How can I use SceneTexture in mobile?

Hello, I want to make use of postprocessing in mobile.
My PostProcessing material keep warning me SceneTexture cannot be used in ES 2.
Then How can I use SceneTexture in ES3.1?
I have tried tick “Mobile HDR”, and I preview my scene in ES3.1,
All I get is something like this:

This image span the whole screen.
When I turn back to SM5, then It is normal.
So does the SceneTexture node still not be supported in ES3.1?