Hiding walls which cover the player in a 3D RPG

2025-03-03

Introduction

This year I started a 3D rpg project in godot. Choosing to make the project both in 3D, and multiplayer lead to some quite interesting new ideas and problems to solve. This is one such problem.

Taking a scene from an RPG such as Chrono Trigger. There is some floor, some walls, some furniture, and the main character - Crono. The measurements aren’t hard to make as the game uses a 16x16 pixel grid, and we can quickly get a rough idea of what the 3d level will look like:

Download chrono_room_back.xcf

Let’s say we want a view of the room from the other side. I imagine that would look something like this:

It looks alright, but we can’t see Crono, or any of the furniture because the front wall is covering it. This would work well if we were viewing the house from the outside, but not when we’re inside. So let’s remove the closest wall. Let’s also add the walls that would be visible from this angle:

Notice how which walls are shown depends on which direction the camera is facing. When viewing from the front, only the back wall with the window is shown. When viewed from the back however, 3 additional walls need to be drawn, and the previous ‘back’ wall is hidden.

The challenge here comes from determining which walls are obscuring the level from view and should be hidden, and which walls do not obscure the level and should be shown. In other words, ‘how to hide walls which obscure the level’.

Raycasts

At first I used raycasts to detect which walls were covering the floor. For each vertex of the wall, I cast a ray away from the camera, and checked for intersection with the floor.

This method was not very reliable, as it did not detect a wall as obsctructing the floor when the wall was higher than the floor. Additionally, when the wall vertex was on the same plane as the floor, the collision was not always detected.

Wall normals

The other solution I wrote with the help of the following few resources¹:

• Viewing 3D Rooms with Backface Culling for Walls by (Interactive Web Demo) by Ritchie Lozada
• I made a shader to apply backface culling to an entire wall mesh at once. How’s it look? : r/Unity3D by 2DArray

What caught my attention was this animated gif in the reddit post which achieved exactly the effect I wanted:

From then on it was a (fairly) simple matter of implementation.

1. Each wall is assigned a normal (a direction which points to where the walls’ ‘inside’ is.

2. Some maths is used to determine if the camera is looking at the back face of a wall. If the camera is looking at the back face of a wall, the wall is hidden.

3. In some cases, this still shows a wall that is covering the floor. I go over each wall that is still shown after the previous step, and perform a few dozen² raycasts for each wall

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