Rendering screenshots on EC2 linux results in 1x1 pixel images

I want to render unity4.2 screenshots of my unity3d scene using EC2 linux. When I run my unity3d application on my macbook, I get rendered scenes as expected. When I render on EC2 linux, I get a 1x1 black pixel.

I tried rendering via:

      Application.CaptureScreenshot("/tmp/Screenshot.png");

and also:

     var width = Screen.width;
     var height = Screen.height;
     var tex = new Texture2D( width, height, TextureFormat.RGB24, false );
     // Read screen contents into the texture
     tex.ReadPixels( Rect(0, 0, width, height), 0, 0 );
     tex.Apply();
 
     // Encode texture into PNG
     var bytes = tex.EncodeToPNG();
     var fs : FileStream = FileStream(str_path, FileMode.CreateNew);
     var w : BinaryWriter = BinaryWriter(fs);
     w.Write(bytes);

I ran the command via:

1. ./et_linux_headless_64bit.x86_64 -batchmode

2. xvfb-run --auto-servernum --server-args='-screen 0 640x480x24:32' ./et_linux_headless_64bit.x86_64 -batchmode

They both just produced the 1x1 black pixel.

I tried EC2 linux "small" instance and the "GPU g2.2xlarge" instance, with the same results.

What can I do to render my screenshots on my EC2 instance? I want to use EC2 so that I can scale up the instances rapidly, and use set up auto-scale to instantly start new server instances to do rendering. Thanks!