WebGL笔记：图形缩放的原理和实现

缩放

1 ）原理

缩放可以理解为对向量长度的改变，或者对向量坐标分量的同步缩放
- 如下图，比如
- 让向量OA 收缩到点B的位置，也就是从OA变成OB，缩放了一半

2 ）公式

已知
- 点A的位置是(ax,ay,az)
- 点A基于原点內缩了一半
求
- 点A內缩了一半后的bx、by、bz位置B

解

bx = ax * 0.5
by = ay * 0.5
bz = az * 0.5

在着色器中缩放

1 ）核心代码

可以对gl_Position 的x、y、z依次缩放

<script id="vertexShader" type="x-shader/x-vertex">
  attribute vec4 a_Position;
  float scale = 1.2; // 注意这里声明了浮点型，一点要用浮点数，否则会导致 UseProgram: program not valid 的警告
  void main() {
    gl_Position.x = a_Position.x * scale;
    gl_Position.y = a_Position.y * scale;
    gl_Position.z = a_Position.z * scale;
    gl_Position.w = 1.0; // 注意 w 的值，默认1.0
  }
</script>

也可以从a_Position中抽离出由x、y、z组成的三维向量，对其进行一次性缩放

<script id="vertexShader" type="x-shader/x-vertex">
  attribute vec4 a_Position;
  float scale = 1.2;
  void main() {
    gl_Position = vec4(vec3(a_Position) * scale, 1.0);
  }
</script>

2 ）完整代码

<canvas id="canvas"></canvas>
<script id="vertexShader" type="x-shader/x-vertex">
  attribute vec4 a_Position;
  float scale = 1.0;
  void main() {
      gl_Position = vec4(vec3(a_Position) * scale, 1.0);
  }
</script>
<script id="fragmentShader" type="x-shader/x-fragment">
  void main(){
      gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
  }
</script>
<script type="module">
  import { initShaders } from './utils.js';

  const canvas = document.getElementById('canvas');
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
  const gl = canvas.getContext('webgl');
  const vsSource = document.getElementById('vertexShader').innerText;
  const fsSource = document.getElementById('fragmentShader').innerText;
  initShaders(gl, vsSource, fsSource);

  const vertices = new Float32Array([
    0.0, 0.1,
    -0.1, -0.1,
    0.1, -0.1
  ])

  const vertexBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
  const a_Position = gl.getAttribLocation(gl.program, 'a_Position');
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
  gl.enableVertexAttribArray(a_Position);
  gl.clearColor(0.0, 0.0, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.drawArrays(gl.TRIANGLES, 0, 3);
</script>

用js缩放图形

1 ）核心代码

同样的我们也可以把缩放系数暴露给js，通过js 缩放图形

建立uniform变量

<script id="vertexShader" type="x-shader/x-vertex">
    attribute vec4 a_Position;
    uniform float u_Scale;
    void main() {
        gl_Position = vec4(vec3(a_Position) * u_Scale, 1.0);
    }
</script>

使用js获取并修改uniform 变量

const u_Scale = gl.getUniformLocation(gl.program, 'u_Scale')
gl.uniform1f(u_Scale, 1.0)

添加动画让其动起来

let angle = 0
!(function animate() {
    angle += 0.05;
    const scale = Math.sin(n) + 1; // 借助三角函数正弦进行缩放 (-1, 1) + 1 => (0, 2)
    gl.uniform1f(u_Scale, scale);
    gl.clear(gl.COLOR_BUFFER_BIT);
    gl.drawArrays(gl.TRIANGLES, 0, 3);
    requestAnimationFrame(animate)
})()

2 ）完整代码

<canvas id="canvas"></canvas>
<script id="vertexShader" type="x-shader/x-vertex">
  attribute vec4 a_Position;
  uniform float u_Scale;
  void main() {
      gl_Position = vec4(vec3(a_Position) * u_Scale, 1.0);
  }
</script>
<script id="fragmentShader" type="x-shader/x-fragment">
  void main() {
      gl_FragColor = vec4(1.0,1.0,0.0,1.0);
  }
</script>
<script type="module">
  import { initShaders } from './utils.js';

  const canvas = document.getElementById('canvas');
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
  const gl = canvas.getContext('webgl');

  const vsSource = document.getElementById('vertexShader').innerText;
  const fsSource = document.getElementById('fragmentShader').innerText;
  initShaders(gl, vsSource, fsSource);
  
  const vertices = new Float32Array([
    0.0, 0.1,
    -0.1, -0.1,
    0.1, -0.1
  ])

  const vertexBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);
  const a_Position = gl.getAttribLocation(gl.program, 'a_Position');
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
  gl.enableVertexAttribArray(a_Position);

  const u_Scale = gl.getUniformLocation(gl.program, 'u_Scale')
  gl.uniform1f(u_Scale, 1);

  gl.clearColor(0.0, 0.0, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.drawArrays(gl.TRIANGLES, 0, 3);

  let angle = 0;
  !(function animate() {
    angle += 0.05;
    const scale = Math.sin(angle) + 1;
    gl.uniform1f(u_Scale, scale);
    gl.clear(gl.COLOR_BUFFER_BIT);
    gl.drawArrays(gl.TRIANGLES, 0, 3);
    requestAnimationFrame(animate);
  })()
</script>