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>