Delve into the theory: reflection is basically an image of a scene taken from a certain position. Therefore, if you want the flat grid to serve as a mirror, you will need to add the camera to this position so that it turns the scene into a texture in the animation loop, and then use this texture in the planar mesh material, I would also recommend watching http: / /stemkoski.imtqy.com/Three.js/Reflection.html in addition to the examples mentioned by WestLangley.
In addition, play with the settings; for a less reflective effect, for example, try:
var mirrorMaterial = new THREE.MeshBasicMaterial( { color: 0x111111, envMap: mirrorCamera.renderTarget } );
or
var mirrorMaterial = new THREE.MeshPhongMaterial( { emissive: 0x111111, envMap: mirrorCamera.renderTarget } );
Lee stemkoski
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