(1)线性变换:
通过建立灰度映射来调整源图像的灰度。
k>1增强图像的对比度；k=1调节图像亮度，通过改变d值达到调节亮度目的；0<k<1减弱对比度；k<0图像较亮区域会变暗，较暗区域会变亮。
i = imread(‘theatre.jpg‘);
i = im2double(rgb2gray(i));
[m,n]=size(i);
%增加对比度
Fa = 1.25; Fb = 0;
O = Fa.*i + Fb/255;
figure(1), subplot(221), imshow(O);
title(‘Fa = 1.25, Fb = 0, contrast increasing‘);
figure(2),subplot(221), [H,x]=imhist(O, 64);
stem(x, (H/m/n), ‘.‘);
title(‘Fa = 1.25, Fb = 0, contrast increasing‘);
%减小对比度
Fa =0.5; Fb = 0;
O = Fa.*i + Fb/255;
figure(1), subplot(222),imshow(O);
title(‘Fa = 0.5, Fb = 0, contrast decreasing‘);
figure(2), subplot(222), [H,x] = imhist(O, 64);
stem(x, (H/m/n), ‘.‘);
title(‘Fa = 0.5, Fb = 0, contrast decreasing‘);
%线性亮度增加
Fa = 0.5; Fb = 50;
O = Fa.*i + Fb/255;
figure(1), subplot(223), imshow(O);
title(‘Fa = 0.5, Fb = 50, brightness control‘);
figure(2), subplot(223), [H,x]=imhist(O,64);
stem(x, (H/m/n), ‘.‘);
title(‘Fa = 0.5, Fb = 50, brightness control‘);
%反相显示
Fa = -1; Fb = 255;
O = Fa.*i + Fb/255;
figure(1), subplot(224), imshow(O);
title(‘Fa = -1, Fb = 255, reversal processing‘);
figure(2), subplot(224),[H,x]=imhist(O, 64);
stem(x, (H/m/n), ‘.‘);
title(‘Fa = -1, Fb = 255, reversal processing‘);
(2)对数变换：
增强低灰度，减弱高灰度值。
i = imread(‘theatre.jpg‘);
i = rgb2gray(i);
i = double(i);
out1 = log(1+i)/0.065;
out2 = log(1+i)/0.035;
out1(find(out1>255)) = 255;
out2(find(out2>255)) = 255;
out1 = uint8(out1);
out2 = uint8(out2);
(3)幂次变换：
次数小于1时，增强低灰度，减弱高灰度；次数大于1时增强高灰度，减弱低灰度。
i = rgb2gray(imread(‘theatre.jpg‘));
i = double(i);
y1 = 255*(i/255).^2.5;
y2 = 255*(i/255).^0.4;
y1 = uint8(y1);
y2 = uint8(y2);
(4) 指数变换：
增强高灰度，减弱低灰度。
i = imread(‘theatre.jpg‘);
i = rgb2gray(i);
i = double(i);
y1 = 1.5.^(i*0.070)-1;
y2 = 1.5.^(i*0.050)-1;
y1(find(y1>255)) = 255;
y2(find(y2>255)) = 255;
y1 = uint8(y1);
y2 = uint8(y2);
(5)灰度拉伸：
有时图像灰度集中在某小块区域，需要改变图像对比度。
i = imread(‘theatre.jpg‘);
i = rgb2gray(i);
L = imadjust(i,[ ],[50/255;150/255]);
J = imadjust(L,[50/255;150/255 ],[20/255;230/255]);
(6)灰度均衡：
i = rgb2gray(imread(‘theatre.jpg‘));
LC = imadjust(i,[ ],[50/255;150/255]);
HE1 = histeq(LC);%均衡函数
(7)直方图规定化：
实现局部的灰度均衡。
img = rgb2gray(imread(‘theatre.jpg‘));
img_ref = rgb2gray(imread(‘rpic.jpg‘));%参考图，按照这个的的直方图进行规定化
[hgram, x] = imhist(img_ref);
J = histeq(img, hgram);