AVPlayerView播放HDR视频亮度偏高，如何禁用HDR或转SDR？

我通过AVQueuePlayer、AVPlayerViewController和AVPlayerLooper加载本地缓存文件夹中的视频播放，代码如下。目前播放正常，但HDR视频亮度过高，在QuickTime中播放显示正常，请问如何禁用HDR或将HDR转换为SDR？

internal void LoadVideoUsingURL(string url)
{
    // 在后台线程加载资源避免卡顿
    DispatchQueue.GetGlobalQueue(DispatchQualityOfService.Background).DispatchAsync(() =>
    {
        var avasset = AVAsset.FromUrl(NSUrl.FromFilename(url));
        avasset.LoadValuesAsynchronously(new string[] { "duration", "playable" }, () =>
        {
            DispatchQueue.MainQueue.DispatchAsync(() =>
            {
                var item = new AVPlayerItem(avasset);
                if (this.queuePlayer.CurrentItem != item)
                {
                    this.queuePlayer.ReplaceCurrentItemWithPlayerItem(item);
                    this.looperPlayer = AVPlayerLooper.FromPlayer(this.queuePlayer, item);
                }
            });
        });
    });
}

private void LoadView()
{
    try
    {
        queuePlayer.Volume = SoundON ? 1.0f : 0;
        playerViewController.ShowsPlaybackControls = false;
        queuePlayer.ActionAtItemEnd = AVPlayerActionAtItemEnd.None;
        playerViewController.VideoGravity = AVLayerVideoGravity.Resize;
        playerViewController.View.Layer.MasksToBounds = true;
        ContentView.AddSubview(playerViewController.View);
        playerViewController.Player = queuePlayer;
        ContentView.AddSubview(BlackBackground);
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
}

public virtual void StartPlaying()
{
    if (queuePlayer.CurrentItem != null && !isPlaying)
    {
        if (queuePlayer.Status == AVPlayerStatus.ReadyToPlay)
        {
            queuePlayer.Play();
            queuePlayer.Volume = Volume;
            isPlaying = true;
        }
        else
        {
            this.queuePlayer.CurrentItem.AddObserver(this, new NSString("status"),
NSKeyValueObservingOptions.New | NSKeyValueObservingOptions.Initial, StatusObservationContext.Handle);
        }
    }
}

针对HDR视频播放过亮的问题，有两种可行的处理方式：

1. 播放时强制使用SDR渲染（无需转码） #

通过设置AVPlayerItem的preferredDisplayCriteria，指定使用SDR显示标准，让AVFoundation自动将HDR内容转换为SDR输出。修改LoadVideoUsingURL方法中创建AVPlayerItem的部分：

var item = new AVPlayerItem(avasset);
// 添加SDR显示偏好设置
if (AVDisplayCriteria.SDRDisplayCriteria != null)
{
    item.PreferredDisplayCriteria = AVDisplayCriteria.SDRDisplayCriteria;
}
if (this.queuePlayer.CurrentItem != item)
{
    this.queuePlayer.ReplaceCurrentItemWithPlayerItem(item);
    this.looperPlayer = AVPlayerLooper.FromPlayer(this.queuePlayer, item);
}

这种方式无需修改原视频文件，直接在播放阶段处理，适合临时播放场景。

2. 将HDR视频转码为SDR（永久修改） #

如果需要长期使用SDR版本的视频，可通过AVAssetExportSession将HDR视频转码为SDR格式，示例代码如下：

void ConvertHDRToSDR(AVAsset asset, string outputPath)
{
    var exportSession = AVAssetExportSession.FromAsset(asset, AVAssetExportPresetHighestQuality);
    exportSession.OutputUrl = NSUrl.FromFilename(outputPath);
    exportSession.OutputFileType = AVFileType.Mpeg4;
    
    // 设置输出为SDR参数
    var outputSettings = new AVVideoSettingsCompressed();
    outputSettings.ColorProperties = new AVVideoColorProperties()
    {
        TransferFunction = AVVideoTransferFunction.SDRVideo,
        YCbCrMatrix = AVVideoYCbCrMatrix.BT601,
        ColorPrimaries = AVVideoColorPrimaries.SMPTE_C
    };
    exportSession.VideoSettings = outputSettings.Dictionary;
    
    exportSession.ExportAsynchronously(() =>
    {
        if (exportSession.Status == AVAssetExportSessionStatus.Completed)
        {
            // 转码完成后加载SDR视频
            LoadVideoUsingURL(outputPath);
        }
        else
        {
            Console.WriteLine($"转码失败: {exportSession.Error?.LocalizedDescription}");
        }
    });
}

在LoadVideoUsingURL的后台线程中，可先判断视频是否为HDR，再决定是否转码：

avasset.LoadValuesAsynchronously(new string[] { "duration", "playable", "commonMetadata" }, () =>
{
    DispatchQueue.MainQueue.DispatchAsync(() =>
    {
        // 判断视频是否为HDR
        bool isHDR = avasset.CommonMetadata.Any(meta => 
            meta.CommonKey == AVMetadataCommonKey.ColorSpace && 
            meta.StringValue?.Contains("HDR") == true);
        
        if (isHDR)
        {
            string tempSDRPath = Path.Combine(Path.GetTempPath(), "temp_sdr_video.mp4");
            ConvertHDRToSDR(avasset, tempSDRPath);
        }
        else
        {
            var item = new AVPlayerItem(avasset);
            if (this.queuePlayer.CurrentItem != item)
            {
                this.queuePlayer.ReplaceCurrentItemWithPlayerItem(item);
                this.looperPlayer = AVPlayerLooper.FromPlayer(this.queuePlayer, item);
            }
        }
    });
});

这种方式适合需要重复播放同一视频的场景，避免实时转换带来的性能开销。

额外优化（iOS 16+/macOS Ventura+） #

如果系统版本符合要求，可直接设置AVPlayerViewController的动态范围偏好：

playerViewController.PreferredVideoDynamicRange = AVPlayerViewControllerVideoDynamicRange.SDR;

该属性可直接控制播放器的动态范围输出，实现更简洁的HDR转SDR播放。

内容的提问来源于stack exchange，提问作者Ahmed Mohamed Anas