目录
1. 具体实例
通过WebGL，可以渲染生成DEM(数字高程模型)。DEM(数字高程模型)是网格点组成的模型，每个点都有x，y，z值；x，y根据一定的间距组成网格状，同时根据z值的高低来选定每个点的颜色RGB。通过这个例子可以熟悉WebGL颜色渲染的过程。
2. 解决方案
1) DEM数据.XYZ文件
这里使用的DEM文件的数据组织如下，如下图所示。
其中每一行表示一个点，前三个数值表示位置XYZ，后三个数值表示颜色RGB。
2) showDEM.html
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title> 显示地形 </title>
<script src="lib/webgl-utils.js"></script>
<script src="lib/webgl-debug.js"></script>
<script src="lib/cuon-utils.js"></script>
<script src="lib/cuon-matrix.js"></script>
<script src="showDEM.js"></script>
</head>
<body>
<div><input type = 'file' id = 'demFile' ></div>
<!-- <div><textarea id="output" rows="300" cols="200"></textarea></div> -->
<div>
<canvas id ="demCanvas" width="600" height="600">
请使用支持WebGL的浏览器
</canvas>
</div>
</body>
</html>
3) showDEM.js
// Vertex shader program
var VSHADER_SOURCE =
//'precision highp float;\n' +
'attribute vec4 a_Position;\n' +
'attribute vec4 a_Color;\n' +
'uniform mat4 u_MvpMatrix;\n' +
'varying vec4 v_Color;\n' +
'void main() {\n' +
' gl_Position = u_MvpMatrix * a_Position;\n' +
' v_Color = a_Color;\n' +
'}\n';
// Fragment shader program
var FSHADER_SOURCE =
'#ifdef GL_ES\n' +
'precision mediump float;\n' +
'#endif\n' +
'varying vec4 v_Color;\n' +
'void main() {\n' +
' gl_FragColor = v_Color;\n' +
'}\n';
//
var col = 89; //DEM宽
var row = 245; //DEM高
// Current rotation angle ([x-axis, y-axis] degrees)
var currentAngle = [0.0, 0.0];
//当前lookAt()函数初始视点的高度
var eyeHight = 2000.0;
//setPerspective()远截面
var far = 3000;
//
window.onload = function () {
var demFile = document.getElementById('demFile');
if (!demFile) {
console.log("Error!");
return;
}
//demFile.onchange = openFile(event);
demFile.addEventListener("change", function (event) {
//判断浏览器是否支持FileReader接口
if (typeof FileReader == 'undefined') {
console.log("你的浏览器不支持FileReader接口！");
return;
}
//
var reader = new FileReader();
reader.onload = function () {
if (reader.result) {
//
var stringlines = reader.result.split("\n");
verticesColors = new Float32Array(stringlines.length * 6);
//
var pn = 0;
var ci = 0;
for (var i = 0; i < stringlines.length; i++) {
if (!stringlines[i]) {
continue;
}
var subline = stringlines[i].split(',');
if (subline.length != 6) {
console.log("错误的文件格式！");
return;
}
for (var j = 0; j < subline.length; j++) {
verticesColors[ci] = parseFloat(subline[j]);
ci++;
}
pn++;
}
if (ci < 3) {
console.log("错误的文件格式！");
}
//
var minX = verticesColors[0];
var maxX = verticesColors[0];
var minY = verticesColors[1];
var maxY = verticesColors[1];
var minZ = verticesColors[2];
var maxZ = verticesColors[2];
for (var i = 0; i < pn; i++) {
minX = Math.min(minX, verticesColors[i * 6]);
maxX = Math.max(maxX, verticesColors[i * 6]);
minY = Math.min(minY, verticesColors[i * 6 + 1]);
maxY = Math.max(maxY, verticesColors[i * 6 + 1]);
minZ = Math.min(minZ, verticesColors[i * 6 + 2]);
maxZ = Math.max(maxZ, verticesColors[i * 6 + 2]);
}
//包围盒中心
var cx = (minX + maxX) / 2.0;
var cy = (minY + maxY) / 2.0;
var cz = (minZ + maxZ) / 2.0;
//根据视点高度算出setPerspective()函数的合理角度
var fovy = (maxY - minY) / 2.0 / eyeHight;
fovy = 180.0 / Math.PI * Math.atan(fovy) * 2;
startDraw(verticesColors, cx, cy, cz, fovy);
}
};
//
var input = event.target;
reader.readAsText(input.files[0]);
});
}
function startDraw(verticesColors, cx, cy, cz, fovy) {
