OnRenderImage() possibly didn't write anything to the destination texture!

Graphics Shaders & Materials
1

So lately I’ve been getting this “OnRenderImage() possibly didn’t write anything to the destination texture!” warning every time I start up my app. It’s a one time warning (thankfully) so it doesn’t spam me or anything, but I hate seeing warnings for any reason though.

What exactly triggers this warning? What are the conditions that would cause this warning to appear?

It first appeared when I decided to do a hack to work around the VR showDeviceView limitation described in a thread I posted earlier: Any way to get VRSettings.showDeviceView to render into a Render Texture? - Unity Engine - Unity Discussions

The code sorta looks like this:

  public class VRCamera : MonoBehaviour
   {
     private Camera _camera;
     private int _frameCount;

     public RenderTexture MirrorTexture { get; private set; }

     void Awake()
     {
       _camera = GetComponent<Camera>();
       MirrorTexture = new RenderTexture(_camera.pixelWidth, _camera.pixelHeight, 0);
     }

     private void OnRenderImage(RenderTexture source, RenderTexture destination)
     {
       Graphics.Blit(source, destination);

       if (++_frameCount % 2 == 0)
       {
         Graphics.Blit(source, MirrorTexture);
       }
     }
   }

If I take out the second Blit, the warning goes away. Otherwise I still get the warning even if I do the blit every frame, every other frame (for left eye only), or whatever.

2 Likes
2

i had the same issue with u.
i solved like this…

1 Like
3

I somewhat followed this suggestion on a related script. The error disappeared alright but the effect I wanted initially did not also work. This may be a bug in Unity’s engine itself as reported here.

1 Like
4

From https://docs.unity3d.com/Manual/PostProcessingWritingEffects.html

“When OnRenderImage finishes, Unity expects that the destination render texture is the active render target. Generally, a Graphics.Blit or manual rendering into the destination texture should be the last rendering operation.”

So your last blit should use the destination texture.

5

if for whatever reason it’s inconvenient to have the destination texture be the target of the final blit, simply calling

Graphics.SetRenderTarget(destination);

seems to fix it