问题原因
我公司里有一个项目，需要能够一个矩形的四个顶点能够随意移动，而且上面还绘制的纹理。然后，我按照以前的方式书写，问题来了，由于一个矩形是由两个三角形组成的，然后就出现下面喜闻乐见的效果： 
我们会发现，两个三角形的交汇处，有一道折线，所以比较尴尬了。
而我们需要的是什么呢： 
我们需要的是上面这种，具有透视效果，而且图形不会有折线的效果。
即使是随意的扭曲，也不会出现折线，这就是所谓的透视纹理绘制。 
思路
- 首先，我需要先获取到当前绘制的那一点的坐标x，y两个轴的，然后计算这一个点和四个矩形顶点的位置关系，计算出这个点需要绘制的纹理图片的位置。关系图如下：
查看图形，其实我们唯一不知道的就是过图形的线和两个点切割获得的比例，通过高中的学的图形知识，将五个点代进去，计算一下，最后出来的就是这个点。
由于(c1-v1)/(v0-v1) = (c2-v3)/(v2-v3)这个比例保证每一个点在纹理坐标中确定需要绘制的纹理。
然后通过两个点确定一条直线，根据两点方程(x-c1.x)/(c2.x-c1.x) = (y-c1.y)/(c2.y-c1.y)然后将所有的值代入进入，再通过一元二次方程的解析，最后得出比例。
以下是js版本的代码：
function calc(p1,p2,p3,p4,p0) {
/*
* p1-------p3
* | |
* | |
* | |
* p2-------p4
*假设四个点的坐标分别是(x1,y1)、(x2,y2)、(x3,y3)、(x4,y4)
* 则下式成立平行
* （x1-x2)/(y1-y2)=(x3-x4)/(y3-y4)
*
*/
/*
* 根据两点式方程
* (y-y1)/(y2-y1) = (x-x1)/(x2-x1)
* 得出
* (y-y1)(x2-x1) = (y2-y1)(x-x1)
* yx2-yx1-y1x2+y1x1 = y2x-y2x1-y1x+y1x1
* yx2-yx1-y1x2+y1x1-(y2x-y2x1-y1x+y1x1) = 0;
* yx2-yx1-y1x2+y1x1-y2x+y2x1+y1x-y1x1 = 0;
* yx2-yx1-y1x2-y2x+y2x1+y1x = 0;\
* x2(y-y1)+x1(y2-y)+x(y1-y2)=0
*
* //两个点的位置为
* p12Xlen = p2.x-p1.x;
* p12Ylen = p2.y-p1.y;
* p34Xlen = p4.x-p3.x;
* p34Ylen = p4.y-p3.y;
* p12.x = x1 = p1.x+p12Xlen*s;
* p12.y = y1 = p1.y+p12Ylen*s;
* p34.x = x2 = p3.x+p34Xlen*s;
* p34.y = y2 = p3.y+p34Ylen*s;
* x = p0.x;
* y = p0.y;
*
* 由此得出
* x2(y-y1) = (p3.x+p34Xlen*s)(p0.y-(p1.y+p12Ylen*s));
* x2(y-y1) = (p3.x + p34Xlen*s)(p0.y - p1.y - p12Ylen*s);
* x2(y-y1) = p3.x*p0.y-p3.x*p1.y-p3.x*p12Ylen*s+p0.y*p34Xlen*s-p1.y*p34Xlen*s-p12Ylen*p34Xlen*s*s;
* x2(y-y1) = p3.x*p0.y-p3.x*p1.y+(-p3.x*p12Ylen+p0.y*p34Xlen-p1.y*p34Xlen)*s-p12Ylen*p34Xlen*s*s;
*
* x1(y2-y) = (p1.x+p12Xlen*s)*(p3.y+p34Ylen*s-p0.y);
* x1(y2-y) = (p1.x+p12Xlen*s)*(p3.y-p0.y+p34Ylen*s);
* x1(y2-y) = p1.x*p3.y-p1.x*p0.y+p1.x*p34Ylen*s+p3.y*p12Xlen*s-p0.y*p12Xlen*s+p12Xlen*p34Ylen*s*s;
* x1(y2-y) = p1.x*p3.y-p1.x*p0.y+(p1.x*p34Ylen+p3.y*p12Xlen-p0.y*p12Xlen)*s+p12Xlen*p34Ylen*s*s;
*
* x(y1-y2) = p0.x(p1.y+p12Ylen*s-(p3.y+p34Ylen*s));
* x(y1-y2) = p0.x(p1.y+p12Ylen*s-p3.y-p34Ylen*s);
* x(y1-y2) = p0.x(p1.y-p3.y+p12Ylen*s-p34Ylen*s);
* x(y1-y2) = p0.x(p1.y-p3.y)+p0.x(p12Ylen-p34Ylen)*s;
*
* 得出
*
* A = p12Xlen*p34Ylen-p12Ylen*p34Xlen;
*
* B = p1.x*p34Ylen+p3.y*p12Xlen-p0.y*p12Xlen-p3.x*p12Ylen+p0.y*p34Xlen-p1.y*p34Xlen+p0.x(p12Ylen-p34Ylen);
*
* C = p3.x*p0.y-p3.x*p1.y+p1.x*p3.y-p1.x*p0.y+p0.x(p1.y-p3.y)
*
* Ass + Bs +C = 0;
* */
var p12Xlen = p2.x-p1.x;
var p12Ylen = p2.y-p1.y;
var p34Xlen = p4.x-p3.x;
var p34Ylen = p4.y-p3.y;