// Retrieve <canvas> element
var canvas = document.getElementById('demCanvas');
// Get the rendering context for WebGL
var gl = getWebGLContext(canvas);
if (!gl) {
console.log('Failed to get the rendering context for WebGL');
return;
}
// Initialize shaders
if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
console.log('Failed to intialize shaders.');
return;
}
// Set the vertex coordinates and color (the blue triangle is in the front)
n = initVertexBuffers(gl, verticesColors); //, verticesColors, n
if (n < 0) {
console.log('Failed to set the vertex information');
return;
}
// Get the storage location of u_MvpMatrix
var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
if (!u_MvpMatrix) {
console.log('Failed to get the storage location of u_MvpMatrix');
return;
}
// Register the event handler
initEventHandlers(canvas);
// Specify the color for clearing <canvas>
gl.clearColor(0, 0, 0, 1);
gl.enable(gl.DEPTH_TEST);
// Start drawing
var tick = function () {
//setPerspective()宽高比
var aspect = canvas.width / canvas.height;
//
draw(gl, n, aspect, cx, cy, cz, fovy, u_MvpMatrix);
requestAnimationFrame(tick, canvas);
};
tick();
}
//
function initEventHandlers(canvas) {
var dragging = false; // Dragging or not
var lastX = -1, lastY = -1; // Last position of the mouse
// Mouse is pressed
canvas.onmousedown = function (ev) {
var x = ev.clientX;
var y = ev.clientY;
// Start dragging if a moue is in <canvas>
var rect = ev.target.getBoundingClientRect();
if (rect.left <= x && x < rect.right && rect.top <= y && y < rect.bottom) {
lastX = x;
lastY = y;
dragging = true;
}
};
//鼠标离开时
canvas.onmouseleave = function (ev) {
dragging = false;
};
// Mouse is released
canvas.onmouseup = function (ev) {
dragging = false;
};
// Mouse is moved
canvas.onmousemove = function (ev) {
var x = ev.clientX;
var y = ev.clientY;
if (dragging) {
var factor = 100 / canvas.height; // The rotation ratio
var dx = factor * (x - lastX);
var dy = factor * (y - lastY);
// Limit x-axis rotation angle to -90 to 90 degrees
//currentAngle[0] = Math.max(Math.min(currentAngle[0] + dy, 90.0), -90.0);
currentAngle[0] = currentAngle[0] + dy;
currentAngle[1] = currentAngle[1] + dx;
}
lastX = x, lastY = y;
};
//鼠标缩放
canvas.onmousewheel = function (event) {
var lastHeight = eyeHight;
if (event.wheelDelta > 0) {
eyeHight = Math.max(1, eyeHight - 80);
} else {
eyeHight = eyeHight + 80;
}
far = far + eyeHight - lastHeight;
};
}
function draw(gl, n, aspect, cx, cy, cz, fovy, u_MvpMatrix) {
//模型矩阵
var modelMatrix = new Matrix4();
modelMatrix.rotate(currentAngle[0], 1.0, 0.0, 0.0); // Rotation around x-axis
modelMatrix.rotate(currentAngle[1], 0.0, 1.0, 0.0); // Rotation around y-axis
modelMatrix.translate(-cx, -cy, -cz);
//视图矩阵
var viewMatrix = new Matrix4();
viewMatrix.lookAt(0, 0, eyeHight, 0, 0, 0, 0, 1, 0);
//投影矩阵
var projMatrix = new Matrix4();
projMatrix.setPerspective(fovy, aspect, 10, far);
//模型视图投影矩阵
var mvpMatrix = new Matrix4();
mvpMatrix.set(projMatrix).multiply(viewMatrix).multiply(modelMatrix);
// Pass the model view projection matrix to u_MvpMatrix
gl.uniformMatrix4fv(u_MvpMatrix, false, mvpMatrix.elements);
// Clear color and depth buffer
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
// Draw the cube
gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_SHORT, 0);
}
