Custom AVVideoCompositing class not working as expected

Question

I'm attempting to apply a CIFilter to an AVAsset, and then save it with the filter applied. The way that I am doing this is by using an AVAssetExportSession with videoComposition set to an AVMutableVideoComposition object with a custom AVVideoCompositing class.

I am also setting the instructions of my AVMutableVideoComposition object to a custom composition instruction class (conforming to AVMutableVideoCompositionInstruction). This class is passed a track ID, along with a few other unimportant variables.

Unfortunately, I've run into a problem - the startVideoCompositionRequest: function in my custom video compositor class (conforming to AVVideoCompositing) is not being called correctly.

When I set the passthroughTrackID variable of my custom instruction class to the track ID, the startVideoCompositionRequest(request) function in my AVVideoCompositing is not called.

Yet, when I do not set the passthroughTrackID variable of my custom instruction class, the startVideoCompositionRequest(request) is called, but not correctly - printing request.sourceTrackIDs results in an empty array, and request.sourceFrameByTrackID(trackID) results in a nil value.

Something interesting that I found was that the cancelAllPendingVideoCompositionRequests: function is always called twice when attempting to export the video with filters. It is either called once before startVideoCompositionRequest: and once after, or just twice in a row in the case that startVideoCompositionRequest: is not called.

I've created three classes for exporting the video with filters. Here's the utility class, which basically just includes an export function and calls all of the required code

class VideoFilterExport{

    let asset: AVAsset
    init(asset: AVAsset){
        self.asset = asset
    }

    func export(toURL url: NSURL, callback: (url: NSURL?) -> Void){
        guard let track: AVAssetTrack = self.asset.tracksWithMediaType(AVMediaTypeVideo).first else{callback(url: nil); return}

        let composition = AVMutableComposition()
        let compositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)

        do{
            try compositionTrack.insertTimeRange(track.timeRange, ofTrack: track, atTime: kCMTimeZero)
        }
        catch _{callback(url: nil); return}

        let videoComposition = AVMutableVideoComposition(propertiesOfAsset: composition)
        videoComposition.customVideoCompositorClass = VideoFilterCompositor.self
        videoComposition.frameDuration = CMTimeMake(1, 30)
        videoComposition.renderSize = compositionTrack.naturalSize

        let instruction = VideoFilterCompositionInstruction(trackID: compositionTrack.trackID)
        instruction.timeRange = CMTimeRangeMake(kCMTimeZero, self.asset.duration)
        videoComposition.instructions = [instruction]

        let session: AVAssetExportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetMediumQuality)!
        session.videoComposition = videoComposition
        session.outputURL = url
        session.outputFileType = AVFileTypeMPEG4

        session.exportAsynchronouslyWithCompletionHandler(){
            callback(url: url)
        }
    }
}

Here's the other two classes - I'll put them both into one code block to make this post shorter

// Video Filter Composition Instruction Class - from what I gather,
// AVVideoCompositionInstruction is used only to pass values to
// the AVVideoCompositing class

class VideoFilterCompositionInstruction : AVMutableVideoCompositionInstruction{

    let trackID: CMPersistentTrackID
    let filters: ImageFilterGroup
    let context: CIContext


    // When I leave this line as-is, startVideoCompositionRequest: isn't called.
    // When commented out, startVideoCompositionRequest(request) is called, but there
    // are no valid CVPixelBuffers provided by request.sourceFrameByTrackID(below value)
    override var passthroughTrackID: CMPersistentTrackID{get{return self.trackID}}
    override var requiredSourceTrackIDs: [NSValue]{get{return []}}
    override var containsTweening: Bool{get{return false}}


    init(trackID: CMPersistentTrackID, filters: ImageFilterGroup, context: CIContext){
        self.trackID = trackID
        self.filters = filters
        self.context = context

        super.init()

        //self.timeRange = timeRange
        self.enablePostProcessing = true
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}


