webgl第24课-几何图形的装配和光栅化

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上一节课我们学习了颜色与纹理中-将非坐标数据传入顶点着色器的另外一种方式--交错组织

这一节课我们将学习颜色与纹理中-几何图形的装配和光栅化

我们前面的课程了解到顶点着色器是控制的几何图形的位置、尺寸甚至指定几何图形的颜色，最终片元着色器来决定几何图形的颜色！但是我们并不知道他们之间的具体过程！

下面我们来看一下顶点着色器和片元着色器之间的千丝万缕：

需要了解的属性：

顶点着色器到片元着色器的过程：

前面的gl.deawArrays()函数的执行过程（n=3）：

示例代码：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8" />
    <title>webgl第24课-几何图形的装配和光栅化</title>
  </head>

  <body onload="main()">
    <canvas id="webgl" width="400" height="400">
    Please use a browser that supports "canvas"
    </canvas>

    <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="HelloTriangle_FragCoord.js"></script>
  </body>
</html>

// HelloTriangle_FragCoord.js (c) 2012 matsuda
// Vertex shader program
var VSHADER_SOURCE =
  'attribute vec4 a_Position;\n' +
  'void main() {\n' +
  '  gl_Position = a_Position;\n' +
  '}\n';

// Fragment shader program
var FSHADER_SOURCE =
  'precision mediump float;\n' +
  'uniform float u_Width;\n' +
  'uniform float u_Height;\n' +
  'void main() {\n' +
'  gl_FragColor = vec4(gl_FragCoord.x/u_Width, 0.0, gl_FragCoord.y/u_Height, 1.0);\n' +
  '}\n';

function main() {
  // Retrieve <canvas> element
  var canvas = document.getElementById('webgl');

  // 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;
  }

  // Write the positions of vertices to a vertex shader
  var n = initVertexBuffers(gl);
  if (n < 0) {
    console.log('Failed to set the positions of the vertices');
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(1.0, 1.0, 1.0, 1.0);

  // Clear <canvas>
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Draw the rectangle
  gl.drawArrays(gl.TRIANGLES, 0, n);
}

function initVertexBuffers(gl) {
  var vertices = new Float32Array([
    0, 0.5,   -0.5, -0.5,   0.5, -0.5
  ]);
  var n = 3; // The number of vertices

  // Create a buffer object
  var vertexBuffer = gl.createBuffer();
  if (!vertexBuffer) {
    console.log('Failed to create the buffer object');
    return -1;
  }

  // Bind the buffer object to target
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  // Write date into the buffer object
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

  // Pass the position of a point to a_Position variable
  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, 0, 0);

  var u_Width = gl.getUniformLocation(gl.program, 'u_Width');
  if (!u_Width) {
    console.log('Failed to get the storage location of u_Width');
    return;
  }

  var u_Height = gl.getUniformLocation(gl.program, 'u_Height');
  if (!u_Height) {
    console.log('Failed to get the storage location of u_Height');
    return;
  }

  // Pass the width and hight of the <canvas>
  gl.uniform1f(u_Width, gl.drawingBufferWidth);
  gl.uniform1f(u_Height, gl.drawingBufferHeight);

  // Enable the generic vertex attribute array
  gl.enableVertexAttribArray(a_Position);

  // Unbind the buffer object
  gl.bindBuffer(gl.ARRAY_BUFFER, null);

  return n;
}

运行展示：

webgl第24课-几何图形的装配和光栅化

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