目录

选中一个表面 

示例程序（PickFace.js）

代码详解

示例效果

选中一个表面 

​​​​​​​WebGL 选中物体_山楂树の的博客-CSDN博客可以使用同样的方法来选中物体的某一个表面。这一节在PickObject程序的基础上编写了PickFace程序，后者同样包含一个立方体，但用户可以选中立方体的某一个表面，被选中的表面会变成白色。下图显示了PickFace的运行效果。

如果你理解了PickObject的程序原理，那么PickFace就很简单了。PickObject用户在点击鼠标时，将立方体重绘为红色，然后读取鼠标点击位置的像素颜色，根据其是红色或是黑色来判断点击时鼠标是否在立方体上，即是否选中了立方体。而PickFace则更进一步，在用户点击鼠标时重绘立方体，并将“每个像素属于哪个面”的信息写入到颜色缓冲区的α分量中。下面来看一下示例程序。 

示例程序（PickFace.js）

PickFace.js的代码如下所示。为了简洁，略去了与前例相同的部分，如顶点着色器等。 

// PickFace.js (c) 2012 matsuda and kanda
// Vertex shader program
var VSHADER_SOURCE ='attribute vec4 a_Position;\n' +'attribute vec4 a_Color;\n' +'attribute float a_Face;\n' +   // Surface number (Cannot use int for attribute variable)'uniform mat4 u_MvpMatrix;\n' +'uniform int u_PickedFace;\n' + // Surface number of selected face'varying vec4 v_Color;\n' +'void main() {\n' +'  gl_Position = u_MvpMatrix * a_Position;\n' +'  int face = int(a_Face);\n' + // Convert to int'  vec3 color = (face == u_PickedFace) ? vec3(1.0) : a_Color.rgb;\n' +'  if(u_PickedFace == 0) {\n' + // In case of 0, insert the face number into alpha'    v_Color = vec4(color, a_Face/255.0);\n' +'  } else {\n' +'    v_Color = vec4(color, a_Color.a);\n' +'  }\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 ANGLE_STEP = 20.0; // Rotation angle (degrees/second)function main() {// Retrieve <canvas> elementvar canvas = document.getElementById('webgl');// Get the rendering context for WebGLvar gl = getWebGLContext(canvas);if (!gl) {console.log('Failed to get the rendering context for WebGL');return;}// Initialize shadersif (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {console.log('Failed to intialize shaders.');return;}// Set the vertex informationvar n = initVertexBuffers(gl);if (n < 0) {console.log('Failed to set the vertex information');return;}// Set the clear color and enable the depth testgl.clearColor(0.0, 0.0, 0.0, 1.0);gl.enable(gl.DEPTH_TEST);// Get the storage locations of uniform variablesvar u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');var u_PickedFace = gl.getUniformLocation(gl.program, 'u_PickedFace');if (!u_MvpMatrix || !u_PickedFace) { console.log('Failed to get the storage location of uniform variable');return;}// Calculate the view projection matrixvar viewProjMatrix = new Matrix4();viewProjMatrix.setPerspective(30.0, canvas.width / canvas.height, 1.0, 100.0);viewProjMatrix.lookAt(0.0, 0.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);// Initialize selected surfacegl.uniform1i(u_PickedFace, -1);var currentAngle = 0.0; // Current rotation angle// Register the event handlercanvas.onmousedown = function(ev) {   // Mouse is pressedvar x = ev.clientX, y = ev.clientY;var rect = ev.target.getBoundingClientRect();if (rect.left <= x && x < rect.right && rect.top <= y && y < rect.bottom) {// If Clicked position is inside the <canvas>, update the selected surfacevar x_in_canvas = x - rect.left, y_in_canvas = rect.bottom - y;var face = checkFace(gl, n, x_in_canvas, y_in_canvas, currentAngle, u_PickedFace, viewProjMatrix, u_MvpMatrix);gl.uniform1i(u_PickedFace, face); // Pass the surface number to u_PickedFacedraw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);}}var tick = function() {   // Start drawingcurrentAngle = animate(currentAngle);draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);requestAnimationFrame(tick, canvas);};tick();
}function initVertexBuffers(gl) {// Create a cube//    v6----- v5//   /|      /|//  v1------v0|//  | |     | |//  | |v7---|-|v4//  |/      |///  v2------v3var vertices = new Float32Array([   // Vertex coordinates1.0, 1.0, 1.0,  -1.0, 1.0, 1.0,  -1.0,-1.0, 1.0,   1.0,-1.0, 1.0,    // v0-v1-v2-v3 front1.0, 1.0, 1.0,   1.0,-1.0, 1.0,   1.0,-1.0,-1.0,   1.0, 1.0,-1.0,    // v0-v3-v4-v5 right1.0, 1.0, 1.0,   1.0, 1.0,-1.0,  -1.0, 1.0,-1.0,  -1.0, 1.0, 1.0,    // v0-v5-v6-v1 up-1.0, 1.0, 1.0,  -1.0, 1.0,-1.0,  -1.0,-1.0,-1.0,  -1.0,-1.0, 1.0,    // v1-v6-v7-v2 left-1.0,-1.0,-1.0,   1.0,-1.0,-1.0,   1.0,-1.0, 1.0,  -1.0,-1.0, 1.0,    // v7-v4-v3-v2 down1.0,-1.0,-1.0,  -1.0,-1.0,-1.0,  -1.0, 1.0,-1.0,   1.0, 1.0,-1.0     // v4-v7-v6-v5 back]);var colors = new Float32Array([   // Colors0.32, 0.18, 0.56,  0.32, 0.18, 0.56,  0.32, 0.18, 0.56,  0.32, 0.18, 0.56, // v0-v1-v2-v3 front0.5, 0.41, 0.69,   0.5, 0.41, 0.69,   0.5, 0.41, 0.69,   0.5, 0.41, 0.69,  // v0-v3-v4-v5 right0.78, 0.69, 0.84,  0.78, 0.69, 0.84,  0.78, 0.69, 0.84,  0.78, 0.69, 0.84, // v0-v5-v6-v1 up0.0, 0.32, 0.61,   0.0, 0.32, 0.61,   0.0, 0.32, 0.61,   0.0, 0.32, 0.61,  // v1-v6-v7-v2 left0.27, 0.58, 0.82,  0.27, 0.58, 0.82,  0.27, 0.58, 0.82,  0.27, 0.58, 0.82, // v7-v4-v3-v2 down0.73, 0.82, 0.93,  0.73, 0.82, 0.93,  0.73, 0.82, 0.93,  0.73, 0.82, 0.93, // v4-v7-v6-v5 back]);var faces = new Uint8Array([   // Faces1, 1, 1, 1,     // v0-v1-v2-v3 front2, 2, 2, 2,     // v0-v3-v4-v5 right3, 3, 3, 3,     // v0-v5-v6-v1 up4, 4, 4, 4,     // v1-v6-v7-v2 left5, 5, 5, 5,     // v7-v4-v3-v2 down6, 6, 6, 6,     // v4-v7-v6-v5 back]);var indices = new Uint8Array([   // Indices of the vertices0, 1, 2,   0, 2, 3,    // front4, 5, 6,   4, 6, 7,    // right8, 9,10,   8,10,11,    // up12,13,14,  12,14,15,    // left16,17,18,  16,18,19,    // down20,21,22,  20,22,23     // back]);// Create a buffer objectvar indexBuffer = gl.createBuffer();if (!indexBuffer) {return -1;}// Write vertex information to buffer objectif (!initArrayBuffer(gl, vertices, gl.FLOAT, 3, 'a_Position')) return -1; // Coordinates Informationif (!initArrayBuffer(gl, colors, gl.FLOAT, 3, 'a_Color')) return -1;      // Color Informationif (!initArrayBuffer(gl, faces, gl.UNSIGNED_BYTE, 1, 'a_Face')) return -1;// Surface Information// Unbind the buffer objectgl.bindBuffer(gl.ARRAY_BUFFER, null);// Write the indices to the buffer objectgl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, indices, gl.STATIC_DRAW);return indices.length;
}function checkFace(gl, n, x, y, currentAngle, u_PickedFace, viewProjMatrix, u_MvpMatrix) {var pixels = new Uint8Array(4); // Array for storing the pixel valuegl.uniform1i(u_PickedFace, 0);  // Draw by writing surface number into alpha valuedraw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix);// Read the pixel value of the clicked position. pixels[3] is the surface numbergl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixels);return pixels[3];
}var g_MvpMatrix = new Matrix4(); // Model view projection matrix
function draw(gl, n, currentAngle, viewProjMatrix, u_MvpMatrix) {// Caliculate The model view projection matrix and pass it to u_MvpMatrixg_MvpMatrix.set(viewProjMatrix);g_MvpMatrix.rotate(currentAngle, 1.0, 0.0, 0.0); // Rotate appropriatelyg_MvpMatrix.rotate(currentAngle, 0.0, 1.0, 0.0);g_MvpMatrix.rotate(currentAngle, 0.0, 0.0, 1.0);gl.uniformMatrix4fv(u_MvpMatrix, false, g_MvpMatrix.elements);gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);     // Clear buffersgl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);   // Draw
}var last = Date.now();  // Last time that this function was called
function animate(angle) {var now = Date.now(); // Calculate the elapsed timevar elapsed = now - last;last = now;// Update the current rotation angle (adjusted by the elapsed time)var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;return newAngle % 360;
}function initArrayBuffer (gl, data, type, num, attribute) {// Create a buffer objectvar buffer = gl.createBuffer();if (!buffer) {console.log('Failed to create the buffer object');return false;}// Write date into the buffer objectgl.bindBuffer(gl.ARRAY_BUFFER, buffer);gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);// Assign the buffer object to the attribute variablevar a_attribute = gl.getAttribLocation(gl.program, attribute);if (a_attribute < 0) {console.log('Failed to get the storage location of ' + attribute);return false;}gl.vertexAttribPointer(a_attribute, num, type, false, 0, 0);// Enable the assignment to a_attribute variablegl.enableVertexAttribArray(a_attribute);return true;
}

