颜色特征提取（一）------颜色直方图

        颜色特征是一种全局特征,描述了图像或图像区域所对应的景物的表面性质.一般颜色特征是基于像素点的特征,此时所有属于图像或图像区域的像素都有各自的贡献.由于颜色对图像或图像区域的方向、大小等变化不敏感,所以颜色特征不能很好地捕捉图像中对象的局部特征.另外,仅使用颜色特征查询时,如果数据库很大,常会将许多不需要的图像也检索出来.颜色直方图是最常用的表达颜色特征的方法,其优点是不受图像旋转和平移变化的影响,进一步借助归一化还可不受图像尺度变化的影响,基缺点是没有表达出颜色空间分布的信息.

（颜色直方图）：

       A color histogram of an image represents the distribution of the composition of colors in the image. It shows different types of colors appeared and the number of pixels in each type of the colors appeared. The relation between a color histogram and a luminance histogram is that a color histogram can be also expressed as “Three Color Histograms”, each of which shows the brightness distribution of each individual Red/Green/Blue color channel.

      使用最多的可能就是灰度直方图，而它丢失了很多颜色信息，所以努力实现颜色直方图，它能够直接代表实际图中的颜色的数量。

MATLAB实现：

 

function createColorHistograms(im_str)

if ~isstr(im_str)
    if ndims(im_str)==3
        try
            col_array_vals=double(im_str);
        catch
            disp('Input is not a valid three-dimensional array');
            return;
        end
    end
else
    try
        col_array_vals=double(imread(im_str));
        if ndims(col_array_vals)~=3
            disp('Input is not a valid three-dimensional array');
            return;
        end

    catch
        disp('Input string does not point to a valid image file');
        return;
    end
end

res_val=90;

t_count=res_val*floor(col_array_vals(:,:,1)/res_val)+256*(res_val*floor(col_array_vals(:,:,2)/res_val))+256*256*(res_val*floor(col_array_vals(:,:,3)/res_val));
t_count=sort(t_count(:));

[col_val,ind_first]=unique(t_count,'first');
[col_val,ind_last]=unique(t_count,'last');
disp('Drawing color bars')

disp('Drawing image')
subplot(121);
set(gcf,'position',[5   61   274   236]);
imshow(col_array_vals/255)
colorbars(col_val,ind_last-ind_first,1/3,1/4)


function colorbars(triplet_color,triplet_freq,varargin)

if nargin==2
    color_pow=1/3;
    freq_pow=1/4;
else
    color_pow=varargin{1};
    freq_pow=varargin{2};
end

N_rand=randperm(length(triplet_freq));
triplet_freq=sqrt(triplet_freq(N_rand));
triplet_color=triplet_color(N_rand);

triplet_color=([rem(triplet_color,256) floor(rem(triplet_color,256*256)/255) floor(triplet_color/(256*256))]/255);
triplet_color_norm=triplet_color./repmat(((sum(triplet_color.^(1),2))+.005),1,3);
max(triplet_color_norm)
triplet_diff=sum(abs(triplet_color_norm-repmat(triplet_color_norm(end,:),size(triplet_color_norm,1),1)),2);

triplet_diff=sum(abs(triplet_color_norm-repmat([.9 0 0],size(triplet_color_norm,1),1)),2);

max(triplet_diff)

triplet_diff=(triplet_diff/max(triplet_diff).^(color_pow))+(triplet_freq*0).^(freq_pow);



[d,inds_sort]=sort(triplet_diff);
triplet_freq=(triplet_freq(inds_sort));
triplet_color=(triplet_color(inds_sort,:));

num_bars=length(triplet_color);
max_val=max(triplet_freq);
% close all;
subplot(122);
axis([0 num_bars 0 1]);
%% 
    [~,ind] = max(triplet_freq);
    triplet_color(ind,:)=[];
    triplet_freq(ind,:)=[];
    num_bars = num_bars-1;
%%   
for i=1:num_bars
    tempColor=min(triplet_color(i,:),.9);
    %===
    % Use patch to draw individual bars
    %===
    patch([i-1 i-1 i i],...
        [0 triplet_freq(i)/max_val triplet_freq(i)/max_val 0],...
        tempColor,...
        'edgecolor',...
        tempColor);
    
end
% colorbar('LineWidth',1);

set(gca,'xtick',[0:10:255])
set(gca,'ytick',[0:0.05:1])
set(gcf,'position',[5 378 560 420]);
set(gca,'visible','on')

function y_val=sigmoidVal(x_val,varargin)

if nargin==1
    multip_val=15;
else
    multip_val=varargin{1};
end

y_val=1./(1+exp(-(x_val-.5)*multip_val));