// My custom AVVideoCompositing class. This is where the problem lies -
// although I don't know if this is the root of the problem

class VideoFilterCompositor : NSObject, AVVideoCompositing{

    var requiredPixelBufferAttributesForRenderContext: [String : AnyObject] = [
        kCVPixelBufferPixelFormatTypeKey as String : NSNumber(unsignedInt: kCVPixelFormatType_32BGRA), // The video is in 32 BGRA
        kCVPixelBufferOpenGLESCompatibilityKey as String : NSNumber(bool: true),
        kCVPixelBufferOpenGLCompatibilityKey as String : NSNumber(bool: true)
    ]
    var sourcePixelBufferAttributes: [String : AnyObject]? = [
        kCVPixelBufferPixelFormatTypeKey as String : NSNumber(unsignedInt: kCVPixelFormatType_32BGRA),
        kCVPixelBufferOpenGLESCompatibilityKey as String : NSNumber(bool: true),
        kCVPixelBufferOpenGLCompatibilityKey as String : NSNumber(bool: true)
    ]

    let renderQueue = dispatch_queue_create("co.getblix.videofiltercompositor.renderingqueue", DISPATCH_QUEUE_SERIAL)

    override init(){
        super.init()
    }

    func startVideoCompositionRequest(request: AVAsynchronousVideoCompositionRequest){
       // This code block is never executed when the
       // passthroughTrackID variable is in the above class  

        autoreleasepool(){
            dispatch_async(self.renderQueue){
                guard let instruction = request.videoCompositionInstruction as? VideoFilterCompositionInstruction else{
                    request.finishWithError(NSError(domain: "getblix.co", code: 760, userInfo: nil))
                    return
                }
                guard let pixels = request.sourceFrameByTrackID(instruction.passthroughTrackID) else{
                    // This code block is executed when I comment out the
                    // passthroughTrackID variable in the above class            

                    request.finishWithError(NSError(domain: "getblix.co", code: 761, userInfo: nil))
                    return
                }
                // I have not been able to get the code to reach this point
                // This function is either not called, or the guard
                // statement above executes

                let image = CIImage(CVPixelBuffer: pixels)
                let filtered: CIImage = //apply the filter here

                let width = CVPixelBufferGetWidth(pixels)
                let height = CVPixelBufferGetHeight(pixels)
                let format = CVPixelBufferGetPixelFormatType(pixels)

                var newBuffer: CVPixelBuffer?
                CVPixelBufferCreate(kCFAllocatorDefault, width, height, format, nil, &newBuffer)

                if let buffer = newBuffer{
                    instruction.context.render(filtered, toCVPixelBuffer: buffer)
                    request.finishWithComposedVideoFrame(buffer)
                }
                else{
                    request.finishWithComposedVideoFrame(pixels)
                }
            }
        }
    }

    func renderContextChanged(newRenderContext: AVVideoCompositionRenderContext){
        // I don't have any code in this block
    }

    // This is interesting - this is called twice,
    // Once before startVideoCompositionRequest is called,
    // And once after. In the case when startVideoCompositionRequest
    // Is not called, this is simply called twice in a row
    func cancelAllPendingVideoCompositionRequests(){
        dispatch_barrier_async(self.renderQueue){
            print("Cancelled")
        }
    }
}

I've been looking at Apple's AVCustomEdit sample project a lot for guidance with this, but I can't seem to find in it any reason why this is happening.

How could I get the request.sourceFrameByTrackID: function to call correctly, and provide a valid CVPixelBuffer for each frame?

Answer 1

_{All of the code for this utility is on GitHub}

It turns out that the requiredSourceTrackIDs variable in the custom AVVideoCompositionInstruction class (VideoFilterCompositionInstruction in the question) has to be set to an array containing the track IDs

override var requiredSourceTrackIDs: [NSValue]{
  get{
    return [
      NSNumber(value: Int(self.trackID))
    ]
  }
}

So the final custom composition instruction class is

class VideoFilterCompositionInstruction : AVMutableVideoCompositionInstruction{
    let trackID: CMPersistentTrackID
    let filters: [CIFilter]
    let context: CIContext

    override var passthroughTrackID: CMPersistentTrackID{get{return self.trackID}}
    override var requiredSourceTrackIDs: [NSValue]{get{return [NSNumber(value: Int(self.trackID))]}}
    override var containsTweening: Bool{get{return false}}

    init(trackID: CMPersistentTrackID, filters: [CIFilter], context: CIContext){
        self.trackID = trackID
        self.filters = filters
        self.context = context
    
        super.init()
    
        self.enablePostProcessing = true
    }

    required init?(coder aDecoder: NSCoder){
        fatalError("init(coder:) has not been implemented")
    }
}

All of the code for this utility is also on GitHub