var a = p12Xlen*p34Ylen-p12Ylen*p34Xlen;
var b = p1.x*p34Ylen+p3.y*p12Xlen-p0.y*p12Xlen-p3.x*p12Ylen+p0.y*p34Xlen-p1.y*p34Xlen+p0.x*(p12Ylen-p34Ylen);
var c = p3.x*p0.y-p3.x*p1.y+p1.x*p3.y-p1.x*p0.y+p0.x*(p1.y-p3.y);
//在两条线平行的情况下
if(a === 0){
return -c/b;
}
//在两条线不平行的情况下 获取数组里面再0到1范围内的
return [(-b+Math.sqrt(b*b-4.0*a*c))/(2*a),(-b-Math.sqrt(b*b-4.0*a*c))/(2*a)];
}案例代码
以下是可以运行的案例代码，把文件放到服务里面，设置好图片，就可以直接查看效果：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Title</title>
<style>
body {
margin: 0;
text-align: center;
}
#canvas {
display: block;
}
#div{
width:800px;
height:800px;
position: relative;
margin: 0 auto;
}
</style>
</head>
<body onload="main()">
<div id="div">
<canvas id="canvas" height="800" width="800"></canvas>
</div>
</body>
<script src="https://johnson2heng.github.io/webgl-demo/lib/webgl-utils.js"></script>
<script src="https://johnson2heng.github.io/webgl-demo/lib/webgl-debug.js"></script>
<script src="https://johnson2heng.github.io/webgl-demo/lib/cuon-utils.js"></script>
<script src="https://johnson2heng.github.io/webgl-demo/lib/cuon-matrix.js"></script>
<script>
/*第一部分顶点着色器接收顶点的纹理坐标，传递给片元着色器*/
var VSHADER_SOURCE = "" +
"attribute vec4 a_Position;\n" +//
"varying vec2 v_Position;\n" +
"void main(){\n" +
" gl_Position = a_Position;\n" +
" v_Position = vec2(a_Position);\n" +
"}\n";
var FSHADER_SOURCE = "" +
"precision mediump float;\n" +//
"uniform sampler2D u_Sampler;\n" +//
"varying vec2 v_Position;\n" +
"uniform vec2 u_point0;\n" +
"uniform vec2 u_point1;\n" +
"uniform vec2 u_point2;\n" +
"uniform vec2 u_point3;\n" +
"float calc(vec2 p1,vec2 p2,vec2 p3,vec2 p4,vec2 p0){\n" +
//如果不是平行的两条直线
" float p12Xlen = p2.x-p1.x;\n" +
" float p12Ylen = p2.y-p1.y;\n" +
" float p34Xlen = p4.x-p3.x;\n" +
" float p34Ylen = p4.y-p3.y;\n" +
" float a = p12Xlen*p34Ylen-p12Ylen*p34Xlen;\n" +
" float b = p1.x*p34Ylen+p3.y*p12Xlen-p0.y*p12Xlen-p3.x*p12Ylen+p0.y*p34Xlen-p1.y*p34Xlen+p0.x*(p12Ylen-p34Ylen);\n" +
" float c = p3.x*p0.y-p3.x*p1.y+p1.x*p3.y-p1.x*p0.y+p0.x*(p1.y-p3.y);\n" +
//两条线都垂直于x轴或者y轴的情况下
" if(a == 0.0){\n" +
" return -c/b;\n" +
" }\n" +
//两条线不平行的情况下
" float endA = (-b+sqrt(b*b-4.0*a*c))/(2.0*a);\n" +
" float endB = (-b-sqrt(b*b-4.0*a*c))/(2.0*a);\n" +
" if(endA > 0.0 && endA < 1.0){\n" +
" return endA;\n" +
" }\n" +
" if(endB > 0.0 && endB < 1.0){\n" +
" return endB;\n" +
" }\n" +
" else {\n" +
" return -c/b;\n" +
" }\n" +
"}\n" +
""+
