WebGL的颜色渲染-渲染一张DEM(数字高程模型)

发布时间:2019-05-01 22:00:01发布者:Mr.Zhang阅读(244)

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:

  1. 获取WebGL环境。
  2. 初始化shaders,构建着色器。
  3. 初始化顶点数组,分配到缓冲对象。
  4. 绑定鼠标键盘事件,设置模型视图投影变换矩阵。
  5. 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:

  1. 创建缓冲区对象(gl.createBuffer());
  2. 绑定缓冲区对象(gl.bindBuffer());
  3. 将数据写入缓冲区对象(gl.bufferData);
  4. 将缓冲区对象分配给一个attribute变量(gl.vertexAttribPointer)
  5. 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。





本文转自博客园,原文地址:https://www.cnblogs.com/charlee44/p/10799995.html