代码详解

首先，顶点着色器中添加了attribute变量a_Face，它表示立方体各表面的编号，即当前顶点属于哪个表面（第6行）。鼠标被点击时，这个值就会被“编码”成颜色值的α分量。initVertexBuffers（）函数（第99行）建立了表面编号数组faces，数组中的每个元素对应一个顶点（第127行）。比如，顶点v0-v1-v2-v3定义了1号表面，而顶点v0-v3-v4-v5定义了2号表面，等等。数组faces前4个元素都是1，表示前4个顶点都属于1号表面，以此类推（第128行）。

当某个表面被选中时，就通过u_PickedFace变量来通知顶点着色器这个表面被选中了（第8行）。这样顶点着色器就可以将这个表面绘制成白色，用户就获得了反馈，知道这个表面确实被选中了。

在正常情况下（即不在鼠标被点击的那一刻），顶点着色器会比较当前被选中的表面编号u_PickedFace和当前顶点的表面编号a_Face，如果它们相等，即当前顶点属于被选中的表面，就将color赋为白色，如果不相等，就将其赋为顶点原来的颜色a_Color。此处必须将float类型的a_Face转化为int类型，再与u_PickedFace进行比较，因为attribute变量只能是float类型的。在鼠标点击的那一刻，u_PickedFace被设为0，我们将a_Face的值写入到颜色的α分量中。

main（）函数为u_PickedFace指定初始值-1（第75行）。按照faces变量的定义（第127行），立方体各表面中没有哪一个编号是-1，所以一开始没有任何表面被选中，每一个面的颜色都是初始颜色。

鼠标被点击时，u_PickedFace变量变为了0，顶点着色器就在颜色缓冲区中将每个面绘制成非1的α值，并且α的值取决于表面编号。最关键的逻辑发生在鼠标点击事件的响应函数中：在获取了鼠标点击位置后，调用checkFace（）函数并传入u_PickedFace变量，也就是着色器中同名变量的存储地址（第85行）。

checkFace（）函数的任务是，根据点击位置返回选中表面的编号（第166行）。该函数首先将u_PickedFace变量设为0（第168行），然后立刻调用draw（）函数（在颜色缓冲区中，最终没有显示在屏幕上）进行绘制，此时每个表面的α值就取决于表面的编号。然后，从颜色缓冲区获取鼠标点击处的像素值（第171行），通过pixels［3］获取表面编号（即α分量，索引为3）。checkFace（）函数返回选中的表面编号，执行流程回到鼠标点击事件响应函数中，用选中的表面编号重新绘制立方体（第86～87行），并在屏幕上显示出来，如前所述。

示例效果