图像处理 - WebGL

绘制图片需要使用纹理，WebGL 渲染时需要裁剪空间，渲染纹理时需要纹理坐标，纹理坐标的范围始终是 0.0~1.0

1. 创建程序
2. 查找属性位置
3. 创建 Buffer(接收外部数据)
4. 关联 Buffer 与内部状态
5. 传递数据
6. 设置可视区域尺寸
7. 指定使用程序
8. 开启顶点属性
9. 指定获取外部数据方式
10. 设置分辨率
11. 绘制
12. 像素处理(颜色反转、计算、卷积内核)

"use strict";

function main() {
  var image = new Image();
  image.src = "/webgl/resources/leaves.jpg"; // MUST BE SAME DOMAIN!!!
  image.onload = function () {
    render(image);
  };
}

function render(image) {
  // Get A WebGL context
  /** @type {HTMLCanvasElement} */
  var canvas = document.querySelector("#canvas");
  var gl = canvas.getContext("webgl");
  if (!gl) {
    return;
  }

  // setup GLSL program
  var program = webglUtils.createProgramFromScripts(gl, [
    "vertex-shader-2d",
    "fragment-shader-2d",
  ]);

  // look up where the vertex data needs to go.
  var positionLocation = gl.getAttribLocation(program, "a_position");
  var texcoordLocation = gl.getAttribLocation(program, "a_texCoord");

  // Create a buffer to put three 2d clip space points in
  var positionBuffer = gl.createBuffer();

  // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
  // Set a rectangle the same size as the image.
  setRectangle(gl, 0, 0, image.width, image.height);

  // provide texture coordinates for the rectangle.
  var texcoordBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);
  gl.bufferData(
    gl.ARRAY_BUFFER,
    new Float32Array([
      0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 1.0, 0.0, 1.0, 1.0,
    ]),
    gl.STATIC_DRAW
  );

  // Create a texture.
  var texture = gl.createTexture();
  gl.bindTexture(gl.TEXTURE_2D, texture);

  // Set the parameters so we can render any size image.
  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.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);

  // Upload the image into the texture.
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);

  // lookup uniforms
  var resolutionLocation = gl.getUniformLocation(program, "u_resolution");

  webglUtils.resizeCanvasToDisplaySize(gl.canvas);

  // Tell WebGL how to convert from clip space to pixels
  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // Clear the canvas
  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Tell it to use our program (pair of shaders)
  gl.useProgram(program);

  // Turn on the position attribute
  gl.enableVertexAttribArray(positionLocation);

  // Bind the position buffer.
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  // Tell the position attribute how to get data out of positionBuffer (ARRAY_BUFFER)
  var size = 2; // 2 components per iteration
  var type = gl.FLOAT; // the data is 32bit floats
  var normalize = false; // don't normalize the data
  var stride = 0; // 0 = move forward size * sizeof(type) each iteration to get the next position
  var offset = 0; // start at the beginning of the buffer
  gl.vertexAttribPointer(
    positionLocation,
    size,
    type,
    normalize,
    stride,
    offset
  );

  // Turn on the texcoord attribute
  gl.enableVertexAttribArray(texcoordLocation);

  // bind the texcoord buffer.
  gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBuffer);

  // Tell the texcoord attribute how to get data out of texcoordBuffer (ARRAY_BUFFER)
  var size = 2; // 2 components per iteration
  var type = gl.FLOAT; // the data is 32bit floats
  var normalize = false; // don't normalize the data
  var stride = 0; // 0 = move forward size * sizeof(type) each iteration to get the next position
  var offset = 0; // start at the beginning of the buffer
  gl.vertexAttribPointer(
    texcoordLocation,
    size,
    type,
    normalize,
    stride,
    offset
  );

  // set the resolution
  gl.uniform2f(resolutionLocation, gl.canvas.width, gl.canvas.height);

  // Draw the rectangle.
  var primitiveType = gl.TRIANGLES;
  var offset = 0;
  var count = 6;
  gl.drawArrays(primitiveType, offset, count);
}

function setRectangle(gl, x, y, width, height) {
  var x1 = x;
  var x2 = x + width;
  var y1 = y;
  var y2 = y + height;

  // 其实是画了两个三角形
  gl.bufferData(
    gl.ARRAY_BUFFER,
    new Float32Array([x1, y1, x2, y1, x1, y2, x1, y2, x2, y1, x2, y2]),
    gl.STATIC_DRAW
  );
}

main();