老规矩：直接放效果图，符合就往下看，不符合出门右转。

• cesium官方暂时未提供贴地的圆，添加外轮廓线。

思路能不能写glsl更改材质，让图形显示外轮廓线。设置一个阈值距离圆心距离定值内显示一种颜色，超过这个阈值显示另一种颜色。

资料来资源Shadertoy BETA网站，网友提供的代码。

czm_material czm_getMaterial(czm_materialInput materialInput){
 	czm_material material = czm_getDefaultMaterial(materialInput);
	vec2 st = materialInput.st;
	material.diffuse = color.rgb;
    float dis = distance(st, vec2(0.5, 0.5));
    if(dis>d) {
         material.alpha = 1.0;
     }else {
    	material.alpha = color.a;
  	}
    return material;
}

函数解析

distance 获取两个点之间的距离.
为什么求(0.5,0.5)的距离了，因为坐标系左上角(0,0),所以中心点为(0.5,0.5)

示例代码

viewer.scene.groundPrimitives.add(new GroundPrimitive({
	geometryInstances: new GeometryInstance({
		geometry: new CircleGeometry({
          center: p,
          radius: r,
          extrudedHeight: HeightReference.CLAMP_TO_GROUND
       }),
 	}),
    appearance: new Appearance({
       material: new Material({
           fabric: {
              type: 'stroke',
                uniforms: {
                 	color: Color.fromCssColorString("rgba(0,0,255,0.2)"),
                    d: 0.49
                 },
                 source: `czm_material czm_getMaterial(czm_materialInput materialInput)
                              {
                                czm_material material = czm_getDefaultMaterial(materialInput);
                                vec2 st = materialInput.st;
                                material.diffuse = color.rgb;
                                float dis = distance(st, vec2(0.5, 0.5));
                                if(dis>d) {
                                    material.alpha = 1.0;
                                }else {
                                    material.alpha = color.a;
                                }
                                return material;
                              }`,
                        },
                    })
                })
            }));