刚刚写了imshow, 想了想发现imwrite和imshow是完全一致的， 所以根据上篇文章简单写写imwrite用法。

上篇文章链接： http://blog.csdn.net/watkinsong/article/details/38535341

采用图像：

imwrite() 中， 如果参数为uint8类型， 那么期待的参数像素值范围为0-255， 如果参数矩阵为double类型， 那么期待的像素值范围为0-255.

在imwrite中， 如果你将读取的图像转换为double类型， 直接用imwrite保存图像， 会出现全是白色的图像的情况 ， 因为在转化图像类型的时候， 图像的像素值虽然变为double数据类型， 但是取值范围仍然是0-255， imwrite处理double类型的时候期待的数据范围是0-1， 会把大于1的值当作1处理， 这样几乎所有的像素都变成了白色。

根据上一篇文章， 这里直接简单给出所有的测试代码以及保存的图像结果：

%% this file is used to show how to use imshow or imagesc and so on
% there is some different when you use imshow to display double image or uint8 image
% here all the code will process gray image, RGB color image will not suitable for this file's code
% but you can convert RGB color image to gray image if you want to exercise by this code

% img will be uint8 type
img = imread('syz.bmp');


% imshow: when the parameter img is uint8 type, 
% imshow will default make the range of pixel value as [0-255]
imshow(img);
% this write correct pic
imwrite(img, 'sw1.bmp');
fprintf('Program paused. Press enter to continue.\n');
pause;

% sometimes we need to process the img by double type,
% so, you may convert the data type to double
dimg = double(img);

% but, right now you will not get the correct display,
% because if the parameter of imshow is double type, if will defaultly 
% take range of [0-1], but now the range is [0-255]
% all the value over 1 is ceilled to 1. (clamped to 1)
% so, the displayed image will be a whole white picture.
imshow(dimg);
% this will write a white pic
imwrite(dimg, 'sw2.bmp')
fprintf('Program paused. Press enter to continue.\n');
pause;


% how to correctly display double typed image
% way 1: convert double to uint8
imshow(uint8(dimg));
imwrite(uint8(dimg), 'sw3.bmp')
fprintf('Program paused. Press enter to continue.\n');
pause;



% way 2: change the value of double typed image to [0-1]
maxVal = max(max(dimg));
imshow(dimg / maxVal);
imwrite(dimg / maxVal, 'sw4.bmp');
fprintf('Program paused. Press enter to continue.\n');
pause;

%% some other occurence, the image maybe in double type, 
% but some normalization operation will lead to negative pixel value
% In order to display the image, we need to add a value that make
% all negative value to positive value

% here i just minus a specific value, in real application,
% a face image maybe normalized by substract mean face.
normImg = dimg - 127; % then normImg will have some negative values

minVal = min(min(normImg));
imshow(uint8(normImg - minVal));
imwrite(uint8(normImg - minVal), 'sw5.bmp');
fprintf('Program paused. Press enter to continue.\n');
pause;


%% if you want to save image by imwrite()
% if the image is double type, you need to normalize the value to [0-1]
% if the image is uint8 type, it's ok to save image directly.

sw1: 因为img本身就是uint8类型， 所以保存的图像是正确的

sw2: （其实下面是一张白色图像）， 将图像转换为double以后， 值还是在0-255， 但是imwrite要求是0-1， 所以这个时候超过1的像素都被当作1， 所有图像像素几乎都是白色。。。。其实下面图像可以看到一个黑色的像素点。。。

sw3: 转换像素类型后正确

sw4: 将像素值归一到0-1之间， 正确

sw5: 处理存在像素值为负数的像素值后， 正确