Culling arbitrary pixels using a Compositor Effect?

[arandomkrilios]

Godot Version

Godot v4.4.1 Stable

Question

I have been making a planar reflection script+shader, and it works pretty fine!

However, one glaring issue is the fact that it’s very difficult to cull pixels that are behind the mirror, while setting the near of the camera helps with smaller ones, larger mirrors clipping with geometry that goes well deep into the mirror are visible like so:

Notice the small part that pokes out on the right.

In this angle, the near property cannot do anything, and it’s not something I can fix in the script or shader themselves, however here’s where the main question of this post comes from:
Can you use a Compositor Effect (Set in either pre-opaque or post-opaque, as any pass afterwards will include the sky) to effectively “cull” pixels that are on one side of a plane?
This effect would be attached to the “mirror” camera’s Compositor.

I am sadly completely inexperienced with the Compositor and its language, and the documentation is very spread out and bare, especially for using an Effect in the pre/post-opaque pass, so any kind of indications are appreciated! ^^

[dragonforge-dev]

Dunno. The Compositor is a new feature. Haven’t played with it. Is this a 2D or 3D effect? I’m assuming 3D because the Compositor only affects 3D from what I can see. But you don’t actually say what you’re doing, and I cannot tell from that screenshot. Godot doesn’t support mirror or portal effects very well yet in 3D. I’ve seen some discussions in PRs about it.

You might get more help if you were to tell us your mirroring approach.

If this is 2D, it’s not that complicated to mirror stuff. If it’s 3D, you could apply a shader and set the alpha value above the “mirror” delineator to transparent so the object didn’t show.

Something like this:

[arandomkrilios]

Firstly, yes, it is a 3D effect, and I’ll try to explain a little more in detail:

So firstly, Planar Reflections: I have a plane, such as a flat mirror, that is the point of reference, then I have a somewhat simple script that creates a secondary viewport that has a Camera3D that mirrors the main viewport’s camera, placing itself on the other side of the plane, then a near plane is set to the distance to the plane to cull things that are directly in front of the camera.

Up to this point, simple, there’s just a shader on the mirror itself that displays the viewport texture using the view UV and all works well.

However, the scenario I posted, which now in retrospective probably wasn’t the best picture, but it is a very subtle erroneous effect, shows a major issue, that is that if geometry is underneath the Plane but outside the near plane, they get shown in the reflection as well, hence the tiny part of the pillar that is shown on the top right - or alternatively, if you had something like water, you could see part of the ground underneath the water visible, but only far away or angled geometry is visible like this.

Now the solution you show is good if the player is completely unable to get behind the mirror (Or have a copy of the Mesh that only the mirror can see) - which is not what I want, again, the example of water, if the player could get in it, they should be able to see things underneath it.

A better reference picture.

This is where I loop back to the Compositor Effect idea, I know you can deduce the global position of a pixel using the Depth Map you get on all the passes in the effect, this way it would be posible to know if a pixel is underneath the mirror plane (in 3D Space, which I should’ve explicitly mentioned originally!).

My question is basically if I can “remove pixels” in the Pre/Post-Opaque pass so the Sky pass draws over them. This would be a pretty smooth way (imo) to be able to have perfect Planar Reflections without having to mess with either additional Viewports or having clones of all meshes per mirror, while still allowing the main camera to go to the other side of the plane/mirror/water.
My main problem currently is just how little documentation there’s of the Compositor for now, even though it clearly seems to be very powerful.

I also am pretty sure that without a Compositor there’s no clear solution to this without doing messy work arounds that are either tedious or performance consuming - or both.

I have found another post now that has a similar question to mine, and the only (half) answer that it had was using a third viewport to try and “erase” the extra geometry - and even then, it was still pretty much visible.

[dragonforge-dev]

Thanks for that additional info. So here’s what I would do in your position.

I’d go find the ticket in the Godot Engine project (make sure you’re searching closed tickets) and ask your question on that thread. someone who worked on it will likely get back to you in a few days. When you get your answer, post it here and mark it as the answer - and maybe consider updating the documentation for those who come after you.