BitmapEncoder FlushAsync throws Argument Exception - c#

Question

When I'm using below code for image processing it throws exception at FlushAsync(). Strangely this issue happens for some video files only. Can you help why this happen ?

   private async Task<String> PreProcessVideoToGetTextAsync(StorageFile file, int i)
    {  /*--------To get frame from video----------*/
        var thumbnail = await GetThumbnailAsync(file, i);
        BitmapImage bitmapImage = new BitmapImage();
        StringBuilder ocr = null;
        InMemoryRandomAccessStream randomAccessStream = new InMemoryRandomAccessStream();
        await RandomAccessStream.CopyAsync(thumbnail, randomAccessStream);
        randomAccessStream.Seek(0);
        SoftwareBitmap inputBitmap;
        BitmapDecoder decoder = await BitmapDecoder.CreateAsync(randomAccessStream);

  // create a new stream and encoder for the new image
        InMemoryRandomAccessStream ras = new InMemoryRandomAccessStream();
        BitmapEncoder enc = await BitmapEncoder.CreateForTranscodingAsync(ras, decoder);
        BitmapBounds bounds = new BitmapBounds();

            bounds.Height = 270;//screnheight/4 =270
            bounds.Width = 960;//screenwidth/2 =960
            bounds.X = 960;//screenwidth/2 =960
            bounds.Y = 810;
            enc.BitmapTransform.Bounds = bounds;  
        // write out to the stream
        try
        {
            await enc.FlushAsync();
        }
        catch (Exception ex)
        {
            string exx = ex.ToString();
            var dialog = new MessageDialog(exx);
            await dialog.ShowAsync();
        }
     //Remain portion of code
     }

Answer 1

I've already used the following code snippet to save video thumbnails in my uwp app, hopefully you can apply a little bit of changes as you want to use it.

private async Task PreProcessVideoToGetTextAsync(string videopath)
    {
        try
        {

            SoftwareBitmap softwareBitmap = null;

            StorageFile videofile = await StorageFile.GetFileFromPathAsync(videopath);
            var thumbnail = await GetThumbnailAsync(videofile);
            Windows.Storage.Streams.InMemoryRandomAccessStream randomAccessStream = new Windows.Storage.Streams.InMemoryRandomAccessStream();
            await Windows.Storage.Streams.RandomAccessStream.CopyAsync(thumbnail, randomAccessStream);

            BitmapDecoder decoder = await BitmapDecoder.CreateAsync(randomAccessStream);

            softwareBitmap = await decoder.GetSoftwareBitmapAsync();

            StorageFile outputFile = await ApplicationData.Current.LocalCacheFolder.CreateFileAsync(Path.GetFileNameWithoutExtension(videopath) + ".jpg");//save path for video thumbnail image in app cache folder
            using (Windows.Storage.Streams.IRandomAccessStream stream = await outputFile.OpenAsync(FileAccessMode.ReadWrite))
            {

                BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, stream);

                encoder.SetSoftwareBitmap(softwareBitmap);

                encoder.BitmapTransform.ScaledWidth = 128;//thumbnail width
                encoder.BitmapTransform.ScaledHeight = 128;//thumbnail height

                encoder.IsThumbnailGenerated = true;

                try
                {
                    await encoder.FlushAsync();
                    if (softwareBitmap.BitmapPixelFormat != BitmapPixelFormat.Bgra8 || softwareBitmap.BitmapAlphaMode == BitmapAlphaMode.Straight)
                    {
                        softwareBitmap = SoftwareBitmap.Convert(softwareBitmap, BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied);
                    }

                    var source = new Windows.UI.Xaml.Media.Imaging.SoftwareBitmapSource();
                    await source.SetBitmapAsync(softwareBitmap);

                    return;

                }
                catch (Exception err)
                {
                    switch (err.HResult)
                    {
                        case unchecked((int)0x88982F81): //WINCODEC_ERR_UNSUPPORTEDOPERATION
                                                         // If the encoder does not support writing a thumbnail, then try again
                                                         // but disable thumbnail generation.
                            encoder.IsThumbnailGenerated = false;
                            break;
                        default:
                            throw err;
                    }
                }

                if (encoder.IsThumbnailGenerated == false)
                {
                    await encoder.FlushAsync();
                }

            }
            return;
        }

        catch (Exception)
        {
            return;

        }
        finally
        {

            GC.Collect();
        }

    }
private async Task<Windows.Storage.Streams.IInputStream> GetThumbnailAsync(StorageFile file)
    {
        var mediaClip = await Windows.Media.Editing.MediaClip.CreateFromFileAsync(file);
        var mediaComposition = new Windows.Media.Editing.MediaComposition();
        mediaComposition.Clips.Add(mediaClip);
        return await mediaComposition.GetThumbnailAsync(
            TimeSpan.Zero, 0, 0, Windows.Media.Editing.VideoFramePrecision.NearestFrame);
    }


}