參照網頁的一些demo可知,primitive就是對geometry的二次封裝。
primitive需要指定geometryInstance屬性和appearance屬性,因此脫離entity和primitive來實現圖元的渲染,則需要構造geometry和appearance。
首先來看geometry,查閱API可知有四個屬性
attributes |
GeometryAttributes | Attributes, which make up the geometry's vertices. | |
primitiveType |
PrimitiveType | PrimitiveType.TRIANGLES |
optionalThe type of primitives in the geometry. |
indices |
Uint16Array | Uint32Array | optionalOptional index data that determines the primitives in the geometry. | |
boundingSphere |
BoundingSphere |
GeometryAttributes中一共有6個屬性
bitangent: bitangent屬性(標準化),用於凹凸映射等切線空間效果。//暫未明白如何使用
color : 頂點座標的顏色
normal : 法線,通常用於光照 //暫未明白如何使用
position :頂點位置屬性
st :紋理座標
tangent :正切屬性(規範化),用於凹凸映射等切線空間效果。 //暫未明白
先打開primitiveType 可以發現,其實這裏面的type與webgl原生的draw type就完全一致
以上六個屬性均由GeometryAttribute構造生成,GeometryAttribute的API爲:
componentDatatype |
ComponentDatatype | optionalThe datatype of each component in the attribute, e.g., individual elements in values. | |
componentsPerAttribute |
Number | optionalA number between 1 and 4 that defines the number of components in an attributes. | |
normalize |
Boolean | false |
optionalWhen true and componentDatatype is an integer format, indicate that the components should be mapped to the range [0, 1] (unsigned) or [-1, 1] (signed) when they are accessed as floating-point for rendering. |
values |
TypedArray | optionalThe values for the attributes stored in a typed array. |
componentDatatype:數據類型
componentsPerAttribute:每個元素取的value中的個數,可理解爲webgl中的步長
normalize:是否歸一化
values:座標數組
indice就不用說了,索引座標,索引position。
boundingSphere:
包圍球,查看API可發現有響應的多種方式生成。
根據以上知識:構造三角扇。
var p1 = Cesium.Cartesian3.fromDegrees(120.6822, 50.9247, 10); var p2 = Cesium.Cartesian3.fromDegrees(120.6822, 38.9247, 10); var p3 = Cesium.Cartesian3.fromDegrees(133.6822, 38.9247, 10); var p4 = Cesium.Cartesian3.fromDegrees(133.6822, 50.9247, 10); var positions = new Float64Array([ p1.x, p1.y, p1.z, p2.x, p2.y, p2.z, p3.x, p3.y, p3.z, p4.x, p4.y, p4.z ]); var colors = new Float32Array([ 1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 1.0, 1.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0 ]); var geometry = new Cesium.Geometry({ attributes: { position: new Cesium.GeometryAttribute({ componentDatatype: Cesium.ComponentDatatype.DOUBLE, componentsPerAttribute: 3, values: positions }), color: new Cesium.GeometryAttribute({ componentDatatype: Cesium.ComponentDatatype.FLOAT, componentsPerAttribute: 4, values: colors }) }, //索引 indices: new Uint16Array([ 0, 1, 2, //第一個三角形用的座標點 1, 2, 3 //第二個三角形用的座標點 ]), //繪製類型 primitiveType: Cesium.PrimitiveType.TRIANGLE_FAN , boundingSphere: Cesium.BoundingSphere.fromVertices(positions) });