Compensate For AVLayerVideoGravityResizeAspectFill Height Difference

Question

I have a nested video like this: Live camera feed

When the user takes a photo, the image is offset along the y axis Captured Still image

I do want to capture the WHOLE image and let the user scroll up and down. They can do this currently but I want the starting scroll of the image to be centered to match the camera feed preview. So if they take a picture, the image matches the frame that the video feed was showing.

The problem is, because the aspect on the camera is set to AVLayerVideoGravityResizeAspectFill it's doing some 'cropping' to fit the image into the live preview. Since the height is much bigger than the width, there are top and bottom parts that are captured in the image that are NOT showing up in the live feed (naturally).

What I don't know, however, is how much the top is being cropped so I can offset the previewed image to match this.

So my question is: Do you know how to calculate how much is being cropped from the top of a camera with its aspect ratio set to AVLayerVideoGravityResizeAspectFill? (Objective-C and Swift answers welcome!)

Answer 1

The solution I came up with is this:

  func getVerticalOffsetAdjustment()->CGFloat
    {
        var cropRect:CGRect = _videoPreviewLayer.metadataOutputRectOfInterestForRect(_videoPreviewLayer.bounds) //returns the cropped aspect ratio box so you can use its offset position
        //Because the camera is rotated by 90 degrees, you need to use .x for the actual y value when in portrait mode
        return cropRect.origin.x/cropRect.width * frame.height
    }

Its confusing I admit, but because the camera is rotated 90 degrees when in portrait mode you need to use the width and x values. The cropRect will return a value like (0.125,0,0,75,1.0)(your exact values will be different).

What this tells me, is my my shifted y value (that the video live feed is showing me) is shifted down 12.5% of its total height and that the height of the video feed is only 75% of the total height.

So I take 12.5% and divide by 75% to get the normalized (to my UIWindow) value and then apply that amount to the scrollview offset.

WHEW!!!