[Cesium]Cesium基础学习—

[Cesium]Cesium基础学习——Primitive

Cesium开发高级篇 | 01空间数据可视化之Primitive - 知乎

Primitive由两部分组成：几何体（Geometry）和外观（Appearance）。几何体定义了几何类型、位置和颜色，例如三角形、多边形、折线、点、标签等；外观则定义了Primitive的着色或渲染（Shading），包括GLSL（OpenGL着色语言，OpenGL Shading Language）顶点着色器和片元着色器（ vertex and fragment shaders），以及渲染状态（render state）。粗略地说，几何实例定义了结构和位置，外观定义了视觉特征。

1.几何类型

2.外观

Cesium开发高级篇 | 02材质设置 - 知乎

Cesium中的Material类的内部机制是通过一种json格式的Fabric对象来表达物体的外观特征，而这些外观特征是由漫反射(diffuse)、镜面反射(specular)、法向量(normal)、自发光(emission)以及透明度（alpha）组合（即一个Components）而成。可通过两种方式去获取并设置几何对象材质：

Material.fromType方法
Fabric方法
// Create a color material with fromType:
polygon.material = Cesium.Material.fromType('Color');
polygon.material.uniforms.color = new Cesium.Color(1.0, 1.0, 0.0, 1.0);-----------------------------------------------------------------------------
// Create the default material:
polygon.material = new Cesium.Material();
// Create a color material with full Fabric notation:
polygon.material = new Cesium.Material({fabric : {type : 'Color',uniforms : {color : new Cesium.Color(1.0, 1.0, 0.0, 1.0)}}
});
两种MaterialProperty

ImageMaterialProperty 贴图材质，可以给几何对象表面贴上一张图片。
var imgUrl = "./images/bumpmap.png";var imgMaterial = new Cesium.ImageMaterialProperty({image: imgUrl,repeat: new Cesium.Cartesian2(4, 4),color: Cesium.Color.BLUE,});viewer.entities.add({position: Cesium.Cartesian3.fromDegrees(-65.0, 40.0, 100000.0),ellipse: {semiMajorAxis: 300000.0, // 长半轴距离semiMinorAxis: 200000.0, // 短半轴距离height: 20000.0,material: imgMaterial,},});
PolylineArrowMaterialProperty 带有箭头的线

官方案例Cesium Sandcastle

3.常见方法

getGeometryInstanceAttributes(id)

Returns the modifiable per-instance attributes for a GeometryInstance.

返回一个GeometryInstance的可修改的每个实例属性。

const attributes = primitive.getGeometryInstanceAttributes('an id');
attributes.color = Cesium.ColorGeometryInstanceAttribute.toValue(Cesium.Color.AQUA);
attributes.show = Cesium.ShowGeometryInstanceAttribute.toValue(true);
attributes.distanceDisplayCondition = Cesium.DistanceDisplayConditionGeometryInstanceAttribute.toValue(100.0, 10000.0);
attributes.offset = Cesium.OffsetGeometryInstanceAttribute.toValue(Cartesian3.IDENTITY);

4.贴地或贴模型特性

Cesium Sandcastle 线贴模型

Cesium Sandcastle

Cesium贴地设置_primitive贴地-CSDN博客

跟Entity[HeightReference]类似，Primitive也支持贴地或贴模型的特性，但不一样的是，Primitive是通过classificationType属性控制的。其中GroundPolylineGeometry、GroundPolylinePrimitive结合实现贴地线；

GroundPrimitive实现贴地几何形状，包括CircleGeometry、CorridorGeometry、EllipseGeometry、PolygonGeometry、RectangleGeometry；ClassificationPrimitive可实现贴地或贴模型，包括BoxGeometry、CylinderGeometry、EllipsoidGeometry、PolylineVolumeGeometry、SphereGeometry几何形状。

5.GroupPrimitive

GroupPrimitive表示场景中覆盖在Terrain或3DTiles上的几何体

Support for the WEBGL_depth_texture extension is required to use GeometryInstances with different PerInstanceColors or materials besides PerInstanceColorAppearance.

(支持WEBGL_depth_texture扩展需要使用不同的PerInstanceColors或PerInstanceColor Appearance 材料的GeometryInstances。)

Textured GroundPrimitives were designed for notional patterns and are not meant for precisely mapping textures to terrain - for that use case, use SingleTileImageryProvider

Textured GroundPrimitives是为空想模式设计的，并不是为了精确地将纹理映射到地形。对于这种情况，使用SingleTileImageryProvider。

For correct rendering, this feature requires the EXT_frag_depth WebGL extension. For hardware that do not support this extension, there will be rendering artifacts for some viewing angles.

为了正确渲染，这个特性需要EXT_frag_depth WebGL扩展。对于不支持此扩展的硬件，将会有一些视角的渲染工件。

Valid geometries are CircleGeometry, CorridorGeometry, EllipseGeometry, PolygonGeometry, and RectangleGeometry.

常见方法：

Cesium.GroundPrimitive.initializeTerrainHeights()

Initializes the minimum and maximum terrain heights. This only needs to be called if you are creating the GroundPrimitive synchronously.
Return:A promise that will resolve once the terrain heights have been loaded.

Cesium.GroundPrimitive.isSupported(scene)

Cesium.GroundPrimitive.supportsMaterials(scene)

Checks if the given Scene supports materials on GroundPrimitives. Materials on GroundPrimitives require support for the WEBGL_depth_texture extension.
Return:Checks if the given Scene supports materials on GroundPrimitives. Materials on GroundPrimitives require support for the WEBGL_depth_texture extension.

getGeometryInstanceAttributes(id)