matlab ( octave ) imwrite 保存图像详解

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.

保存的图像：

sw1: 因为img本身就是uint8类型，所以保存的图像是正确的