Three.js Geometry Face Disappearance Issue

Based on your screenshots, it appears you're experiencing a common issue in 3D rendering called "face culling." This happens when Three.js determines which faces to render based on their orientation relative to the camera.

Likely Causes

1. Face Normals: The most common cause is incorrect face normals. Three.js uses normals to determine which side of a face is the "front" and which is the "back".

2. Back-face Culling: By default, Three.js only renders the front faces of geometry (those whose normals point toward the camera) and culls (doesn't render) back faces to improve performance.

Solutions

Try one of these approaches:

1. Disable Back-face Culling:

   JAVASCRIPTCopied
   material.side = THREE.DoubleSide;

   This will render both sides of each face, but may impact performance slightly.

2. Compute Vertex Normals:
   If you're creating a custom geometry, make sure to compute the normals:

   JAVASCRIPTCopied
   geometry.computeVertexNormals();

3. Check Face Winding Order:
   Ensure your vertices are defined in the correct order (typically counter-clockwise for front faces).

4. Flip Normals (if needed):

   JAVASCRIPTCopied
   // For BufferGeometry
   const normals = geometry.attributes.normal.array;
   for (let i = 0; i < normals.length; i++) {
     normals[i] = -normals[i];
   }
   geometry.attributes.normal.needsUpdate = true;

Without seeing your actual code, I'd recommend starting with setting material.side = THREE.DoubleSide as it's the simplest solution, though not always the most efficient for complex scenes.