import { Mesh, BackSide, SphereGeometry, ShaderMaterial, Color } from "three";

const SKY_COLOR = 0x999999;
const GROUND_COLOR = 0x242424;
const SKY_SIZE = 40000;

export function addSkyGradient(scene) {
  const vertexShader = `
      varying vec3 vWorldPosition;
            void main() {
                vec4 worldPosition = modelMatrix * vec4( position, 1.0 );
                vWorldPosition = worldPosition.xyz;
                gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
            }`;
  const fragmentShader = `
      uniform vec3 topColor;
            uniform vec3 bottomColor;
            varying vec3 vWorldPosition;
            void main() {
                float h = (normalize(vWorldPosition).y ) / 2.0 + 0.25;
                gl_FragColor = vec4( mix( bottomColor, topColor, h ), 1.0 );
            }`;
  const uniforms = {
    topColor: { value: new Color(SKY_COLOR) },
    bottomColor: { value: new Color(GROUND_COLOR) },
  };
  const skyGeo = new SphereGeometry(SKY_SIZE, 32, 15);
  const skyMat = new ShaderMaterial({
    uniforms,
    vertexShader,
    fragmentShader,
    side: BackSide,
  });
  const sky = new Mesh(skyGeo, skyMat);
  scene.add(sky);
}