transform portrait video to be aspectFit while preview and exporting

Question

I have been trying to get portrait video to transform in a such what that it could be aspect fit in center of player while keep rest of the screen empty.

Example of How the video should look like

here is the code : Few thing : self.videoRenderSize = CGSize(width: 1920.0, height: 1080.0) videoTrackSize could be (960,720), (1280,720), (1920,1080) ( All Portrait though, so width is height and vice versa)

        let instruction = AVMutableVideoCompositionInstruction()
        instruction.timeRange = passThroughTimeRanges[i]

        let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: currentTrack)

        let assetObject = self.assetsList[i]["assetVideo"] as! Asset
        let asset = assetObject.movieAsset as AVAsset
        let videoTrack:AVAssetTrack = asset.tracksWithMediaType(AVMediaTypeVideo)[0]
        let videoTrackSize = videoTrack.naturalSize
        let preferredTransform = videoTrack.preferredTransform

        let videoOrientationMode = self.isVideoPortrait(asset)

        print(preferredTransform)
        print(videoTrackSize)

        switch videoOrientationMode {
        case "Portrait" :

            if self.videoRenderSize.height > videoTrackSize.height {

            }else if self.videoRenderSize.height == videoTrackSize.height {

            }else {

            }

            break

        case "PortraitUpsideDown":


            break

        case "LandscapeRight", "LandscapeLeft":

            let heightScale = CGFloat(self.videoRenderSize.height / videoTrackSize.height)
            let widthScale = CGFloat(self.videoRenderSize.width / videoTrackSize.width)

            layerInstruction.setTransform(CGAffineTransformMakeScale(widthScale,heightScale), atTime: instruction.timeRange.start)

            break
        default:
            print("Unknown Orientation")
        }

        instruction.layerInstructions = [layerInstruction]
        compositionInstructions.append(instruction)
    }

    videoComposition.instructions = compositionInstructions
    videoComposition.renderSize = self.videoRenderSize
    videoComposition.frameDuration = CMTimeMake(1, 30)
    videoComposition.renderScale = 1.0 // This is a iPhone only option.

Code for isVideoPortrait Function

    func isVideoPortrait(asset : AVAsset) -> String{

    var videoOrientation : String = ""

    let tracks : NSArray  = asset.tracksWithMediaType(AVMediaTypeVideo)

    if(tracks.count > 0) {

        let videoTrack : AVAssetTrack = tracks[0] as! AVAssetTrack

        let t = videoTrack.preferredTransform

        // Portrait
        if(t.a == 0 && t.b == 1.0 && t.c == -1.0 && t.d == 0 || t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0)
        {
            videoOrientation = "Portrait"
        }
        // PortraitUpsideDown
        if(t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0)  {
            videoOrientation = "PortraitUpsideDown"
        }
        // LandscapeRight
        if(t.a == 1.0 && t.b == 0 && t.c == 0 && t.d == 1.0)
        {
            videoOrientation = "LandscapeRight"
        }
        // LandscapeLeft
        if(t.a == -1.0 && t.b == 0 && t.c == 0 && t.d == -1.0)
        {
            videoOrientation = "LandscapeLeft"
        }
    }

    return videoOrientation
}

What would be transformation layerInstruction for making sure video will transfor in center and could be placed like in image attached above.