自动色阶、自动对比度算法实现

引言

自动色阶算法用于图像增强，去雾等，其思想去掉像素值最高的比例，去掉像素值最低的一些比例，然后在将图像的其余像素值进行线性映射或者伽马校正至[0, 255]区间。如下图，在图像处理中是不是很熟悉?!

本文主要参考人在旅途的博客，采用matlab语言实现，算法不在详细叙述，在此代码留做备注。

代码

autolevel：

function imDst = autolevel(varargin)
[I,lowCut,highCut] =parse_inputs(varargin{:});
[hei,wid,~] = size(I);

PixelAmount = wid * hei;
if size(I,3)==3

   [HistRed,~]  = imhist(I(:,:,1));
   [HistGreen,~] = imhist(I(:,:,2));
   [HistBlue,~] = imhist(I(:,:,3));

   CumRed = cumsum(HistRed);
   CumGreen = cumsum(HistGreen);
   CumBlue = cumsum(HistBlue);

   minR =find(CumRed>=PixelAmount*lowCut,1,‘first‘);
   minG = find(CumGreen>=PixelAmount*lowCut,1,‘first‘);
   minB =find(CumBlue>=PixelAmount*lowCut,1,‘first‘);

   maxR =find(CumRed>=PixelAmount*(1-highCut),1,‘first‘);
   maxG =find(CumGreen>=PixelAmount*(1-highCut),1,‘first‘);
   maxB = find(CumBlue>=PixelAmount*(1-highCut),1,‘first‘);

   RedMap = linearmap(minR,maxR);
   GreenMap = linearmap(minG,maxG);
   BlueMap = linearmap(minB,maxB);

   imDst = zeros(hei,wid,3,‘uint8‘);
   imDst(:,:,1) = RedMap (I(:,:,1)+1);
   imDst(:,:,2) = GreenMap(I(:,:,2)+1);
   imDst(:,:,3) = BlueMap(I(:,:,3)+1);

else
   HistGray = imhist(I(:,:));
   CumGray = cumsum(HistRed);
   minGray =find(CumGray>=PixelAmount*lowCut,1,‘first‘);
   maxGray =find(CumGray>=PixelAmount*(1-highCut),1,‘first‘);
   GrayMap = linearmap(minGray,maxGray);

   imDst = zeros(hei,wid,‘uint8‘);
   imDst(:,:) = GrayMap (I(:,:)+1); 
end

%--------------------------------------------------------------------
function map = linearmap(low,high)
map = [0:1:255];
for i=0:255
   if(i<low)
       map(i+1) = 0;
   elseif (i>high)
       map(i+1) = 255;
   else
       map(i+1) =uint8((i-low)/(high-low)*255);
   end
end


%-------------------------------------------------------------------
function [I,lowCut,highCut] = parse_inputs(varargin)
narginchk(1,3)
I = varargin{1};
validateattributes(I,{‘double‘,‘logical‘,‘uint8‘,‘uint16‘,‘int16‘,‘single‘},{},...
             mfilename,‘Image‘,1);

if nargin == 1
   lowCut = 0.005;
   highCut = 0.005;
elseif nargin == 3
   lowCut = varargin{2};
   highCut = varargin{3};
else
   error(message(‘images:im2double:invalidIndexedImage‘,‘single, or logical.‘));
end

测试驱动代码：

I = imread(‘bikes1.jpg‘);
Out = autolevel(I,0.005,0.005); 
imshow([I Out])

结果

一键去雾

低照度图像增强、水下图像增强

评价

自动色阶用来做去雾，还是比较稳定的，特别是对于雾分布均匀，效果很好；缺点也比较明显，不能自适应雾的分布，且通道分离计算，有些时候会带来一些颜色失真现象。现在也有很多种其他增强方法，例如暗通道去雾，优化对比度去雾，基于Color-Lines的去雾，但是这些算法还不是很稳定，当先验知识失效时，处理失真比较严重。顺便一提的是，美图秀秀的一键去雾即是采用此方法。
另外，自动对比度算法与自动色阶及其类似，可以参考更多内容，在此不再详述。

更多阅读

http://www.cnblogs.com/Imageshop/archive/2011/11/13/2247614.html
http://blog.csdn.net/bluecol/article/details/45422653

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