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WebGL入门教程（二十）帧缓冲区对象FBO

发表于2017-12-13
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一、概念

前面都是将图片作为绘制图形时的纹理映射，这一节来看看如何将渲染结果作为纹理，即动态生成纹理，贴在另一个物体上。默认情况下，都是在屏幕提供帧缓冲区中绘制，如果要动态生成纹理，就需要另外新建一个缓冲区对象，来代替默认的缓冲区，在其中进行离屏绘制。

帧缓冲区对象包括：

  1. 颜色关联对象（可以是纹理对象或渲染缓冲区对象）
  2. 深度关联对象（渲染缓冲区对象）
  3. 模板关联对象

要将动态生成的纹理作为贴图，只需要在帧缓冲区的颜色关联对象设置为纹理对象，然后将此纹理对象作为纹理，在绘制其他图形时使用即可。

二、示例

attribute vec4 a_Position;
attribute vec2 a_TexCoord;
uniform mat4 u_MvpMatrix;
varying vec2 v_TexCoord;
void main() ｛
    gl_Position = u_MvpMatrix * a_Position;
    v_TexCoord = a_TexCoord;
｝
#ifdef GL_ES
precision mediump float;
#endif
uniform sampler2D u_Sampler;
varying vec2 v_TexCoord;
void main() ｛
    gl_FragColor = texture2D(u_Sampler, v_TexCoord);
｝
/**
 * 帧缓冲区对象FBO
 * xu.lidong@qq.com
 * */
var SCREEN_WIDTH = 1280;
var SCREEN_HEIGHT = 720;
var OFFSCREEN_WIDTH = 512;
var OFFSCREEN_HEIGHT = 512;
function main() ｛
    var gl = getGL();
    var vsFile = "./res/shader/fbo.vert.glsl";
    var fsFile = "./res/shader/fbo.frag.glsl";
    initShaderProgram(gl, vsFile, fsFile, function (shaderProgram) ｛
        gl.program = shaderProgram;
        var cube = initVertexBuffersForCube(gl);
        var plane = initVertexBuffersForPlane(gl);
        initTextures(gl, function (texture) ｛
            var fbo = initFramebufferObject(gl);
            gl.enable(gl.DEPTH_TEST);
            var pPlane = getPerspectiveProjection(30.0, SCREEN_WIDTH / SCREEN_HEIGHT, 1.0, 100.0);
            var vPlane = lookAt(0.0, 0.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
            var vpPlane = multiMatrix44(pPlane, vPlane);
            var pCube = getPerspectiveProjection(30.0, OFFSCREEN_WIDTH / OFFSCREEN_HEIGHT, 1.0, 100.0);
            var vCube = lookAt(0.0, 2.0, 7.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
            var vpCube = multiMatrix44(pCube, vCube);
            var speed = Math.PI/4;
            var rad = 0.0;
            var listTime = null;
            var tick = function (timestamp) ｛
                var delta = listTime ? (timestamp - listTime) / 1000 : 0;
                listTime = timestamp;
                rad = (rad + speed * delta) % (2 * Math.PI);
                draw(gl, fbo, plane, cube, texture, vpPlane, vpCube, rad);
                requestAnimationFrame(tick);
            ｝;
            requestAnimationFrame(tick);
        ｝);
    ｝);
｝
function initVertexBuffersForCube(gl) ｛
    var vertices = new Float32Array([
        1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0, -1.0, 1.0, 1.0, -1.0, 1.0,
        1.0, 1.0, 1.0, 1.0, -1.0, 1.0, 1.0, -1.0, -1.0, 1.0, 1.0, -1.0,
        1.0, 1.0, 1.0, 1.0, 1.0, -1.0, -1.0, 1.0, -1.0, -1.0, 1.0, 1.0,
        -1.0, 1.0, 1.0, -1.0, 1.0, -1.0, -1.0, -1.0, -1.0, -1.0, -1.0, 1.0,
        -1.0, -1.0, -1.0, 1.0, -1.0, -1.0, 1.0, -1.0, 1.0, -1.0, -1.0, 1.0,
        1.0, -1.0, -1.0, -1.0, -1.0, -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0
    ]);
    var texCoords = new Float32Array([
        1.0, 1.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0,
        0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 1.0,
        1.0, 0.0, 1.0, 1.0, 0.0, 1.0, 0.0, 0.0,
        1.0, 1.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0,
        0.0, 0.0, 1.0, 0.0, 1.0, 1.0, 0.0, 1.0,
        0.0, 0.0, 1.0, 0.0, 1.0, 1.0, 0.0, 1.0
    ]);
    var indices = new Uint8Array([
        0, 1, 2, 0, 2, 3,
        4, 5, 6, 4, 6, 7,
        8, 9, 10, 8, 10, 11,
        12, 13, 14, 12, 14, 15,
        16, 17, 18, 16, 18, 19,
        20, 21, 22, 20, 22, 23
    ]);
    var cube = ｛｝;
    cube.vertexBuffer = initArrayBufferForLaterUse(gl, vertices, 3, gl.FLOAT);
    cube.texCoordBuffer = initArrayBufferForLaterUse(gl, texCoords, 2, gl.FLOAT);
    cube.indexBuffer = initElementArrayBufferForLaterUse(gl, indices, gl.UNSIGNED_BYTE);
    cube.numIndices = indices.length;
    gl.bindBuffer(gl.ARRAY_BUFFER, null);
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);
    return cube;
｝
function initVertexBuffersForPlane(gl) ｛
    var vertices = new Float32Array([
        1.0, 1.0, 0.0,  -1.0, 1.0, 0.0,  -1.0,-1.0, 0.0,   1.0,-1.0, 0.0
    ]);
    var texCoords = new Float32Array([1.0, 1.0,   0.0, 1.0,   0.0, 0.0,   1.0, 0.0]);
    var indices = new Uint8Array([0, 1, 2,   0, 2, 3]);
    var plane = ｛｝;
    plane.vertexBuffer = initArrayBufferForLaterUse(gl, vertices, 3, gl.FLOAT);
