WebGL 绘制Line的bug(三)

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。

如果你对WebGL 感兴趣，可以了解下我们用WebGL开发的3D机房项目：
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