Bounding Box from VNDetectRectangleRequest is not correct size when used as child VC

First let’s look at boundingBox, which is a “normalized” rectangle. Apple says

The coordinates are normalized to the dimensions of the processed image, with the origin at the image’s lower-left corner.

This means that:

• The origin is at the bottom-left, not the top-left
• The origin.x and width are in terms of a fraction of the entire image’s width
• The origin.y and height are in terms of a fraction of the entire image’s height

Hopefully this diagram makes it clearer:

Your function above converts boundingBox to the coordinates of finalFrame, which I assume is the entire view’s frame. That is much larger than your small CameraCaptureVC.

Also, your CameraCaptureVC‘s preview layer probably has a aspectFill video gravity. You will also need to account for the overflowing parts of the displayed image.

Try this converting function instead.

func getConvertedRect(boundingBox: CGRect, inImage imageSize: CGSize, containedIn containerSize: CGSize) -> CGRect {
    
    let rectOfImage: CGRect
    
    let imageAspect = imageSize.width / imageSize.height
    let containerAspect = containerSize.width / containerSize.height
    
    if imageAspect > containerAspect { /// image extends left and right
        let newImageWidth = containerSize.height * imageAspect /// the width of the overflowing image
        let newX = -(newImageWidth - containerSize.width) / 2
        rectOfImage = CGRect(x: newX, y: 0, width: newImageWidth, height: containerSize.height)
        
    } else { /// image extends top and bottom
        let newImageHeight = containerSize.width * (1 / imageAspect) /// the width of the overflowing image
        let newY = -(newImageHeight - containerSize.height) / 2
        rectOfImage = CGRect(x: 0, y: newY, width: containerSize.width, height: newImageHeight)
    }
    
    let newOriginBoundingBox = CGRect(
    x: boundingBox.origin.x,
    y: 1 - boundingBox.origin.y - boundingBox.height,
    width: boundingBox.width,
    height: boundingBox.height
    )
    
    var convertedRect = VNImageRectForNormalizedRect(newOriginBoundingBox, Int(rectOfImage.width), Int(rectOfImage.height))
    
    /// add the margins
    convertedRect.origin.x += rectOfImage.origin.x
    convertedRect.origin.y += rectOfImage.origin.y
    
    return convertedRect
}

This takes into account the frame of the image view and also aspect fill content mode.

Example (I’m using a static image instead of a live camera feed for simplicity):

/// inside your Vision request completion handler...
guard let image = self.imageView.image else { return }

let convertedRect = self.getConvertedRect(
    boundingBox: observation.boundingBox,
    inImage: image.size,
    containedIn: self.imageView.bounds.size
)
self.drawBoundingBox(rect: convertedRect)

func drawBoundingBox(rect: CGRect) {
    let uiView = UIView(frame: rect)
    imageView.addSubview(uiView)
        
    uiView.backgroundColor = UIColor.clear
    uiView.layer.borderColor = UIColor.orange.cgColor
    uiView.layer.borderWidth = 3
}

I made an example project here.