//两个点的位置，第一个calc（v0,v2,v1,v3,p0） 第二个calc(v1,v0,v3,v2,p0)
"void main(){\n" +
" gl_FragColor = texture2D(u_Sampler,vec2(calc(u_point0,u_point2,u_point1,u_point3,v_Position),calc(u_point1,u_point0,u_point3,u_point2,v_Position)));\n" +//
"}\n";
//四个顶点的位置
var v0 = {x:-0.5,y:0.5};
var v1 = {x:-0.5,y:-0.5};
var v2 = {x:0.5,y:0.5};
var v3 = {x:0.5,y:-0.5};
//图片的地址设置
var imgSrc = "../image/door/001.png";
var gl;
/*第二部分 main()方法 初始化着色器，设置顶点信息，调用配置纹理方法*/
function main() {
var canvas = document.getElementById("canvas");
gl = getWebGLContext(canvas);
if(!gl){
console.log("你的电脑不支持WebGL！");
return;
}
if(!initShaders(gl,VSHADER_SOURCE,FSHADER_SOURCE)){
console.log("初始化着色器失败！");
return;
}
//设置顶点的相关信息
var n = initVertexBuffers(gl);
if(n < 0){
console.log("无法获取到点的数据");
return;
}
//配置纹理
if(!initTextures(gl,n)){
console.log("无法配置纹理");
return;
}
}
/*第三部分 initVertexBuffers() 设置顶点坐标和纹理坐标 调用initTextures()进行下一步处理*/
function initVertexBuffers(gl) {
/*
* v0-------v2
* | |
* | |
* | |
* v1-------v3
*
* [v0.x,v0.y,v1.x,v1.y,v2.x,v2.y,v3.x,v3.y]
* config.Positions[0].Config 数组由八个数据组成，代表四个点的位置，距离中心点的偏移量
* */
var verticesSizes = new Float32Array([
//四个顶点的位置
v0.x,v0.y,
v1.x,v1.y,
v2.x,v2.y,
v3.x,v3.y
]);
initArrayBuffer(gl, verticesSizes, 2, gl.FLOAT, "a_Position");
//将四个顶点位置传入
var point0 = gl.getUniformLocation(gl.program, "u_point0");
var point1 = gl.getUniformLocation(gl.program, "u_point1");
var point2 = gl.getUniformLocation(gl.program, "u_point2");
var point3 = gl.getUniformLocation(gl.program, "u_point3");
if(point0 < 0){
alert("无法获取到存储的位置");
}
gl.uniform2f(point0,v0.x,v0.y);
gl.uniform2f(point1,v1.x,v1.y);
gl.uniform2f(point2,v2.x,v2.y);
gl.uniform2f(point3,v3.x,v3.y);
var indexSize = new Uint8Array([0,1,3,0,2,3]);
//将索引写入缓冲区对象
var indexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER,indexBuffer);
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER,indexSize, gl.STATIC_DRAW);
return indexSize.length;
}
//创建变量缓冲区
function initArrayBuffer(gl, data, num, type, attribute) {
//创建缓冲区对象
var buffer = gl.createBuffer();
if (!buffer) {
console.log("无法创建缓冲区对象");
return -1;
}
//绑定缓冲区对象并写入数据
gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);
//获取顶点位置变量位置
var a_attribue = gl.getAttribLocation(gl.program, attribute);
if (a_attribue < 0) {
console.log("无法获取顶点位置的存储变量");
return -1;
}
//对位置的顶点数据进行分配，并开启
gl.vertexAttribPointer(a_attribue, num, type, false, 0, 0);
gl.enableVertexAttribArray(a_attribue);
}
