Cropping visible part of UIImage in UIImageView for saliency - swift

Question

I'm doing attentionBased saliency and should pass image to the request. When the contentMode is ScaleAspectFill, the result of the request is not correct, because I use full image (not visible on screen part)
I'm trying to crop UIImage, but this method doesn't crop correctly
let newImage = cropImage(imageToCrop: imageView.image, toRect: imageView.frame)
func cropImage(imageToCrop: UIImage?, toRect rect: CGRect) -> UIImage? {
guard let imageRef = imageToCrop?.cgImage?.cropping(to: rect) else {
return nil
}
let cropped: UIImage = UIImage(cgImage: imageRef)
return cropped
}
How can I make saliency request only for the visible part of the image (which changes when change contentMode)?

Answer 1

If I understand your goal correctly...
Suppose we have this 640 x 360 image:
and we display it in a 240 x 240 image view, using .scaleAspectFill...
It looks like this (the red outline is the image view frame):
and, with .clipsToBounds = true:
we want to generate this new 360 x 360 image (that is, we want to keep the original image resolution... we don't want to end up with a 240 x 240 image):
To crop the visible portion of the image, we need to calculate the scaled rect, including the offset:
func cropImage(imageToCrop: UIImage?, toRect rect: CGRect) -> UIImage? {
guard let imageRef = imageToCrop?.cgImage?.cropping(to: rect) else {
return nil
}
let cropped: UIImage = UIImage(cgImage: imageRef)
return cropped
}
func myCrop(imgView: UIImageView) -> UIImage? {
// get the image from the imageView
guard let img = imgView.image else { return nil }
// image view rect
let vr: CGRect = imgView.bounds
// image size -- we need to account for scale
let imgSZ: CGSize = CGSize(width: img.size.width * img.scale, height: img.size.height * img.scale)
let viewRatio: CGFloat = vr.width / vr.height
let imgRatio: CGFloat = imgSZ.width / imgSZ.height
var newRect: CGRect = .zero
// calculate the rect that needs to be clipped from the full image
if viewRatio > imgRatio {
// image has a wider aspect ratio than the image view
// so top and bottom will be clipped
let f: CGFloat = imgSZ.width / vr.width
let h: CGFloat = vr.height * f
newRect.origin.y = (imgSZ.height - h) * 0.5
newRect.size.width = imgSZ.width
newRect.size.height = h
} else {
// image has a narrower aspect ratio than the image view
// so left and right will be clipped
let f: CGFloat = imgSZ.height / vr.height
let w: CGFloat = vr.width * f
newRect.origin.x = (imgSZ.width - w) * 0.5
newRect.size.width = w
newRect.size.height = imgSZ.height
}
return cropImage(imageToCrop: img, toRect: newRect)
}
and call it like this:
if let croppedImage = myCrop(imgView: theImageView) {
// do something with the new image
}