Read an image file asynchronously

Question

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, rather than doing it asynchronously and stops the game from updating.

I notice more lag this way than just using File.ReadAllBytes which is odd. I can't use that method that still causes some lag and I would like to not have a stop in frame rate.

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. When this is called the game stops updating completely rather than 
    public void AddImage(string filePath)
    {
        Task<byte[]> readImageTask = AsyncImageReader.ReadImageAsync(filePath);
        byte [] imageBytes = await readImageTask;
        // other things that use the bytes....
    }

your code will not even compile because your AddImage method should be async — Muhammad Touseef, Sep 16 '18 at 20:29
Note that `async` methods always run on the calling thread until an actual `await` statement is reached (actual meaning that it is IO bound). Also note, that when this `await` finishes, work returns to the same (calling) thread. This might theoretically be the cause of the lag. — Martin Zikmund, Sep 17 '18 at 04:52
@touseefbsb I know IDE told me. This was a copy of the code. — user14492, Sep 17 '18 at 08:23
@PoulBak not so much file is big but CPU is crap. Trying to work with Hololens — user14492, Sep 17 '18 at 08:24
Are you sure the 'OnCapturedPhotoToDisk' event is triggered just as often, when you do it synchronously? — Poul Bak, Sep 17 '18 at 08:28
@user14492 Read image async? Task in not required. Must your solution involve Task? — Programmer, Sep 17 '18 at 08:45
@Programmer no I’m open to any other solutions. Got any? I just want to spread out my read operations over multiple frames. — user14492, Sep 17 '18 at 09:37

score 0 · Answer 1 · answered Sep 17 '18 at 10:24

You don't need Task to load image asynchronously. You can use Unity's UnityWebRequest and DownloadHandlerTexture API to load. The UnityWebRequestTexture.GetTexture simplifies this since it setups up DownloadHandlerTexture for you automatically. This is done in another thread under the hood but you use coroutine to control it.

public RawImage image;

void Start()
{
    StartCoroutine(LoadTexture());
}

IEnumerator LoadTexture()
{
    string filePath = "";//.....
    UnityWebRequest uwr = UnityWebRequestTexture.GetTexture(filePath);
    yield return uwr.SendWebRequest();

    if (uwr.isNetworkError || uwr.isHttpError)
    {
        Debug.Log(uwr.error);
    }
    else
    {
        Texture texture = ((DownloadHandlerTexture)uwr.downloadHandler).texture;
        image.texture = texture;
    }
}

You can also load the file in another Thread with the Thread API. Even better, use the ThreadPool API. Any of them should work. Once you load the image byte array, to load the byte array into Texture2D, you must do it in the main Thread since you can't use Unity's API in the main Thread.

To use Unity's API from another Thread when you are done loading the image bytes, you would need a wrapper to do so. The example below is done with C# ThreadPool and my UnityThread API that simplifies using Unity's API from another thread. You can grab UnityThread from here.

public RawImage image;

void Awake()
{
    //Enable Callback on the main Thread
    UnityThread.initUnityThread();
}

void ReadImageAsync()
{
    string filePath = "";//.....

    //Use ThreadPool to avoid freezing
    ThreadPool.QueueUserWorkItem(delegate
    {
        bool success = false;

        byte[] imageBytes;
        FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, true);

        try
        {
            int length = (int)fileStream.Length;
            imageBytes = new byte[length];
            int count;
            int sum = 0;

            // read until Read method returns 0
            while ((count = fileStream.Read(imageBytes, sum, length - sum)) > 0)
                sum += count;

            success = true;
        }
        finally
        {
            fileStream.Close();
        }

        //Create Texture2D from the imageBytes in the main Thread if file was read successfully
        if (success)
        {
            UnityThread.executeInUpdate(() =>
            {
                Texture2D tex = new Texture2D(2, 2);
                tex.LoadImage(imageBytes);
            });
        }
    });
}

Read an image file asynchronously

1 Answers1