/*第四部分 initTextures() 创建纹理对象 并调用纹理绘制方法*/
function initTextures(gl,n) {
var texture = gl.createTexture();//创建纹理对象
if(!texture){
console.log("无法创建纹理对象");
return;
}
//获取u_Sampler的存储位置
var u_Sampler = gl.getUniformLocation(gl.program,"u_Sampler");
if(u_Sampler < 0){
console.log("无法获取变量的存储位置");
return;
}
//创建Image对象，并绑定加载完成事件
var image = new Image();
image.onload = function () {
loadTexture(gl,n,texture,u_Sampler,image);
//初始化交互事件
initMoveBtn();
};
image.src = imgSrc;
return true;
}
function initMoveBtn() {
var canvas = document.querySelector("#canvas");
var div = document.querySelector("#div");
var canvasWidth = canvas.width;
var canvasHeight = canvas.height;
addBtn(v0);
addBtn(v1);
addBtn(v2);
addBtn(v3);
function addBtn(obj) {
var x = (obj.x+1)/2;
var y = 1-(obj.y+1)/2;
var btn = document.createElement("div");
btn.style.cssText = "height:10px; width:10px; transform:translate(-50%,-50%); position:absolute; background:green; cursor:pointer;";
btn.style.left = x *canvasWidth+"px";
btn.style.top = y * canvasHeight +"px";
div.appendChild(btn);
var downX,downY,btnX,btnY;
btn.addEventListener("mousedown",function (event) {
//鼠标按下时的位置
downX = event.clientX;
downY = event.clientY;
//按下时btn的位置
btnX = parseFloat(btn.style.left);
btnY = parseFloat(btn.style.top);
document.addEventListener("mousemove",move,true);
document.addEventListener("mouseup",up,true);
},true);
function move(event) {
event.preventDefault();
var moveX = event.clientX;
var moveY = event.clientY;
var x = moveX-downX + btnX;
var y = moveY - downY + btnY;
btn.style.left = x+"px";
btn.style.top = y + "px";
obj.x = x/canvasWidth*2-1;
obj.y = (1-y/canvasHeight)*2-1;
//重新绘制
reload();
}
function up() {
document.removeEventListener("mousemove",move,true);
document.removeEventListener("mouseup",up,true);
}
}
}
/*第五部分 设置纹理相关信息供WebGL使用，并进行绘制*/
function loadTexture(gl,n,texture,u_Sampler,image) {
//对纹理图像进行y轴反转
gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL,1);
//开启0号纹理单元
gl.activeTexture(gl.TEXTURE0);
//向target绑定纹理对象
gl.bindTexture(gl.TEXTURE_2D,texture);
//配置纹理参数
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
//配置纹理图像
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
//将0号纹理传递给着色器
gl.uniform1i(u_Sampler,0);
draw(n);
}
//重新绘制的方法
function reload(){
var n = initVertexBuffers(gl);
//绘制
draw(n);
}
function draw(n) {
//绘制
gl.clearColor(0.0,0.0,0.0,1.0);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
}
</script>
</html>