【WebGL】《WebGL编程指南》读书笔记——第五章

var VSHADER_SOURCE =
  ‘attribute vec4 a_Position;\n‘ +
  ‘attribute vec2 a_TexCoord;\n‘ +  // 声明一个attribute变量
  ‘varying vec2 v_TexCoord;\n‘ +    // 声明一个varying变量
  ‘void main() {\n‘ +
  ‘  gl_Position = a_Position;\n‘ +
  ‘  v_TexCoord = a_TexCoord;\n‘ +  // attribute变量赋给varying变量
  ‘}\n‘;

var FSHADER_SOURCE =
  ‘#ifdef GL_ES\n‘ +
  ‘precision mediump float;\n‘ +
  ‘#endif\n‘ +
‘uniform sampler2D u_Sampler;\n‘ +
  ‘varying vec2 v_TexCoord;\n‘ +
  ‘void main() {\n‘ +
  // texture2D(sampler2D sampler, vec2 coord)
  // (纹理单元编号，纹理坐标) 这里是赋值的关键
  ‘  gl_FragColor = texture2D(u_Sampler, v_TexCoord);\n‘ + 
  ‘}\n‘;

function main() {  
  var canvas = document.getElementById(‘webgl‘);
  var gl = getWebGLContext(canvas);
  if (!gl) {
    console.log(‘Failed to get the rendering context for WebGL‘);
    return;
  }
  if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
    console.log(‘Failed to intialize shaders.‘);
    return;
  }

  // 设置顶点缓存
  var n = initVertexBuffers(gl);
  if (n 0) {
    console.log(‘Failed to set the vertex information‘);
    return;
  }

  gl.clearColor(0.0, 0.0, 0.0, 1.0);

  // 设置纹理
  if (!initTextures(gl, n)) {
    console.log(‘Failed to intialize the texture.‘);
    return;
  }
}

function initVertexBuffers(gl) {
  var verticesTexCoords = new Float32Array([
    //webgl顶点坐标, 纹理坐标相应点
    -0.5,  0.5,   0.0, 1.0,
    -0.5, -0.5,   0.0, 0.0,
     0.5,  0.5,   1.0, 1.0,
     0.5, -0.5,   1.0, 0.0,
  ]);
  var n = 4; 
// 创建缓存区对象
  var vertexTexCoordBuffer = gl.createBuffer();
  if (!vertexTexCoordBuffer) {
    console.log(‘Failed to create the buffer object‘);
    return -1;
  }

  gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);

  var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
  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, 2, gl.FLOAT, false, FSIZE * 4, 0);
  gl.enableVertexAttribArray(a_Position);  
  // 将纹理坐标分配给该存储位置并开启
  var a_TexCoord = gl.getAttribLocation(gl.program, ‘a_TexCoord‘);
  if (a_TexCoord 0) {
    console.log(‘Failed to get the storage location of a_TexCoord‘);
    return -1;
  }  
  gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
  gl.enableVertexAttribArray(a_TexCoord);  

  return n;
}

function initTextures(gl, n) {
// Step1：设置纹理对象
  var texture = gl.createTexture();   
  if (!texture) {
    console.log(‘Failed to create the texture object‘);
    return false;
  }

  // Step2: 获取u_Sampler（取样器）存储位置
  var u_Sampler = gl.getUniformLocation(gl.program, ‘u_Sampler‘);
  if (!u_Sampler) {console.log(‘Failed to get the storage location of u_Sampler‘);
    return false;
  }
  // Step3: 创建图片对象
  var image = new Image();  
  if (!image) {console.log(‘Failed to create the image object‘);
    return false;
  }

  image.onload = function(){ loadTexture(gl, n, texture, u_Sampler, image); };
  image.src = ‘../resources/sky.jpg‘;
  return true;
}

function loadTexture(gl, n, texture, u_Sampler, image) {
// Step1：对图像进行y轴反转
  gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1);  
  // Step2: 开启0号纹理单元（textunit0~7）
  gl.activeTexture(gl.TEXTURE0);
// Step3: 绑定纹理对象（target，texture）   
  // target可以是：gl.TEXTURE或gl.TEXTURE_CUBE_MAP
  gl.bindTexture(gl.TEXTURE_2D, texture);

  // Step4: 设置纹理参数(target,pname,param)
// gl.TEXTURE_MAG_FILTER (纹理放大) 默认值： gl.LINEAR
  // gl.TEXTURE_MIN_FILTER (纹理缩小) 默认值： gl.NEAREST_MIPMAP_LINEAR
  // gl.TEXTURE_WRAP_S (纹理水平填充)  默认值： gl.REPEAT(平铺式) 
  //                                         gl.MIRRORED_REPEAT (镜像对称)
  //                                         gl.CLAMP_TO_EDGE (使用纹理图像边缘值)
  // gl.TEXTURE_WRAP_T (纹理垂直填充)  默认值： gl.REPEAT
 gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
// Step5：配置纹理图片(target,level,internalformat,format,type,image)
  // level: 0
  // internalformat：图像的内部格式
  // format: 纹理数据的格式，必须与internalformat一致
  // type: 纹理数据的类型
  // image：包含纹理的图像的image对象
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image); 
// Step6：将0号纹理传递至取样器
  gl.uniform1i(u_Sampler, 0);
  
  gl.clear(gl.COLOR_BUFFER_BIT);  
  gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); 
}