Custom off-screen buffer prepass

[Bcompartment]

Im not rendering expert, so i try to find example of material writing some data(vec4) to custom texture before main rendering. And use this texture in screen post FX just like i can use depth texture. Its should be simular to stencil buffer, so material writes to texture only if it visible (not behind object). In example image torus is both rendered and used as mask with custom data.