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[操作系统]美图秀秀滤镜之对比度


  对比度指的是一幅图像中明暗区域最亮的白和最暗的黑之间不同亮度层级的测量,即指一幅图像灰度反差的大小。

  在GPUImage中通过GPUImageContrastFilter来实现

  片段着色器

 varying highp vec2 textureCoordinate; uniform sampler2D inputImageTexture; uniform lowp float contrast; void main() {   lowp vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);     gl_FragColor = vec4(((textureColor.rgb - vec3(0.5)) * contrast + vec3(0.5)), textureColor.w); }

 

每个像素点的RGB值-0.5 乘以对比度值 然后+0.5生成新的像素点RGB

+ (UIImage *)changeValueForContrastFilter:(float)value image:(UIImage *)image;{  GPUImageContrastFilter *filter = [[GPUImageContrastFilter alloc] init];  filter.contrast = value;  [filter forceProcessingAtSize:image.size];  GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:image];  [pic addTarget:filter];    [pic processImage];  [filter useNextFrameForImageCapture];  return [filter imageFromCurrentFramebuffer];}

 

 

 

效果

  

          对比度低                          对比度高