灰度级数k，k=2^b，称该图像为b比特图像。

降低灰度级数是靠2的幂次方

网上代码：https://blog.csdn.net/silence2015/article/details/68927360

function changereduce_factor(imgpath,reduce_factor)
% Write a computer program capable of reducing the number of intensity levels in an image from 256 to 2, 
% in integer powers of 2. The desired number of intensity levels needs to be a variable input to your program.
    f = imread(imgpath);
    if reduce_factor<0
        reduce_factor=0
    else if reduce_factor>8
            reduce_factor=8
        end
    end

    dfactor=uint8(2^reduce_factor);
    f_trans=(f/dfactor)*dfactor;
    subplot(1,2,1);
    imshow(f);
    subplot(1,2,2);
    imshow(f_trans);

end

其中reduce_factor=8表示原图像是一张8bit图像，有灰度级dfactor是256，

所以（f/dfactor）取整只有两个结果0或者1，然后再乘灰度级就变成了黑白图片（只有两个灰度级）

那么当reduce不等于比特图像那个值的时候会出现什么结果，如果大于，那么图像是全黑的0，小于的话取整运算的结果将不只是0，1，还会出现更多数

但是还是小于比特图像数，也就做到了将灰度值降低。

pic=imread('pic/coltogray/1.jpg');
##gray_pic=rgb2gray(pic);
##figure(1)
##imshow(gray_pic)

[x,y,z]=size(pic);

graypic=zeros(x,y);

level=4

dfactor=uint8(2^level)
max1=0;
min1=0;
for i=1:x
  for j=1:y
    sum=0;
    for k =1:z
      sum=sum+pic(i,j,k)/3;
    end
    graypic(i,j)=sum;
    graypic1(i,j)=(sum/dfactor)*dfactor;
  end
end

graypic=uint8(graypic);
graypic1=uint8(graypic1);

figure(1);
imshow(graypic);
figure(2);
imshow(graypic1);
%将三维图像压缩至一维即可以看作是灰度图像？