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WebGL的颜色渲染-渲染一张DEM(数字高程模型)

程序员文章站 2023-10-31 22:01:46
通过渲染一张DEM的具体例子，了解在WebGL中颜色渲染的过程。 ......

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。
WebGL的颜色渲染-渲染一张DEM(数字高程模型)
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：
WebGL的颜色渲染-渲染一张DEM(数字高程模型)

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。

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