>> Img=imread('18.bmp');
[h,w]=size(Img)
IMG1 = double(Img);
IMG2=zeros(h,w);
for i = 1 : h
for j = 1 : w

IMG2(i,j,1) = 4 * IMG1(i,j,1); %RED Channel

IMG2(i,j,2) = 4 * IMG1(i,j,2); %GREEN Channel

IMG2(i,j,3) = 4 * IMG1(i,j,3); %BLUE Channle
if (IMG2(i,j,1) >= 255)
IMG2(i,j,1) = 255;
end
if (IMG2(i,j,2) >= 255)
IMG2(i,j,2) = 255;
end
if (IMG2(i,j,3) >= 255)
IMG2(i,j,3) = 255;
end

end

end

IMG2 = uint8(IMG2); %不转换为8位就无法正常显示

subplot(1,2,2); %分为1行2列的子图显示，并且在2显示调整对比度后的图

imshow(IMG2);

title('Processed Image');

h =

389

w =

1758

试图访问 IMG1(1,587,1)；由于 size(IMG1)=[389,586,3]，索引超出范围。