网上有很多效果都是基于后处理来实现的
如bloom,雷达扫描,圆形扩散等;尽管前期学习了一些关于着色器的知识,但是要直接上手写代码还是比较困难
实际上,理解的都比较困难
因此在痛苦的折磨之后,终于算是摸清了一点点门道。下面这篇文章对于与PostProcessStage做出了解释,
文中的每一句话我都懂,但最后读代码就不懂了。
https://www.cnblogs.com/webgl-angela/p/9272810.html
先上代码:
let viewer = new Cesium.Viewer('cesiumContainer',{
imageryProvider:new Cesium.WebMapTileServiceImageryProvider({
url:"http://t0.tianditu.com/img_w/wmts?service=wmts&request=GetTile&version=1.0.0&LAYER=img&tileMatrixSet=w&TileMatrix={TileMatrix}&TileRow={TileRow}&TileCol={TileCol}&style=default&format=tiles"+"&tk=token",
layer: "tdtBasicLayer",
style: "default",
format: "image/jpeg",
maximumLevel:18, //天地图的最大缩放级别
tileMatrixSetID: "GoogleMapsCompatible",
show: false
}),
selectionIndicator : false,
infoBox : false,
//terrainProvider: Cesium.createWorldTerrain(),
shouldAnimate : true,
baseLayerPicker:true,
// selectedImageryProviderViewModel:cs[7],
});
var lat = 39.70524201566827;// 42.006;
var lon = 117.01296152309055;//128.055;
viewer.scene.globe.depthTestAgainstTerrain = true;
//取消双击事件
viewer.cesiumWidget.screenSpaceEventHandler.removeInputAction(Cesium.ScreenSpaceEventType.LEFT_DOUBLE_CLICK);
/*
添加扫描线 depth关闭 lon:-74.01296152309055 lat:40.70524201566827 height:129.14366696393927
viewer
cartographicCenter 扫描中心
maxRadius 最大半径 米
scanColor 扫描颜色
duration 持续时间 毫秒
*/
function AddCircleScanPostStage(viewer, cartographicCenter, maxRadius, scanColor, duration) {
var ScanSegmentShader =
"uniform sampler2D colorTexture;\n" + //颜色纹理
"uniform sampler2D depthTexture;\n" + //深度纹理
"varying vec2 v_textureCoordinates;\n" + //纹理坐标
"uniform vec4 u_scanCenterEC;\n" + //扫描中心
"uniform vec3 u_scanPlaneNormalEC;\n" + //扫描平面法向量
"uniform float u_radius;\n" + //扫描半径
"uniform vec4 u_scanColor;\n" + //扫描颜色
//根据二维向量和深度值 计算距离camera的向量
"vec4 toEye(in vec2 uv, in float depth)\n" +
" {\n" +
" vec2 xy = vec2((uv.x * 2.0 - 1.0),(uv.y * 2.0 - 1.0));\n" +//
" vec4 posInCamera =czm_inverseProjection * vec4(xy, depth, 1.0);\n" +//看看源码中关于此函数的解释是,cesium系统自动生成的4*4的反投影变换矩阵,从clip坐标转为眼睛坐标,clip坐标是指顶点着色器的坐标系统gl_position输出的
" posInCamera =posInCamera / posInCamera.w;\n" + //将视角坐标除深度分量
" return posInCamera;\n" +
" }\n" +
//点在平面上的投影,输入参数为 平面法向量,平面起始点,测试点
"vec3 pointProjectOnPlane(in vec3 planeNormal, in vec3 planeOrigin, in vec3 point)\n" +
"{\n" +
//计算测试点与平面起始点的向量
"vec3 v01 = point -planeOrigin;\n" +
//平面法向量与 测试点与平面上的点 点积 点积的几何意义,b在a上的投影长度,
//即v01在平面法向量上的长度
"float d = dot(planeNormal, v01) ;\n" +
//planeNormal * d 即为v01在平面法向量上的投影向量
//根据三角形向量相加为0的原则 即可得点在平面上的投影
"return (point - planeNormal * d);\n" +
"}\n" +
//获取深度值,根据纹理坐标获取深度值
"float getDepth(in vec4 depth)\n" +
"{\n" +
"float z_window = czm_unpackDepth(depth);\n" + //源码解释将一个vec4向量还原到0,1内的一个数
"z_window = czm_reverseLogDepth(z_window);\n" + //czm_reverseLogDepth解开深度
"float n_range = czm_depthRange.near;\n" +//
"float f_range = czm_depthRange.far;\n" +
"return (2.0 * z_window - n_range - f_range) / (f_range - n_range);\n" +
"}\n" +
"void main()\n" +
"{\n" +
"gl_FragColor = texture2D(colorTexture, v_textureCoordinates);\n" + //片元颜色
