GPUImage中亮度调整的实现——GPUImageBrightnessFilter

亮度brightness其实是对RGB的调整，RGB值越大，效果越亮；反之则越暗。

GPUImage中提供了对图像亮度调整的Filter，其核心代码如下(fragment)：

  varying highp vec2 textureCoordinate;

  uniform sampler2D inputImageTexture;
  uniform lowp float brightness;

  void main()
  {
      lowp vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);

      gl_FragColor = vec4((textureColor.rgb + vec3(brightness)), textureColor.w);
  }

其中brightness的取值范围为[-1,1]