AVFoundation: scaling a video change colors

Question

I'm creating a video composition by scaling a single video using AVFoundation and an AVMutableComposition. When I apply any scaling other than 1 (either bigger or smaller), I get a color shift on the end result. I have this issue regardless of the video used as source or the preset/format used as destination (MOV/MP4, ProRes/H24...).

Here is the relevant code, widely inspirited from another StackOverflow question (Swift Video Resizer AVAsset)

func resizer(inputURL: URL , completion: @escaping (URL?) -> Void ){
    let videoAsset = AVAsset(url: inputURL) as AVAsset
    let clipVideoTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo).first! as AVAssetTrack

    let composition = AVMutableComposition()
    composition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())

    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = clipVideoTrack.naturalSize
    videoComposition.frameDuration = CMTimeMake(1, 30)

    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRange(start: kCMTimeZero, duration: clipVideoTrack.asset!.duration)

    let transformer : AVMutableVideoCompositionLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: clipVideoTrack)

    // If anything that 1 on both, colors are shifted!
    let transform = CGAffineTransform(scaleX: 1.001, y: 1.001)
    transformer.setTransform(transform, at: kCMTimeZero)
    instruction.layerInstructions = [transformer]
    videoComposition.instructions = [instruction]

    // Export part, left for facility    
    let exporter = AVAssetExportSession(asset: videoAsset, presetName: AVAssetExportPresetHighestQuality)!
    exporter.videoComposition = videoComposition
    let outputURL = URL(fileURLWithPath: "/path/to/destination/video.mov")
    exporter.outputURL = outputURL
    exporter.outputFileType = AVFileTypeQuickTimeMovie

    exporter.exportAsynchronously(completionHandler: { () -> Void in
        completion(outputURL)
    })
}

I am facing same problem and the isssue is only with the iphone. Some gray color layer is coming on the appended video is that right ? — Purushottam Sain, Feb 28 '18 at 10:57

score 0 · Answer 1 · answered Mar 01 '18 at 12:43

A solution is to reset the transformation at the end of the video:

// Add this line to fix the issue:
transformer.setTransform(CGAffineTransform.identity, at: kCMTimeZero + clipVideoTrack.asset!.duration)

// Before this lines:
instruction.layerInstructions = [transformer]
videoComposition.instructions = [instruction]

Not sure why, but also works if you have multiple tracks used for composition.

AVFoundation: scaling a video change colors

1 Answers1