Running an OnRenderImage effect before the post processing stack (v2)

Hi. How would I execute a full screen effect written for OnRenderImage before the post processing stack renders? In the default setup any OnRenderImage methods are called after the post processing stack.

[ImageEffectOpaque]
protected void OnRenderImage(RenderTexture source, RenderTexture destination)

This can be a first help, but won’t affect transparent. Still looking for proper answer