    plane.texCoordBuffer = initArrayBufferForLaterUse(gl, texCoords, 2, gl.FLOAT);
    plane.indexBuffer = initElementArrayBufferForLaterUse(gl, indices, gl.UNSIGNED_BYTE);
    plane.numIndices = indices.length;
    gl.bindBuffer(gl.ARRAY_BUFFER, null);
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);
    return plane;
｝
function initArrayBufferForLaterUse(gl, data, num, type) ｛
    var buffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
    gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);
    buffer.num = num;
    buffer.type = type;
    return buffer;
｝
function initElementArrayBufferForLaterUse(gl, data, type) ｛
    var buffer = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, buffer);
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, data, gl.STATIC_DRAW);
    buffer.type = type;
    return buffer;
｝
function initTextures(gl, cb) ｛
    var image = new Image();
    image.onload = function() ｛
        gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);
        var texture = gl.createTexture();
        gl.bindTexture(gl.TEXTURE_2D, texture);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
        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);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
        var u_Sampler = gl.getUniformLocation(gl.program, 'u_Sampler');
        gl.uniform1i(u_Sampler, 0);
        gl.bindTexture(gl.TEXTURE_2D, null);
        cb(texture);
    ｝;
    image.src = './res/image/sky.jpg';
｝
function initFramebufferObject(gl) ｛
    var framebuffer = gl.createFramebuffer();
    // 新建纹理对象作为帧缓冲区的颜色缓冲区对象
    var texture = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    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);
    framebuffer.texture = texture;
    // 新建渲染缓冲区对象作为帧缓冲区的深度缓冲区对象
    var depthBuffer = gl.createRenderbuffer();
    gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);
    gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT);
    gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
    gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, texture, 0);
    gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);
    // 检测帧缓冲区对象的配置状态是否成功
    var e = gl.checkFramebufferStatus(gl.FRAMEBUFFER);
    if (gl.FRAMEBUFFER_COMPLETE !== e) ｛
        console.log('Frame buffer object is incomplete: ' + e.toString());
        return;
    ｝
    gl.bindFramebuffer(gl.FRAMEBUFFER, null);
    gl.bindTexture(gl.TEXTURE_2D, null);
    gl.bindRenderbuffer(gl.RENDERBUFFER, null);
    return framebuffer;
｝
function draw(gl, fbo, plane, cube, texture, vpPlane, vpCube, rad) ｛
    var u_MvpMatrix = gl.getUniformLocation(gl.program, 'u_MvpMatrix');
    var rot = getRotationMatrix(rad, 0.0, 1.0, 0.0);
    // 在帧缓冲区的颜色关联对象即纹理对象中绘制立方体，纹理使用图片
    gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);// 绑定帧缓冲区对象后绘制就会在绑定帧缓冲区中进行绘制
    gl.viewport(0, 0, OFFSCREEN_WIDTH, OFFSCREEN_HEIGHT);
    gl.clearColor(0.2, 0.2, 0.4, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
    var mvpCube = multiMatrix44(vpCube, rot);
    gl.uniformMatrix4fv(u_MvpMatrix, false, mvpCube);
    drawTexturedObject(gl, cube, texture);// 使用图片纹理绘制立方体
    // 在canvas上绘制矩形，纹理使用上一步在纹理对象中绘制的图像
    gl.bindFramebuffer(gl.FRAMEBUFFER, null);// 接触绑定之后，会在默认的颜色缓冲区中绘制
    gl.viewport(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
    /*
    * 默认绘制图形的正方两个面，所以可以看到平面的正反两个面都贴的有纹理
    * 使用下面代码可以开启消隐功能，不再绘制背面
    * */
    // gl.enable(gl.CULL_FACE);
    var mvpPlane = multiMatrix44(vpPlane, rot);
    gl.uniformMatrix4fv(u_MvpMatrix, false, mvpPlane);
    drawTexturedObject(gl, plane, fbo.texture);// 使用在帧缓冲绘制的纹理绘制矩形
｝
function drawTexturedObject(gl, o, texture) ｛
    initAttributeVariable(gl, 'a_Position', o.vertexBuffer);
    initAttributeVariable(gl, 'a_TexCoord', o.texCoordBuffer);
    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, o.indexBuffer);
    gl.drawElements(gl.TRIANGLES, o.numIndices, o.indexBuffer.type, 0);
｝
function initAttributeVariable(gl, attributeName, buffer) ｛
    gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
    var attribute = gl.getAttribLocation(gl.program, attributeName);
    gl.vertexAttribPointer(attribute, buffer.num, buffer.type, false, 0, 0);
    gl.enableVertexAttribArray(attribute);
｝

如图：

可以看两矩形的两面都有贴面，开启隐藏面消除之后，就只会绘制一面：

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WebGL渲染JavaScriptGLSL纹理映射