"float depth = getDepth( texture2D(depthTexture, v_textureCoordinates));\n" +//根据纹理获取深度值
"vec4 viewPos = toEye(v_textureCoordinates, depth);\n" +//根据纹理坐标和深度值获取视点坐标
//点在平面上的投影,平面法向量,平面中心,视点坐标
"vec3 prjOnPlane = pointProjectOnPlane(u_scanPlaneNormalEC.xyz, u_scanCenterEC.xyz, viewPos.xyz);\n" +
//计算投影坐标到视点中心的距离
"float dis = length(prjOnPlane.xyz - u_scanCenterEC.xyz);\n" +
//如果在扫描半径内,则重新赋值片元颜色
"if(dis < u_radius)\n" +
"{\n" +
//计算与扫描中心的距离并归一化
"float f = dis/ u_radius;\n" +
//原博客如下,实际上可简化为上式子
//"float f = 1.0 -abs(u_radius - dis) / u_radius;\n" +
//四次方
"f = pow(f, 2.0);\n" +
//mix(x, y, a): x, y的线性混叠, x(1-a) + y*a;,
//效果解释:在越接近扫描中心时,f越小,则片元的颜色越接近原来的,相反则越红
"gl_FragColor = mix(gl_FragColor, u_scanColor, f);\n" +
"}\n" +
"}\n";
var _Cartesian3Center = Cesium.Cartographic.toCartesian(cartographicCenter);
var _Cartesian4Center = new Cesium.Cartesian4(_Cartesian3Center.x, _Cartesian3Center.y, _Cartesian3Center.z, 1);
var _CartographicCenter1 = new Cesium.Cartographic(cartographicCenter.longitude, cartographicCenter.latitude, cartographicCenter.height + 500);
var _Cartesian3Center1 = Cesium.Cartographic.toCartesian(_CartographicCenter1);
var _Cartesian4Center1 = new Cesium.Cartesian4(_Cartesian3Center1.x, _Cartesian3Center1.y, _Cartesian3Center1.z, 1);
var _time = (new Date()).getTime();
var _scratchCartesian4Center = new Cesium.Cartesian4();
var _scratchCartesian4Center1 = new Cesium.Cartesian4();
var _scratchCartesian3Normal = new Cesium.Cartesian3();
var ScanPostStage = new Cesium.PostProcessStage({
fragmentShader: ScanSegmentShader,
uniforms: {
u_scanCenterEC: function () {
return Cesium.Matrix4.multiplyByVector(viewer.camera._viewMatrix, _Cartesian4Center, _scratchCartesian4Center);
},
u_scanPlaneNormalEC: function () {
var temp = Cesium.Matrix4.multiplyByVector(viewer.camera._viewMatrix, _Cartesian4Center, _scratchCartesian4Center);
var temp1 = Cesium.Matrix4.multiplyByVector(viewer.camera._viewMatrix, _Cartesian4Center1, _scratchCartesian4Center1);
_scratchCartesian3Normal.x = temp1.x - temp.x;
_scratchCartesian3Normal.y = temp1.y - temp.y;
_scratchCartesian3Normal.z = temp1.z - temp.z;
Cesium.Cartesian3.normalize(_scratchCartesian3Normal, _scratchCartesian3Normal);
return _scratchCartesian3Normal;
},
u_radius: function () {
return maxRadius * (((new Date()).getTime() - _time) % duration) / duration;
},
u_scanColor: scanColor
}
});
viewer.scene.postProcessStages.add(ScanPostStage);
}
viewer.camera.setView({
destination: Cesium.Cartesian3.fromDegrees(lon, lat, 300000)
});
var CartographicCenter = new Cesium.Cartographic(Cesium.Math.toRadians(lon), Cesium.Math.toRadians(lat), 0);
var scanColor = new Cesium.Color(1.0, 0.0, 0.0, 1);
AddCircleScanPostStage(viewer, CartographicCenter, 1500, scanColor, 4000);
首先要明确一点:webgl中顶点着色器与片元着色器总是成对出现
而该API是仅提供了片元着色器代码的入口,实际上该API会默认获取当前视角下的视图范围作为顶点着色器;
将 着色器中 "if(dis < u_radius)\n" 注释掉之后就会发现,整个视图内基本都变成红色了
关于这一点,在首次接触这段代码的时候困扰了我很久
实现该效果的原理就是:
判断片元与圆心的距离是否大于半径,如果小于半径,则更改该片元的颜色,否则使用原来的片元颜色;
我们看到的是一个圆形,实际上,该圆形也是由若干个片元构成的