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推荐：用 NSDT编辑器 快速搭建可编程3D场景

在Three.js应用开发中，有时你可能需要为3D场景中的网格绘制2D的包围框，应该怎么做？

朴素的想法是把网格的3D包围框投影到屏幕空间，例如，下图中的绿色框 3D包围框， 当将其投影为 2D 时，得到的红色2D包围框，显然比想要的蓝色框大很多：

正确的做法是先将3D网格投影到屏幕空间，再计算2D包围框。

1、计算单个网格的2D包围框

只需将所有顶点转换为屏幕空间并从中创建一个 2d 边界框：

function computescreenspaceboundingbox(mesh, camera) {var vertices = mesh.geometry.vertices;var vertex = new three.vector3();var min = new three.vector3(1, 1, 1);var max = new three.vector3(-1, -1, -1);for (var i = 0; i < vertices.length; i++) {var vertexworldcoord = vertex.copy(vertices[i]).applymatrix4(mesh.matrixworld);var vertexscreenspace = vertexworldcoord.project(camera);min.min(vertexscreenspace);max.max(vertexscreenspace);}return new three.box2(min, max);
}

生成的 box2 位于标准化屏幕坐标 [-1, 1] 中， 可以通过乘以渲染器高度和宽度的一半来获得像素：

function normalizedtopixels(coord, renderwidthpixels, renderheightpixels) {var halfscreen = new three.vector2(renderwidthpixels/2, renderheightpixels/2)return coord.clone().multiply(halfscreen);
}

2、计算3DObject的2D包围框

更完善的实现需要考虑组、子节点等各种3DObject，其中可能包含多个网格，这需要一个递归实现，代码如下：

function computescreenspaceboundingbox(obj, camera) {var min;var max;// is this an array of objects?if(array.isarray(obj)) {for(var i = 0; i < obj.length; ++i) {let box2 = computescreenspaceboundingbox(obj[i], camera);if(min === undefined) {min = box2.min.clone();max = box2.max.clone();} else {min.min(box2.min);max.max(box2.max);}}}// does this object have geometry?if(obj.geometry !== undefined) {var vertices = obj.geometry.vertices;if(vertices === undefined&& obj.geometry.attributes !== undefined&& 'position' in obj.geometry.attributes) {// buffered geometryvar vertex = new three.vector3();       var pos = obj.geometry.attributes.position;for(var i = 0; i < pos.count * pos.itemsize; i += pos.itemsize){vertex.set(pos.array[i], pos.array[i + 1], pos.array[1 + 2]);var vertexworldcoord = vertex.applymatrix4(obj.matrixworld);var vertexscreenspace = vertexworldcoord.project(camera);if(min === undefined) {min = vertexscreenspace.clone();max = vertexscreenspace.clone();}min.min(vertexscreenspace);max.max(vertexscreenspace);}} else {// regular geometryvar vertex = new three.vector3();       for(var i = 0; i < vertices.length; ++i) {var vertexworldcoord = vertex.copy(vertices[i]).applymatrix4(obj.matrixworld);var vertexscreenspace = vertexworldcoord.project(camera);if(min === undefined) {min = vertexscreenspace.clone();max = vertexscreenspace.clone();}min.min(vertexscreenspace);max.max(vertexscreenspace);}}}// does this object have children?if(obj.children !== undefined) {for(var i = 0; i < obj.children.length; ++i) {let box2 = computescreenspaceboundingbox(obj.children[i], camera);if(min === undefined) {min = box2.min.clone();max = box2.max.clone();} else {min.min(box2.min);max.max(box2.max);}}}return new three.box2(min, max);
}

原文链接：计算3D对象的2D包围框 — BimAnt