title: How to generate 3D shape/plane from 2d pixel points/map?
body: <p>For a 2d grey image, I want to extract each pixel's grey/brightness value as depth/z-index to generate a smooth 3d plane, something like a brightness level depth map.</p>
  
  <p>My simple idea is that loop add each pixel as a point in the scene, and set z-index with grey value. 
  But, first how to add points(a very small one to represent a pixel) with Three.js?
  And, is this a good way? is this a smooth plane? what if a large image(e.g. 2000*4800, I don't think memory can hold that much points)?</p>