上一篇已经讲述了通过面模拟线条时候，每一个顶点的顶点数据包括：端点坐标、偏移量、前一个端点坐标、后一个端点坐标，当然如果我们通过索引的方式来绘制的话，还包括索引数组，下面的代码通过传递一组线条的端点数组来创建上述相关数据：
```
bk.Line3D = function (points,colors){
this.points = points;
this.colors = colors;
}
bk.Line3D.prototype.computeData = function() {
var len = this.points.length;
var count = len * 3 * 2;
var position = new Float32Array(count);
var positionPrev = new Float32Array(count);
var positionNext = new Float32Array(count);
var color = new Float32Array(count);
var offset = new Float32Array(len * 2);
var indicesCount = 3 * 2 * (len - 1);
var indices = new Uint16Array(indicesCount);
var triangleOffset = 0,vertexOffset = 0;
for(var i = 0; i < len; i ++){
var i3 = i * 3 * 2;
var point = this.points[i];
position[i3 + 0] = point.x;
position[i3 + 1] = point.y;
position[i3 + 2] = point.z;
position[i3 + 3] = point.x;
position[i3 + 4] = point.y;
position[i3 + 5] = point.z;
var r = (i + 1) / len;
var g = Math.random();
var b = Math.random();
g = r;
b = 0;
r = 1- r;
color[i3 + 0] = r;
color[i3 + 1] = g;
color[i3 + 2] = b;
color[i3 + 3] = r;
color[i3 + 4] = g;
color[i3 + 5] = b;
if (i < count - 1) {
var i3p = i3 + 6;
positionNext[i3p + 0] = point.x;
positionNext[i3p + 1] = point.y;
positionNext[i3p + 2] = point.z;
positionNext[i3p + 3] = point.x;
positionNext[i3p + 4] = point.y;
positionNext[i3p + 5] = point.z;
}
if (i > 0) {
var i3n = i3 - 6;
positionPrev[i3n + 0] = point.x;
positionPrev[i3n + 1] = point.y;
positionPrev[i3n + 2] = point.z;
positionPrev[i3n + 3] = point.x;
positionPrev[i3n + 4] = point.y;
positionPrev[i3n + 5] = point.z;
}
var idx = 3 * i;
var i2 = i * 2;
offset[i2 + 0] = 5;
offset[i2 + 1] = -5;
}
var end = count - 1;
for(i = 0;i < 6 ;i ++){
positionNext[i] = positionNext[i + 6];
positionPrev[end - i] = positionPrev[end - i - 6];
}
for(i = 0;i < indicesCount ;i ++){
if(i % 2 == 0){
indices[triangleOffset ++] = i;
indices[triangleOffset ++] = i + 1;
indices[triangleOffset ++] = i + 2;
}else{
indices[triangleOffset ++] = i + 1;
indices[triangleOffset ++] = i;
indices[triangleOffset ++] = i + 2;
}
}
this.position = position;
this.positionNext = positionNext;
this.positionPrev = positionPrev;
this.color = color;
this.offset = offset;
this.indices = indices;
};
```
代码首先定义了一个类，该类构造函数可以传入端点数组；在该类上定义了一个方法 computeData，用来计算顶点数组，每个顶点包括上文所述的4个信息，另外增加了一个颜色信息。
读者，可以结合第二篇的思路和上面的代码来来理解，此处不再详述 代码的细节。
另外一个比较重要的代码是顶点着色器中，通过传入的这些顶点信息来计算最终的顶点坐标，代码如下：
```
var lineVS = `
attribute vec3 aPosition;
attribute vec3 aPositionPre;
attribute vec3 aPositionNext;
attribute float aOffset;
attribute vec3 aColor;
varying vec3 vColor;
uniform mat4 uWorldViewProjection;
uniform vec4 uViewport;
uniform float uNear;
uniform mat4 uViewMatrix;
uniform mat4 uProjectMatrix;
vec4 clipNear(vec4 p1,vec4 p2){
float n = (p1.w - uNear) / (p1.w - p2.w);
return vec4(mix(p1.xy,p2.xy,n),-uNear,uNear);
}
void main(){
vec4 prevProj = uWorldViewProjection * vec4(aPositionPre, 1.0);
vec4 currProj = uWorldViewProjection * vec4(aPosition, 1.0);
vec4 nextProj = uWorldViewProjection * vec4(aPositionNext, 1.0);
if (currProj.w < 0.0) {
if (prevProj.w < 0.0) {
currProj = clipNear(currProj, nextProj);
}else {
currProj = clipNear(currProj, prevProj);
}
}
vec2 prevScreen = (prevProj.xy / abs(prevProj.w) + 1.0) * 0.5 * uViewport.zw;
vec2 currScreen = (currProj.xy / abs(currProj.w) + 1.0) * 0.5 * uViewport.zw;
vec2 nextScreen = (nextProj.xy / abs(nextProj.w) + 1.0) * 0.5 * uViewport.zw;
vec2 dir;
float len = aOffset;
if(aPosition == aPositionPre){
dir = normalize(nextScreen - currScreen);
}else if(aPosition == aPositionNext){
dir = normalize(currScreen - prevScreen);
}else {
vec2 dirA = normalize(currScreen - prevScreen);
vec2 dirB = normalize(nextScreen - currScreen);
vec2 tanget = normalize(dirA + dirB);
float miter = 1.0 / max(dot(tanget,dirA),0.5);
len *= miter;
dir = tanget;
}
dir = vec2(-dir.y,dir.x) * len;
currScreen += dir;
currProj.xy = (currScreen / uViewport.zw - 0.5) * 2.0 * abs(currProj.w);
vec4 pos = uProjectMatrix * uViewMatrix * vec4(aPosition,1.0);
vColor = aColor;
gl_Position = currProj;
}
`;
```
计算的原理，也可以参考第二篇的论述，此处需要注意的是，为了能够计算顶点在屏幕上的最终位置，需要把canvans的尺寸大小传递给着色器（uniform 变量 uViewport），同样为了计算裁剪，需要把镜头的near值传递给着色器（uniform 变量 uNear），而变量uWorldViewProjection表示模型视图透视变换的矩阵，熟悉WebGL的同学一定清楚。
