The reason why I want to write this series of articles is because the function of the "Layer Management" module on the project has been changed. It involves a layer data in GeoJson format.
For the data obtained from the interface, the data format was already determined when the interface was designed. The form of a standard GeoJson object.
Then, I used the GeoJsonDataSource method officially provided by Cesium to directly use this json object. So I managed to see a lot of billboard icons on the map. However, this icon is not the one I want. It is an icon that is not in my project resources. It is the default icon of Cesium or who changed the default icon displayed by the Cesium source code before.
Therefore, I can only traverse its entities again after the dataSource is created, get each entity and reset the image of the billboard.
After I was done, I kept thinking that Cesium knew that I didn't set icons for these points, so it used the default icons. So, what should I do to make Cesium use the icon I set? I looked at the source code with doubts.
Here's the extra action for my project:
loadGeoJsonWithImg: function (geoJsonObj,name,imgPath = null) {
const dataSource = new Cesium.GeoJsonDataSource(name); // 创建并取名
dataSource.load(geoJsonObj,{ clampToGround:true }).then(function (data) {
viewer.dataSources.add(data); // 添加这个datasource
const entities = data.entities.values; // 拿到所有实体
entities.forEach(entity => {
// 重置图片，图片大小，大小单位为米
entity.billboard.image = imgPath || entity.properties.style.getValue().image;
entity.billboard.width = 30;
entity.billboard.height = 30;
entity.billboard.sizeInMeters = true;
})
})
},In Cesium's official example, it loads a geojson file, each of which has a different icon. I tried to print that file. Official example:
Cesium Sandcastle
https://sandcastle.cesium.com/index.html?src=GeoJSON%2520simplestyle.html
The text content of the file of the official example:
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"properties":{"title":"wetland","marker-symbol":"wetland","marker-color":"#FF42EA"}},{"type":"Feature","geometry":{"type":"Point","coordinates":[0.30000000000000004,-1]},"properties":{"title":"zoo","marker-symbol":"zoo","marker-color":"#9195A1"}}]}Content acquisition method, add the following code to the official sample code and run it, and view it in the console card window:
Cesium.Resource.fetchJson("../SampleData/simplestyles.geojson").then(function (result){
console.log(JSON.stringify(result));
})In the official geojson file format, each feature structure looks like this:
{
"type":"Feature",
"geometry":{
"type":"Point",
"coordinates":[0.30000000000000004,-1]
},
"properties":{
"title":"zoo",
"marker-symbol":"zoo",
"marker-color":"#9195A1"
}
}It is such a feature that defines the effect of the official example, the gray giraffe icon in the last row.
Then look at how the first empty icon with no content in the first row is defined
{
"type":"Feature",
"geometry":{
"type":"Point",
"coordinates":[0,0]
},
"properties":{
"title":"Unnamed",
"marker-color":"#B9EB14"
}
}In addition to the different coordinate values in the geometry, that is the difference in the properties attribute. The first empty icon in the first row has no marker-symbol field. The marker-color determines the color of the icon, so you can guess that there are two situations:
1. When Cesium processes point-type features, it will provide a default icon billboard that can change the color, and the content inside is determined by the marker-symbol field.
2. When Cesium processes point-type features, it will provide a default icon billboard that can change the color, and the content inside is determined by the title field.
From the official website to enter the source code view, you will find such a method:
function createPoint(dataSource, geoJson, crsFunction, coordinates, options) {
let symbol = options.markerSymbol;
let color = options.markerColor;
let size = options.markerSize;
const properties = geoJson.properties;
if (defined(properties)) {
const cssColor = properties["marker-color"];
if (defined(cssColor)) {
color = Color.fromCssColorString(cssColor);
}
size = defaultValue(sizes[properties["marker-size"]], size);
const markerSymbol = properties["marker-symbol"];
if (defined(markerSymbol)) {
symbol = markerSymbol;
}
}
let canvasOrPromise;
if (defined(symbol)) {
if (symbol.length === 1) {
canvasOrPromise = dataSource._pinBuilder.fromText(
symbol.toUpperCase(),
color,
size
);
} else {
canvasOrPromise = dataSource._pinBuilder.fromMakiIconId(
symbol,
color,
size
);
}
} else {
canvasOrPromise = dataSource._pinBuilder.fromColor(color, size);
}
const billboard = new BillboardGraphics();
billboard.verticalOrigin = new ConstantProperty(VerticalOrigin.BOTTOM);
// Clamp to ground if there isn't a height specified
if (coordinates.length === 2 && options.clampToGround) {
billboard.heightReference = HeightReference.CLAMP_TO_GROUND;
}
const entity = createObject(
geoJson,
dataSource._entityCollection,
options.describe
);
entity.billboard = billboard;
entity.position = new ConstantPositionProperty(crsFunction(coordinates));
const promise = Promise.resolve(canvasOrPromise)
.then(function (image) {
billboard.image = new ConstantProperty(image);
})
.catch(function () {
billboard.image = new ConstantProperty(
dataSource._pinBuilder.fromColor(color, size)
);
});
dataSource._promises.push(promise);
}The parameters of this method:
The first dataSource: the created dataSource, and the created things will be added later.
