TiffBitmapEncoder: Making bitmap encoding asynchronous

Question

Problem: how can I save/compress bitmap images (several of them stacked in tiff file) asynchronously with respect to the main thread?

My setting: (synchronous & slow working solution: when the compression is performed the thread remains stuck for a few seconds. I cannot afford this.)

 ...in the body of a window class 

private void afunction(){ 
     //called with a given frequency and updating this.colorBitmap
     ...
     this.colorBitmap = something;          
     savePictureStackTiff();
}

private int stack_pict_count = 0;
private int PICT_PER_FILE = 45;        
private TiffBitmapEncoder encoder;        

private string savePictureStackTiff()
{
      initializeTiffEncoder();
      //make a local copy of the image I want to put in the tiff binder
      WriteableBitmap localCopy = new WriteableBitmap(this.colorBitmap);    

      encoder.Frames.Add(BitmapFrame.Create(localCopy));  

      stack_pict_count++;
      if (stack_pict_count % PICT_PER_FILE == 0) 
      //Once I have enough files stacked I ask for compression
      {              
          stack_pict_count = 0;
          pict_metadata = "";            
          try
          {
              using (FileStream fs = new FileStream(path, FileMode.Create))
              {
                  encoder.Save(fs); //<<<== LINE WHICH I'D LIKE TO BE RUN ASYNC
              }                    
           }
           catch (IOException)
           {}               
        }
 }

 private void initializeTiffEncoder()
 {
    if (stack_pict_count == 0)
    {                
        encoder = new TiffBitmapEncoder();
        encoder.Compression = TiffCompressOption.Zip;                
    }
 }

What I've been trying: I would like the compression (the call encoder.save(fs)) to be performed in another thread than the main one.

I tried to put the call encoder.save(fs) in a BackgroundWorker which preventively copies the encoder to a local version (not sure if it worked though) and then makes the call. I receive an error like 'The calling thread cannot access this object because a different thread owns it'.

If I use a Dispatcher.Invoke (provided I do it correctly) the execution becomes again very slow.

Am I make some stupid mistake?

EDIT: (work in progress following the suggestions of @meilke and @user7116)

I now moved the allocation and the execution of the compressor in a BackgroundWorker. Although now colorBitmap which is passed, owns to another thread. I tried to freeze it, but it doesn't look enough; I still get a 'The calling thread cannot access this object because a different thread owns it'.

        tiffCompressorWorker = new BackgroundWorker();
        tiffCompressorWorker.DoWork += (s, a) =>
        {

            initializeTiffEncoder();
            WriteableBitmap localCopy = new WriteableBitmap((WriteableBitmap)a.Argument);
            localCopy.Freeze();
            encoder.Frames.Add(BitmapFrame.Create(localCopy));

            stack_pict_count++;
            if (stack_pict_count % PICT_PER_FILE == 0)
            {
                stack_pict_count = 0;                    
                try
                {
                    using (FileStream fs = new FileStream(path, FileMode.Create))
                    {
                        saving_encoder.Save(fs);
                    }
                }
                catch (IOException)
                {
                    ..
                }
            }            
        };

I am having the same problem. – 9301293 Dec 22 '16 at 18:21 — 9301293, Dec 22 '16 at 18:21

score 2 · Answer 1 · edited May 23 '17 at 11:45

2

You have to put the whole TIFF operation into the background worker. And then pass a copy of the input image as an argument to RunWorkerAsync. Here is a link to one of the many solutions available on the web on how to do it: Copying from BitmapSource to WritableBitmap. Put that code into a helper method and use it to copy the image before saving it to disk.

edited May 23 '17 at 11:45

Community

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answered Sep 13 '13 at 18:42

meilke

3,192
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12
30

The key is the `TiffBitmapEncoder`/`WriteableBitmap` needs to be *created* in the same thread it will be used in, and likely the `colorBitmap` will need to be frozen or copied too. – user7116 Sep 13 '13 at 18:53
@user7116 I guess I now implemented it this way, but now how can I deepcopy `colorBitmap` among the threads? – Acorbe Sep 13 '13 at 18:58
@user7116, I really cannot manage. Could you please elaborate a bit more? – Acorbe Sep 13 '13 at 19:52
I adjusted my answer to include a link on how to deep copy the image. – meilke Sep 14 '13 at 12:28

score 0 · Answer 2 · edited May 23 '17 at 12:24

0

I am no computer scientist, but I found this post, which states:

WriteableBitmap inherits DispatcherObject, and must be accessed only within the dispatcher thread that owns it.

And, it looks to me like you are steping outside of the DispatcherObject's permissions.

edited May 23 '17 at 12:24

Community

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answered Dec 22 '16 at 18:48

9301293

484
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TiffBitmapEncoder: Making bitmap encoding asynchronous

2 Answers2