function initVertexBuffers(gl, verticesColors) {
//DEM的一个网格是由两个三角形组成的
// 0------1 1
// | |
// | |
// col col------col+1
var indices = new Uint16Array((row - 1) * (col - 1) * 6);
var ci = 0;
for (var yi = 0; yi < row - 1; yi++) {
for (var xi = 0; xi < col - 1; xi++) {
indices[ci * 6] = yi * col + xi;
indices[ci * 6 + 1] = (yi + 1) * col + xi;
indices[ci * 6 + 2] = yi * col + xi + 1;
indices[ci * 6 + 3] = (yi + 1) * col + xi;
indices[ci * 6 + 4] = (yi + 1) * col + xi + 1;
indices[ci * 6 + 5] = yi * col + xi + 1;
ci++;
}
}
//创建缓冲区对象
var vertexColorBuffer = gl.createBuffer();
var indexBuffer = gl.createBuffer();
if (!vertexColorBuffer || !indexBuffer) {
return -1;
}
// 将缓冲区对象绑定到目标
gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
// 向缓冲区对象中写入数据
gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
//
var FSIZE = verticesColors.BYTES_PER_ELEMENT;
// 向缓冲区对象分配a_Position变量
var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
if (a_Position < 0) {
console.log('Failed to get the storage location of a_Position');
return -1;
}
gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
//开启a_Position变量
gl.enableVertexAttribArray(a_Position);
// 向缓冲区对象分配a_Color变量
var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
if (a_Color < 0) {
console.log('Failed to get the storage location of a_Color');
return -1;
}
gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
//开启a_Color变量
gl.enableVertexAttribArray(a_Color);
// 写入并绑定顶点数组的索引值
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
return indices.length;
}
4) 运行结果
用chrome打开showDEM.html，选择DEM文件，界面就会显示DEM的渲染效果：
3. 详细讲解
1) 读取文件
程序的第一步是通过JS的FileReader（）函数读取DEM文件，在其回调函数中读取到数组verticesColors中，它包含了位置和颜色信息。读取完成后调用绘制函数startDraw（）。
//
var reader = new FileReader();
reader.onload = function () {
if (reader.result) {
//
var stringlines = reader.result.split("\n");
verticesColors = new Float32Array(stringlines.length * 6);
//
var pn = 0;
var ci = 0;
for (var i = 0; i < stringlines.length; i++) {
if (!stringlines[i]) {
continue;
}
var subline = stringlines[i].split(',');
if (subline.length != 6) {
console.log("错误的文件格式！");
return;
}
for (var j = 0; j < subline.length; j++) {
verticesColors[ci] = parseFloat(subline[j]);
ci++;
}
pn++;
}
if (ci < 3) {
console.log("错误的文件格式！");
}
//
var minX = verticesColors[0];
var maxX = verticesColors[0];
var minY = verticesColors[1];
var maxY = verticesColors[1];
var minZ = verticesColors[2];
var maxZ = verticesColors[2];
for (var i = 0; i < pn; i++) {
minX = Math.min(minX, verticesColors[i * 6]);
maxX = Math.max(maxX, verticesColors[i * 6]);
minY = Math.min(minY, verticesColors[i * 6 + 1]);
maxY = Math.max(maxY, verticesColors[i * 6 + 1]);
minZ = Math.min(minZ, verticesColors[i * 6 + 2]);
maxZ = Math.max(maxZ, verticesColors[i * 6 + 2]);
}
//包围盒中心
var cx = (minX + maxX) / 2.0;
var cy = (minY + maxY) / 2.0;
var cz = (minZ + maxZ) / 2.0;
//根据视点高度算出setPerspective()函数的合理角度
var fovy = (maxY - minY) / 2.0 / eyeHight;
fovy = 180.0 / Math.PI * Math.atan(fovy) * 2;
startDraw(verticesColors, cx, cy, cz, fovy);
}
};
//
var input = event.target;
reader.readAsText(input.files[0]);
2) 绘制函数
绘制DEM跟绘制一个简单三角形的步骤是差不多的：
- 获取WebGL环境。
- 初始化shaders，构建着色器。
- 初始化顶点数组，分配到缓冲对象。
- 绑定鼠标键盘事件，设置模型视图投影变换矩阵。
- 在重绘函数中调用WebGL函数绘制。
其中最关键的步骤是第三步，初始化顶点数组initVertexBuffers()。
