Using Unity Jobs to encode file to PNG and then save it on Main Thread

Question

I have a following code , and i can not retrive encoded Output from Job.

If I Use JOB : The result is not a valid png file. What I get is a 67,2 MB file which can not be opened with any image editor I tried. When I open it with a text editor, it seems to contain only whitespaces.

else i got normal valid png file saved on my disc .

Can we use this approach for encoding inside Unity ?

public class Loger : MonoBehaviour
{
    [SerializeField]
    public bool isLoging = true;

    [SerializeField]
    private RenderTexture _rt;
    private NativeArray<byte> _data;
    private AsyncGPUReadbackRequest _request;

    public bool useJob; 

    private void Start()
    {
        CreateBuffer(ref _data, ref _rt);
        StartCoroutine(StartEncoding(_rt, isLoging));
    
    }

    System.Collections.IEnumerator StartEncoding(RenderTexture oRTs, bool active)
    {
        int i = 0; 
        while (active)
        {
            _request= AsyncGPUReadback.RequestIntoNativeArray(ref _data, oRTs, 0);
        if(_request.hasError)
            {
                Debug.Log("Error in GPU request "); break; 
            }

            yield return new WaitUntil(() => _request.done);
        
            if(useJob)
            {
                Debug.Log("creating  Job");
                EncodeJob job = new EncodeJob
                {
                    input = new NativeArray<byte>(_data, Allocator.TempJob),
                    output = new NativeArray<byte>(_data.Length, Allocator.TempJob),
                    graphicsFormat = oRTs.graphicsFormat,
                    width = oRTs.width,
                    height = oRTs.height,
                    result = new NativeArray<int>(1, Allocator.TempJob)
                };


                JobHandle jobTask = job.Schedule();
                jobTask.Complete();
                Debug.Log("result from job is " + " " + job.result[0]);
                var data = job.output;
                Debug.Log("created PNG file ");
                string pathName = "Logs/" + "FLCW" + "/" + i++ + "JOB" + ".png";
                System.IO.File.WriteAllBytes(pathName, data.ToArray());
                yield return new WaitUntil(() => _request.done);
            }

            else if(!useJob)
            {
                var fileData =ImageConversion.EncodeNativeArrayToPNG(_data, oRTs.graphicsFormat, (uint)oRTs.width, (uint)oRTs.height);
                Debug.Log("created PNG file ");
                string pathName = "Logs/" + "FLCW" + "/" + i++ + "MAIN" +".png" ;
                System.IO.File.WriteAllBytes(pathName, fileData.ToArray());
                yield return new WaitUntil(() => _request.done);
            }
          
            yield return new WaitUntil(() => _request.done);
        }
    }

    void CreateBuffer(ref NativeArray<byte> buffer, ref RenderTexture oRTS)
    {
        buffer = new NativeArray<byte>(oRTS.height * oRTS.width * 4, Allocator.Persistent, NativeArrayOptions.UninitializedMemory);
    }

    private void OnApplicationQuit()
    {
        _data.Dispose();
    }


    struct EncodeJob : IJob
    {
        public NativeArray<byte> input;
        public NativeArray<byte>  output;
        public GraphicsFormat graphicsFormat;
        public NativeArray<int> result;
        public int width;
        public int height;      
            public void Execute()
        {        
            output = ImageConversion.EncodeNativeArrayToPNG(input,graphicsFormat,(uint)width,(uint)height);
            result[0] = width + height;
        }
       
    }


}

Answer 1

After testing I found a few problems.

1. The png format does not save the color information of each pixel. It is a compressed image format. The length of the btyes written to the file is obviously uncompressed.

2. According to 1, you can't know the size of the png before starting the job. The size allocated for the output can only be used as the buffer size.

3. Assignment to member variables during job execution is invalid, that's why you use NativeArray to hold the resulting value. So the assignment to output is also invalid:

   public void Execute()
   {        
       output = ...
   }
   

Fix:

1. Use NativeArray to pass out the length of the resulting array.
2. Use the length value to slice the output and save it to a file.

Code:

EncodeJob job = new EncodeJob
{
    ...
    result = new NativeArray<int>(2, Allocator.TempJob)
};

JobHandle jobTask = job.Schedule();
jobTask.Complete();

Debug.Log("result from job is " + " " + job.result[0]);
var data = new byte[job.result[1]];
NativeArray<byte>.Copy(job.output, data, job.result[1]);
Debug.Log("created PNG file ");
string pathName = "Logs/" + "FLCW" + "/" + i++ + "JOB" + ".png";
System.IO.File.WriteAllBytes(pathName, data);

And:

public void Execute()
{
    var o = ImageConversion.EncodeArrayToPNG(input.ToArray(), graphicsFormat,
    (uint)width, (uint)height);
    NativeArray<byte>.Copy(o, output, o.Length);
    result[0] = width + height;
    result[1] = o.Length;
}