Cesium billboard 自定义shader实现描边效果

      uniform sampler2D u_atlas;
      uniform vec2 dimensions;

      in vec2 v_textureCoordinates;
      in vec4 v_pickColor;
      in vec4 v_color;
      in float v_splitDirection;

      void main()
      {
          if (v_splitDirection < 0.0 && gl_FragCoord.x > czm_splitPosition) discard;
          if (v_splitDirection > 0.0 && gl_FragCoord.x < czm_splitPosition) discard;

          float outlineWidth = 2.0;
          vec2 uv = v_textureCoordinates;

          vec2 uv_up = vec2(uv.x, uv.y + 1./dimensions.y * outlineWidth);
          vec2 uv_down = vec2(uv.x, uv.y - 1./dimensions.y * outlineWidth);

          vec2 uv_left = vec2(uv.x + 1./dimensions.x * outlineWidth, uv.y);
          vec2 uv_right = vec2(uv.x - 1./dimensions.x * outlineWidth, uv.y);

          vec4 color_up = texture(u_atlas, uv_up);
          vec4 color_down = texture(u_atlas, uv_down);
          vec4 color_left = texture(u_atlas, uv_left);
          vec4 color_right = texture(u_atlas, uv_right);

          vec4 outlineColor = color_left + color_right + color_up + color_down;
          // outlineColor = outlineColor / 4.0;
          outlineColor.rgb = vec3(1., 0., 0.);

          vec4 rayColor = texture(u_atlas, uv);

          vec4 color = mix(outlineColor,rayColor, rayColor.a);
          color = czm_gammaCorrect(color);

      // Fully transparent parts of the billboard are not pickable.
      #if !defined(OPAQUE) && !defined(TRANSLUCENT)
          if (color.a < 0.005)   // matches 0/255 and 1/255
          {
              discard;
          }
      #else
      // The billboard is rendered twice. The opaque pass discards translucent fragments
      // and the translucent pass discards opaque fragments.
      #ifdef OPAQUE
          if (color.a < 0.995)   // matches < 254/255
          {
              discard;
          }
      #else
          if (color.a >= 0.995)  // matches 254/255 and 255/255
          {
              discard;
          }
      #endif
      #endif

      #ifdef VECTOR_TILE
          color *= u_highlightColor;
      #endif
          out_FragColor = color;

      #ifdef LOG_DEPTH
          czm_writeLogDepth();
      #endif