function startDraw(verticesColors, cx, cy, cz, fovy) {
// Retrieve <canvas> element
var canvas = document.getElementById('demCanvas');
// Get the rendering context for WebGL
var gl = getWebGLContext(canvas);
if (!gl) {
console.log('Failed to get the rendering context for WebGL');
return;
}
// Initialize shaders
if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
console.log('Failed to intialize shaders.');
return;
}
// Set the vertex coordinates and color (the blue triangle is in the front)
n = initVertexBuffers(gl, verticesColors); //, verticesColors, n
if (n < 0) {
console.log('Failed to set the vertex information');
return;
}
// Get the storage location of u_MvpMatrix
var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
if (!u_MvpMatrix) {
console.log('Failed to get the storage location of u_MvpMatrix');
return;
}
// Register the event handler
initEventHandlers(canvas);
// Specify the color for clearing <canvas>
gl.clearColor(0, 0, 0, 1);
gl.enable(gl.DEPTH_TEST);
// Start drawing
var tick = function () {
//setPerspective()宽高比
var aspect = canvas.width / canvas.height;
//
draw(gl, n, aspect, cx, cy, cz, fovy, u_MvpMatrix);
requestAnimationFrame(tick, canvas);
};
tick();
}
3) 使用缓冲区对象
在函数initVertexBuffers（）中包含了使用缓冲区对象向顶点着色器传入多个顶点数据的过程：
- 创建缓冲区对象（gl.createBuffer()）；
- 绑定缓冲区对象（gl.bindBuffer()）；
- 将数据写入缓冲区对象（gl.bufferData）；
- 将缓冲区对象分配给一个attribute变量(gl.vertexAttribPointer)
- 开启attribute变量(gl.enableVertexAttribArray)；
在本例中，在JS中申请的数组verticesColors分成位置和颜色两部分分配给缓冲区对象，并传入顶点着色器；vertexAttribPointer（）是其关键的函数，需要详细了解其参数的用法。最后，把顶点数据的索引值绑定到缓冲区对象，WebGL可以访问索引来间接访问顶点数据进行绘制。
function initVertexBuffers(gl, verticesColors) {
//DEM的一个网格是由两个三角形组成的
// 0------1 1
// | |
// | |
// col col------col+1
var indices = new Uint16Array((row - 1) * (col - 1) * 6);
var ci = 0;
for (var yi = 0; yi < row - 1; yi++) {
for (var xi = 0; xi < col - 1; xi++) {
indices[ci * 6] = yi * col + xi;
indices[ci * 6 + 1] = (yi + 1) * col + xi;
indices[ci * 6 + 2] = yi * col + xi + 1;
indices[ci * 6 + 3] = (yi + 1) * col + xi;
indices[ci * 6 + 4] = (yi + 1) * col + xi + 1;
indices[ci * 6 + 5] = yi * col + xi + 1;
ci++;
}
}
//创建缓冲区对象
var vertexColorBuffer = gl.createBuffer();
var indexBuffer = gl.createBuffer();
if (!vertexColorBuffer || !indexBuffer) {
return -1;
}
// 将缓冲区对象绑定到目标
gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
// 向缓冲区对象中写入数据
gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);
//
var FSIZE = verticesColors.BYTES_PER_ELEMENT;
// 向缓冲区对象分配a_Position变量
var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
if (a_Position < 0) {
console.log('Failed to get the storage location of a_Position');
return -1;
}
gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
//开启a_Position变量
gl.enableVertexAttribArray(a_Position);
// 向缓冲区对象分配a_Color变量
var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
if (a_Color < 0) {
console.log('Failed to get the storage location of a_Color');
return -1;
}
gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
//开启a_Color变量
gl.enableVertexAttribArray(a_Color);
// 写入并绑定顶点数组的索引值
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);
return indices.length;
}
4. 其他
1.这里用到了几个《WebGL编程指南》书中提供的JS组件。全部源代码(包含DEM数据)地址链接：https://share.weiyun.com/5cvt8PJ ，密码：4aqs8e。
2.如果关心如何设置模型视图投影变换矩阵，以及绑定鼠标键盘事件，可参看这篇文章：WebGL或OpenGL关于模型视图投影变换的设置技巧。
3.渲染的结果如果加入光照，效果会更好。