Sunlight in the caves

[ddel-rio]

Describe the bug Within a terrain surrounded by walls, sunlight can pass through them and cast strange shadows. This also occurs at the surface when the sun is "below the ground".

To reproduce In the MRP dig a cave and wait for dusk.

mrp.zip

Expected behavior The walls should prevent any external light source from entering.

Screenshots

Environment

• OS: Windows 11
• GPU: NVIDIA GeForce RTX 4070 Laptop
• Godot Version: v4.3.dev.custom_build [f6a78f83a]
• Module Version: a0628c7d
• Renderer: Vulkan (Forward+)

[Zylann]

When using shadowmapping with lights, this is a known side effect of how Godot implements them. Godot projects shadows when lights encounter front faces. So while the terrain surface will project shadows under overhangs or caves (when ground is still loaded), no shadows will be projected if the sun comes from below (only encountering backfaces) or when there is no surface anymore to project them in deep caves, or on the edges of unloaded chunks.

This is not a trivial issue to solve, and that might mean you have to do some compromises at the moment.

Some ideas:

• Set all your opaque materials to be double-sided, so shadows are projected from both sides. This will come at the cost of a bit more work for the graphics card to rasterize pixels that would otherwise not be there. It also doesn't solve the issue fully, as light could still leak a little due to shadowmapping being an approximtaion with a degree of error.
• Instead of making everything double-sided, change light shadow settings. You can tell it to project shadows from back faces instead of front faces with shadow_reverse_cull_face. But that still has the issue that there will be leaks. A workaround to leaks would be to somehow detect in the shader when we are rendering a shadowmap, and move vertices slightly towards the sun to artificially create "thickness" between the actuall walls of the cave and what the shadow will "hit". Unfortunately, Godot doesn't expose anything (like a #define?) that would allow you to do this in your shader. The closest hack I could think of is to detect if the projection matrix is orthogonal. Also, Godot doesn't expose a uniform to get sun direction from within the vertex shader. You would have to set this yourself.
• Make your world fixed-height. Your VoxelViewer can be fixed to the same Y coordinate, so the world always has a fixed vertical height loaded. That ensures there will usually always be ground to project shadows over caves during the day. However sunsets and nights might be a problem. Minecraft shaderpacks have similar issues, though maybe mods tried to workaround that somehow. The original game doesn't have shadowmapping after all.
• Never allow shadows to project from below. When the sun sets, fade away its shadows, at the same time as its light intensity. Eventually make then go up again as the moon rises, instead of letting the sun project from below.
• Turn off the sun when deep enough. In practice that could work for a while, though of course if someone carves a gigantic canyon it might look a little strange from below, unless the surface is too far to be loaded anyways?
• Add invisible "shadow occluder quads" between the "filled" sides of non-empty chunks. These would be setup in a way that makes them rendered only by shadowmapping, to ensure that shadows get projected. Unfortunately this still wouldn't always work: if your VoxelViewer is allowed to move indefinitely vertically, if you dig a hole that's suposed to be covered at the surface, but that surface is no longer in range, then the sun can still shine through the hole. The same goes with caves large enough to remain open throughout the whole loaded area, which can be indistinguishable from being at the surface with some sort of floating island on top. I also don't know how much performance overhead this would cause, and I'm not aware of an official rendering feature to "only render on the shadowmap" (though there are hacks to make it functionally work, at the expense of still paying the cost as if it was still rendering everywhere else). Doing this could be attempted with a script, but might require work on the C++ side of the engine to be efficient.
• Have LOD do the work. Instead of disappearing, far-away ground can remain present as lower-resolution and bigger chunk size, which also has the effect of providing cover for shadows to project over larger distances, with minor eventual "popping" as LODs switch. This might be one of the most natural solutions. Unfortunately, blocky voxels don't support LOD yet. It's a big piece of work, which comes with its own caveats (notably affecting generators). This is something planned longer term, but isn't available for now. Also, unless your world is a planet, there is still the issue of the night, which however you could mitigate as described earlier.

[Zylann]

I pushed a branch shadow_occluders which adds an option to use the hack of the same name mentionned earlier (https://github.com/Zylann/godot_voxel/commit/0e6fafdfe643c0e65811b6536f2ccbf6285f8f4b).

It adds 6 extra properties to VoxelMesherBlocky that toggle these occluders for each desired chunk side. If the sun only looks down, the bottom occluder is not necessary. If the sun only looks angled towards the same side, then the opposite occluder is also not needed. If the sun is perfectly vertical, half of lateral occluders might not be needed either etc. Not bullet proof, edges of the map can still get lit in some cases as mentionned.

Not yet in the main branch because I'm not sure if it's useful enough (you/others have to test it to find out if it works well enough), and also because I'm waiting for Godot 4.2.2 to be released before adding new features to the main branch.

[ddel-rio]

Thank you very much! I've been busy with other things that have more priority in the game so I haven't delved into this topic. But I just tested the branch and it works quite well. In my case now the engine behaves exactly as I expected and I have not found any bugs, so I have nothing to object to.

[JekSun97]

Thank you very much! I've been busy with other things that have more priority in the game so I haven't delved into this topic. But I just tested the branch and it works quite well. In my case now the engine behaves exactly as I expected and I have not found any bugs, so I have nothing to object to.

Could you send me a before and after screenshot?

[ddel-rio]

Could you send me a before and after screenshot?

Of course, I also made an mrp to test the new feature:

mrp.zip

Before

With the best configuration I found:

• DirectionalLight3D > Reverse Cull Face = true
• Voxel Terrain > Cast Shadow = Double-Sided

By digging too deep, light leaks around the edges.

After

Using shadow occluders.

Now the light edges have disappeared.

[JekSun97]

@ddel-rio Thanks a lot)

[Zylann]

Regarding your answer, I thought of another idea that wouldn't require rendering shadow occluders, and might have the advantage of rendering a bit faster. However this is a workaround to do on your side, and since Godot is not exposing enough things, it is more work:

Instead of making everything double-sided, change light shadow settings. You can tell it to project shadows from back faces instead of front faces with shadow_reverse_cull_face. But that still has the issue that there will be leaks. A workaround to leaks would be to somehow detect in the shader when we are rendering a shadowmap, and move vertices slightly towards the sun to artificially create "thickness" between the actuall walls of the cave and what the shadow will "hit". Unfortunately, Godot doesn't expose anything (like a #define?) that would allow you to do this in your shader. The closest hack I could think of is to detect if the projection matrix is orthogonal. Also, Godot doesn't expose a uniform to get sun direction from within the vertex shader. You would have to set this yourself.

Haven't tested that, just an idea