cesium画一个渐变颜色的多边形

方式一：用一张颜色渐变的图片作为材质，结合color属性，可设置多边形的颜色，达到渐变效果。图片指向正北方向。

 viewer.entities.add({polygon: {hierarchy: Cesium.Cartesian3.fromDegreesArray([115.000454, 37.000292,114.999768, 37.00046,114.999322, 37.000266,114.999351, 36.999761,115.000598, 36.999793,]),material: new Cesium.ImageMaterialProperty({image: 'img.png',color: Cesium.Color.fromCssColorString('#f00')}),}});

方式二：如果没有图片，可用canvas实现，此方式控制颜色比较灵活，可以设置颜色区间以及方向等。

viewer.entities.add({polygon: {hierarchy: Cesium.Cartesian3.fromDegreesArray([115.000454, 37.000292,114.999768, 37.00046,114.999322, 37.000266,114.999351, 36.999761,115.000598, 36.999793,]),material: new Cesium.ImageMaterialProperty({image: drawImage(0)}),}});function drawImage(rotate) {var canvas = document.createElement("canvas");let ctx = canvas.getContext("2d");var grad = ctx.createLinearGradient(0, 0, 0, 300)grad.addColorStop(0, `rgba(255,0,0,0)`);grad.addColorStop(1, `rgba(255,0,0,1)`);ctx.fillStyle = grad; ctx.fillRect(0, 0, 300, 300); return canvas;}