ScrollView でズームした部分画像

        guard let ciImage = self.imageViewOut.image?.ciImage else { return }
        guard let imageSize = self.imageViewOut.image?.size else { return }
        
        let frameSize = self.scrollView.frame.size
        let ciContext = CIContext(options: nil)

        let zoomScale = self.scrollView.zoomScale
        // ズームしない場合、contentSizeがセットれさず、ゼロのまま。
        // そこで、frameSizeと、zoomScaleから contentSizeを計算する。
        // let contentSize = self.scrollView.contentSize
        let contentSize = CGSize(width: frameSize.width * zoomScale, height: frameSize.height * zoomScale)
        let contentOffset = self.scrollView.contentOffset

        let x = contentOffset.x * imageSize.width / contentSize.width
        let y1 = contentSize.height - contentOffset.y - frameSize.height
        let y2 = imageSize.height / contentSize.height
        let y = y1 * y2
        let newOffset = CGPoint(x: x, y: y)
        
        let newSize = CGSize(width: imageSize.width/zoomScale, height: imageSize.height/zoomScale)
        let newExtent = CGRect(origin: newOffset, size: newSize)
        
        let cgImage = ciContext.createCGImage(ciImage, from: newExtent)
        let uiImageToSave = UIImage(cgImage: cgImage!, scale: 1.0, orientation: .up)
        
        UIImageWriteToSavedPhotosAlbum(uiImageToSave,
                                       self, #selector(didFinishSavingImage), nil)