Coordinate system for CIFaceFeature - iphone

Question

I'm using the CIFeature Class Reference for Face Detection and I'm more than a little confused by the Core Graphics coordinates and the regular UIKit coordinates. This is my code:
UIImage *mainImage = [UIImage imageNamed:#"facedetectionpic.jpg"];
CIImage *image = [[CIImage alloc] initWithImage:mainImage];
NSDictionary *options = [NSDictionary dictionaryWithObject:CIDetectorAccuracyHigh forKey:CIDetectorAccuracy];
CIDetector *detector = [CIDetector detectorOfType:CIDetectorTypeFace context:nil options:options];
NSArray *features = [detector featuresInImage:image];
CGRect faceRect;
for (CIFaceFeature *feature in features)
{
faceRect= [feature bounds];
}
It's pretty standard. Now according to the official documentation it says:
bounds The rectangle that holds discovered feature. (read-only)
Discussion The rectangle is in the coordinate system of the image.
When I directly output FaceRect, I get: get rect {{136, 427}, {46, 46}}. When I apply the CGAffineTransfer to flip it the right way, I get negative coordinates which doesn't seem right. The image I am working with is in an ImageView.
So in which coordinate system are these coordinates in? The image? The ImageView? Core Graphic coordinates? Regular coordinates?

Answer 1

I finally figured it out. As the documentation points out, the rectangle drawn by the CIFaceFeature is in the coordinate system of the image. This means that the rectangle has the coordinates of the original image. If you have the Autoresize option checked, that means your image gets scaled down to fit in UIImageView. So you need to convert the old image coordinates to the new image coordinates.
This nifty piece of code that I adapted from here does that for you:
- (CGPoint)convertPointFromImage:(CGPoint)imagePoint {
CGPoint viewPoint = imagePoint;
CGSize imageSize = self.setBody.image.size;
CGSize viewSize = self.setBody.bounds.size;
CGFloat ratioX = viewSize.width / imageSize.width;
CGFloat ratioY = viewSize.height / imageSize.height;
UIViewContentMode contentMode = self.setBody.contentMode;
if (contentMode == UIViewContentModeScaleAspectFit)
{
if (contentMode == UIViewContentModeScaleAspectFill)
{
CGFloat scale;
if (contentMode == UIViewContentModeScaleAspectFit) {
scale = MIN(ratioX, ratioY);
}
else /*if (contentMode == UIViewContentModeScaleAspectFill)*/ {
scale = MAX(ratioX, ratioY);
}
viewPoint.x *= scale;
viewPoint.y *= scale;
viewPoint.x += (viewSize.width - imageSize.width * scale) / 2.0f;
viewPoint.y += (viewSize.height - imageSize.height * scale) / 2.0f;
}
}
return viewPoint;
}