Read an image file asynchronously.

dontHurtMeNow · September 2018

Hi all,

I would like to read an image file asynchronously and Im trying to achieve that using async/await from C#. However, I still notice a significant lag with this implementation. Using the profiler I'm pretty confident it is it the FileStream.ReadAsync() method that takes ages to complete and causes the game from updating.

Here's some code.

// Static class for reading the image.
class AsyncImageReader
{
    public static async Task<byte[]> ReadImageAsync(string filePath)
        {
            byte[] imageBytes;
            didFinishReading = false;
            using (FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, true))
            {
                imageBytes = new byte[fileStream.Length];
                await fileStream.ReadAsync(imageBytes, 0, (int) fileStream.Length);

            }

            didFinishReading = true;

            return imageBytes;
        }
 }

    // Calling function that is called by the OnCapturedPhotoToDisk event.
    public void AddImage(string filePath)
    {
        Task<byte[]> readImageTask = AsyncImageReader.ReadImageAsync(filePath);
        byte [] imageBytes = await readImageTask;
    }

dontHurtMeNow · October 2018

For anyone else who stumbled through this dangerous sludge of confusion here's what I ended up with. Not perfect, since I still notice a tracking loss but better than nothing. In the calling function don't use the await keyword, just fire and forget. Let the callback do the work that depends on the read file. This works with any other file type too, I think/hope.

public static async Task ReadImageBytesAsync(string filepath, Action<byte[]> callback)
{
    didFinishReading = false;
    byte[] imageBytes;
    using (FileStream sourceStream = new FileStream(filepath, FileMode.Open, FileAccess.Read, FileShare.None, bufferSize, FileOptions.Asynchronous))
    {
        imageBytes = new byte[sourceStream.Length];
        await sourceStream.ReadAsync(imageBytes, 0, (int) sourceStream.Length);
    }

    didFinishReading = true;

    callback(imageBytes);

}

Dwight_Goins_EE_MVP · September 2018

First let me ask where is this code running? Unity or a C# directX app? If it's Unity, Unity basically requires all reference classes to be accessed from the main UI thread. Value types can be accessed from background threads. What this means is your app will basically wait regardless of your async/await patterns. You'll need to implement a dispatcher pattern in your Unity class. It if it's C# DX then another note, have you tried chunking your reads? Read in chunks at a time, not the whole file all at once?

Just some thoughts to help you out.

dontHurtMeNow · September 2018

@Dwight_Goins_EE_MVP Thanks. Yes it is in Unity. I will have to look up dispatcher pattern to implement this.

As for anyone else who might come here looking for a quick fix, what I did what hide all the holograms while it's reading so you don't notice the tracking loss/no rendering during that time.

Dwight_Goins_EE_MVP · September 2018

@dontHurtMeNow - Here is a link to one to help you get started: https://answers.unity.com/questions/305882/how-do-i-invoke-functions-on-the-main-thread.html

dontHurtMeNow · October 2018

For anyone else who stumbled through this dangerous sludge of confusion here's what I ended up with. Not perfect, since I still notice a tracking loss but better than nothing. In the calling function don't use the await keyword, just fire and forget. Let the callback do the work that depends on the read file. This works with any other file type too, I think/hope.

public static async Task ReadImageBytesAsync(string filepath, Action<byte[]> callback)
{
    didFinishReading = false;
    byte[] imageBytes;
    using (FileStream sourceStream = new FileStream(filepath, FileMode.Open, FileAccess.Read, FileShare.None, bufferSize, FileOptions.Asynchronous))
    {
        imageBytes = new byte[sourceStream.Length];
        await sourceStream.ReadAsync(imageBytes, 0, (int) sourceStream.Length);
    }

    didFinishReading = true;

    callback(imageBytes);

}

Read an image file asynchronously.

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