shader 画圆，椭圆

概述

记录一下如何使用shader来画圆

let shader = {
    vs: `
      varying vec2 v_uv;
      void main() {
        v_uv = uv;
        gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4( position, 1.0 );
      }
        `,
    fs: `
      varying vec2 v_uv;
      uniform float u_time;
      void main() {
        vec3 bcol = vec3(0.29412, 0.70196, 0.63921) * (1.0 - 0.6 * length(v_uv));
        // 原理：根据uv值来计算到原点的距离，大于半径值即在圆外，小于半径值在圆内
        float pct = distance(vec2(0.5,0.5), v_uv);
        // 椭圆
        // pct = distance(v_uv,vec2(0.470,0.600)) + distance(v_uv,vec2(0.570,0.540));
        //两个圆相吸
        // pct = distance(v_uv,vec2(0.550,0.520)) * distance(v_uv,vec2(0.010,0.040));
        //两个圆
        // pct = min(distance(v_uv,vec2(0.140,0.130)),distance(v_uv,vec2(0.660,0.600)));
        //两个圆相交部分 - 可以用来做遮罩
        // pct = max(distance(v_uv,vec2(0.550,0.590)),distance(v_uv,vec2(0.6)));
        //半径
        float radius = 0.1;
        // smoothstep(a, b, x)可以用来生成0到1的平滑过渡. 如果a > b 那x为1-》0过渡 ，如果a<b，那位0=》1过渡
        float col = smoothstep(radius + 0.008, radius, pct) ;
        gl_FragColor = vec4(vec3(col + bcol), 1.0);
      }
        `,
    uniform: {
      u_time: {
        value: 0.0
      }
    }
  }

  let geometry = new THREE.PlaneGeometry(20, 20);

  let material = new THREE.ShaderMaterial({
    vertexShader: shader.vs,
    fragmentShader: shader.fs,
    uniforms: shader.uniform,
    transparent: true
  })

  let mesh = new THREE.Mesh(geometry, material);
  // mesh.rotateX(-Math.PI / 2);
  scene.add(mesh);