame.size.height / image.size.height;
float width = scale * image.size.width;
float topLeftX = (myImageView.frame.size.width - width) * 0.5;
boundingRect = CGRectMake(topLeftX, 0, width, myImageView.frame.size.height);
}
else
{
scale = myImageView.frame.size.width / image.size.width;
float height = scale * image.size.height;
float topLeftY = (myImageView.frame.size.height - height) * 0.5;
boundingRect = CGRectMake(0, topLeftY, myImageView.frame.size.width, height);
}
NSDictionary * data = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat:scale], #"scale",
NSStringFromCGRect(boundingRect), #"bounds",
nil];
return data;
}

Answer 1

I completely understand what you are trying to do, but let me offer you a different way to achieve what you want.
you have an over sized image
you know the size of the imageView
ask the image for its CGImage
determine a 'scale' factor, which is the imageView width divided by the image width
multiple this value and your image height, then subtract the result from the imageViewHeight, to get the "empty" height in the imageView, lets call this 'fillHeight'
divide 'fillHeight' by 2 and round to get the 'offset' value used below
using context provided by UIGraphicsBeginImageContextWithOptions(imageView.size, NO, 0), paint the background whatever color you want, then draw your CGImage
CGContextDrawImage (context, CGRectMake(0, offset, imageView.size.width, rintf( image.size.height*scale )), [image CGImage]);
get this new image using:
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
set the image: imageView.image = image;
Now you can exactly map back to your image as you know the EXACT scaling ratio and offsets.

Answer 2

This might be the simple answer you are looking for. If you're x and y coordinates are inverted, you can mirror them yourself. In the below snippet im looping through my returned features and needing to invert the y coordinates, and x coordinate if it's front-facing camera:
for (CIFaceFeature *f in features) {
float newy = -f.bounds.origin.y + self.frame.size.height - f.bounds.size.height;
float newx = f.bounds.origin.x;
if( isMirrored ) {
newx = -f.bounds.origin.x + self.frame.size.width - f.bounds.size.width;
}
[[soups objectAtIndex:rnd] drawInRect:CGRectMake(newx, newy, f.bounds.size.width, f.bounds.size.height)];
}