The second geoJson: the geojson object to process
The third crsFunction: a method
The fourth coordinates: coordinate points
Fifth options: Call the second parameter of Cesium.GeoJsonDataSource.load(data, options).
First, three variables are defined, and the values set in options are used by default. If the geojson to be processed has a properties attribute, use the corresponding value.
Get a conclusion that the "marker-color" in properties determines a certain color value.
Then a variable canvasOrPromise is defined. From the name analysis, this variable either stores a canvas object or an asynchronous promise. If the properties of geojson are set to "marker-symbol", then use it, and then check whether the options set markerSymbol, if the first two are not, then directly use the color and size to create a promise.
I almost understand it when I get here. If the length of "marker-symbol" in properties is only 1, then Cesium will convert it to uppercase, and then use the dataSource._pinBuilder.fromText method to pass in the text content, color and size. Asynchronously drawn into an icon.
dataSource._pinBuilder is known to have three methods, fromText to pass in text, fromMakiIconId to pass in the id value of an icon, and fromColor only do color and size processing.
PinBuilder is a loading method of Cesium:
The id set by "marker-symbol" is actually the icon in this website:
Maki Icons | By Mapbox 
"marker-symbol" is actually the name of the image file in this folder.
In this way, we can understand the processing logic of the official sample file:
If marker-symbol is a string with a length of 1, Cesium will convert the text to uppercase, and then generate an icon with the corresponding text. If the length is not 1, then Cesium will regard it as the address of a picture, and set the color and size for the picture.
So, we can see those numbers and letters, is the generated canvas. And the other icons that cannot be typed out through the keyboard are the image resource addresses used.
The next step is to create an icon billboard and set whether to stick to the ground
Then create an entity that needs to be displayed, and assign the billboard to it.
As you can see here, Cesium uses the promise asynchronous method. If there is no failure or error, the billboard will be set to the corresponding icon. Otherwise, it is the default icon, then the default icon comes from this method: dataSource._pinBuilder.fromColor(color, size)
Therefore, if the backend gives us a bunch of point-type data for the frontend according to the GeoJSON data format, then we should make some requirements for the backend. That is, the properties in each feature set a "marker-symbol value to store the image path corresponding to this point. In this way, we don't need to set the image of each entity's billboard.
In the options parameter of the Cesium.GeoJsonDataSource.load(data, options) method, we have learned that the three values of markerSize, markerSymbol, and markerColor will be used in the createPoint method of the source code. Next, you can look at other creation methods. There are a total of these methods in the source code to create points, lines and surfaces:
createPoint Create a point, which can be regarded as creating a billboard, corresponding to the Point type.
createLineString creates a line, which can be regarded as creating a polyline, corresponding to the LineString type.
createPolygon creates a surface, which can be regarded as creating a polygon, corresponding to the Polygon type.
createObject creates an entity. After all, those points, lines and surfaces need to be placed in the entity.
Multiple points, lines and surfaces are created multiple times in the source code. There are only four bottom-level creation methods.
It also just corresponds to the type defined by the GeoJSON data specification.
First look at the createLineString method, which values of options are used when creating a polyline.
The function of the incoming parameter is the same as that of the createPoint method, so the explanation will not be repeated.
Two variables are set to determine the material and width of the line. In the same way, go to the geojson object to get the value in the properties.
Then create an entity, and then make a polylineGraphics to set the width, position, material and other things.
The summary of the createLineString method is:
// 通过geojson对象来设置线的宽度颜色和透明度
{
"type":"Feature",
"geometry":{
"type":"LineString",
"coordinates": 多个坐标形成的数组
},
"properties":{
"stroke-width":"3", // 宽度。最好使用数字，或者字符串内容为数字
"stroke-opacity":"0.5", // 透明度。范围：0-1.0
"stroke":"#9195A1" // 颜色
}
}Next, look at the createPolygon method, how to create a surface. This is more troublesome.
Three variables are defined, the color of the face outline, the fill material of the face, and the width of the face outline.
Different from creating lines, there are two more attributes, fill color and fill transparency
Another feature of the polygon surface is that the surface can be hollowed out. Cesium handles hollowing like this:
Except for the first value of the coordinate array, the rest are treated as hollow.
The createPolygon method summarizes that if you want to set the drawn surface through the geojson object:
// 通过geojson对象来设置线的宽度颜色和透明度
{
"type":"Feature",
"geometry":{
"type":"Polygon",
"coordinates": // 二维数组，子项为坐标数组
数组[第一个值为面的坐标，后续为镂空.....]
},
"properties":{
"stroke-width":"3", // 轮廓宽度，和线的设置一样
"stroke-opacity":"0.5", // 轮廓线透明度
"stroke":"#9195A1" // 轮廓颜色
// 面独有的设置
"fill":"#9195A1" // 颜色值
"fill-opacity": "0.5" // 填充透明度
}
}This article mainly discusses how to pass GeoJSON objects to better draw the results we want. More will not be studied, and another chapter will be analyzed.
Summarize:
For the point, we can set the icon and color in the json object like this.
{
.....省略
"properties": {
"marker-color": "颜色值",
"marker-symbol": "单个字为单字图标，否则为图片路径"
}
}