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Cesium 地球网格构造

Cesium原理篇：3最长的一帧之地形(2：高度图)

HeightmapTessellator

用于从高程图像创建网格。提供了一个函数 computeVertices，可以根据高程图像创建顶点数组。

该函数的参数包括高程图像、高度数据的结构、网格宽高、边缘裙板高度、矩形范围、相机中心点等。函数的实现使用了许多性能优化技巧，如将函数内常量化、内联等。

该模块的输出为一个对象，包括创建好的顶点数组、最大与最小高度、该网格的边界球、边界方向盒等信息。

1、函数定义

// 声明
static computeVertices(options) {}

// 使用
const width = 5;
const height = 5;
const statistics = Cesium.HeightmapTessellator.computeVertices({heightmap : [1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0],width : width,height : height,skirtHeight : 0.0,nativeRectangle : {west : 10.0,east : 20.0,south : 30.0,north : 40.0}
});const encoding = statistics.encoding;
const position = encoding.decodePosition(statistics.vertices, index);

options 参数:

裙边(skirt)

防止不同层级mesh加载时出现裂缝

实现

  static computeVertices(options) {const cos = Math.cos;const sin = Math.sin;const sqrt = Math.sqrt;const toRadians = CesiumMath.toRadians;const heightmap = options.heightmap;const width = options.width;const height = options.height;const skirtHeight = options.skirtHeight;const hasSkirts = skirtHeight > 0.0;const ellipsoid = defaultValue(options.ellipsoid, Ellipsoid.WGS84);const nativeRectangle = Rectangle.clone(options.nativeRectangle);const rectangle = Rectangle.clone(options.rectangle);const geographicWest = rectangle.west;const geographicSouth = rectangle.south;const geographicEast = rectangle.east;const geographicNorth = rectangle.north;const relativeToCenter = options.relativeToCenter;const includeWebMercatorT = defaultValue(options.includeWebMercatorT, false);const exaggeration = defaultValue(options.exaggeration, 1.0);const exaggerationRelativeHeight = defaultValue(options.exaggerationRelativeHeight,0.0);const hasExaggeration = exaggeration !== 1.0;const includeGeodeticSurfaceNormals = hasExaggeration;const rectangleWidth = Rectangle.computeWidth(nativeRectangle);const rectangleHeight = Rectangle.computeHeight(nativeRectangle);// 每个网格的长宽const granularityX = rectangleWidth / (width - 1);const granularityY = rectangleHeight / (height - 1);const radiiSquared = ellipsoid.radiiSquared;const radiiSquaredX = radiiSquared.x;const radiiSquaredY = radiiSquared.y;const radiiSquaredZ = radiiSquared.z;let minimumHeight = 65536.0;let maximumHeight = -65536.0;const fromENU = Transforms.eastNorthUpToFixedFrame(relativeToCenter,ellipsoid);const minimum = minimumScratch;minimum.x = Number.POSITIVE_INFINITY;minimum.y = Number.POSITIVE_INFINITY;minimum.z = Number.POSITIVE_INFINITY;const maximum = maximumScratch;maximum.x = Number.NEGATIVE_INFINITY;maximum.y = Number.NEGATIVE_INFINITY;maximum.z = Number.NEGATIVE_INFINITY;let hMin = Number.POSITIVE_INFINITY;const gridVertexCount = width * height;const edgeVertexCount = skirtHeight > 0.0 ? width * 2 + height * 2 : 0;const vertexCount = gridVertexCount + edgeVertexCount;const positions = new Array(vertexCount);const heights = new Array(vertexCount);const uvs = new Array(vertexCount);let startRow = 0;let endRow = height;let startCol = 0;let endCol = width;if (hasSkirts) {--startRow;++endRow;--startCol;++endCol;}for (let rowIndex = startRow; rowIndex < endRow; ++rowIndex) {let row = rowIndex;if (row < 0) {row = 0;}if (row >= height) {row = height - 1;}////  ^ latitude(纬度)//  |//  |              North(90)//  |              ---------//  |             |         |//  |  West(-180) |         | East(0)//  |             |         |//  |              ---------//  |              South(-90)//  -----------------------------> longitude(经度)// 地理坐标系下// 当前纬度(latitude) 距离最北头(North) 的距离// 这个值是越来越小的, 随着行数越来越大let latitude = nativeRectangle.north - granularityY * row;latitude = toRadians(latitude);// 当前纬度(latitude) 距离最南头(South) 的百分比(0~1)let v = (latitude - geographicSouth) / (geographicNorth - geographicSouth);v = CesiumMath.clamp(v, 0.0, 1.0);const isNorthEdge = rowIndex === startRow;const isSouthEdge = rowIndex === endRow - 1;const cosLatitude = cos(latitude);const nZ = sin(latitude);const kZ = radiiSquaredZ * nZ;for (let colIndex = startCol; colIndex < endCol; ++colIndex) {let col = colIndex;if (col < 0) {col = 0;}if (col >= width) {col = width - 1;}const terrainOffset = row * width + col;let heightSample = heightmap[terrainOffset]let longitude = nativeRectangle.west + granularityX * col;longitude = toRadians(longitude);let u = (longitude - geographicWest) / (geographicEast - geographicWest);u = CesiumMath.clamp(u, 0.0, 1.0);let index = row * width + col;if (skirtHeight > 0.0) {const isWestEdge = colIndex === startCol;const isEastEdge = colIndex === endCol - 1;const isEdge = isNorthEdge || isSouthEdge || isWestEdge || isEastEdge;const isCorner = (isNorthEdge || isSouthEdge) && (isWestEdge || isEastEdge);if (isCorner) {// Don't generate skirts on the corners.continue;} else if (isEdge) {heightSample -= skirtHeight;if (isWestEdge) {// The outer loop iterates north to south but the indices are ordered south to north, hence the index flip below// 外循环从北到南迭代，但索引按从南到北的顺序排列，因此索引在下面翻转index = gridVertexCount + (height - row - 1);} else if (isSouthEdge) {// Add after west indices. South indices are ordered east to west.// 加在西方指数之后。南方指数是从东向西排列的。index = gridVertexCount + height + (width - col - 1);} else if (isEastEdge) {// Add after west and south indices. East indices are ordered north to south. The index is flipped like above.// 在西部和南部指数后加上。东部指数是从北向南排列的。索引如上所述翻转。index = gridVertexCount + height + width + row;} else if (isNorthEdge) {// Add after west, south, and east indices. North indices are ordered west to east.// 在西部、南部和东部指数后添加。北方指数是从西向东排列的。index = gridVertexCount + height + width + height + col;}}}// 经纬度转笛卡尔坐标系const nX = cosLatitude * cos(longitude);const nY = cosLatitude * sin(longitude);const kX = radiiSquaredX * nX;const kY = radiiSquaredY * nY;const gamma = sqrt(kX * nX + kY * nY + kZ * nZ);const oneOverGamma = 1.0 / gamma;const rSurfaceX = kX * oneOverGamma;const rSurfaceY = kY * oneOverGamma;const rSurfaceZ = kZ * oneOverGamma;const position = new Cartesian3();position.x = rSurfaceX + nX * heightSample;position.y = rSurfaceY + nY * heightSample;position.z = rSurfaceZ + nZ * heightSample;hMin = Math.min(hMin, heightSample);positions[index] = position;uvs[index] = new Cartesian2(u, v);heights[index] = heightSample;}}const aaBox = new AxisAlignedBoundingBox(minimum, maximum, relativeToCenter);const encoding = new TerrainEncoding(relativeToCenter,aaBox,hMin,maximumHeight,fromENU,false,includeWebMercatorT,includeGeodeticSurfaceNormals,exaggeration,exaggerationRelativeHeight);const vertices = new Float32Array(vertexCount * encoding.stride);let bufferIndex = 0;for (let j = 0; j < vertexCount; ++j) {bufferIndex = encoding.encode(vertices,bufferIndex,positions[j],uvs[j],heights[j],undefined,undefined,undefined);}return {vertices: vertices,};}