matlab imshow显示图像详解

最近在用octave (类似于matlab的计算软件， 函数和matlab一致) 写程序的时候， 在显示图像和保存图像的时候遇到了一些小问题， 所以简单的总结了一下。

本文用的图像为灰度图像：

imread() 返回的图像类型是uint8类型， 这时用imshow显示图像的时候， imshow会认为输入矩阵的范围在0-255， 如果imshow的参数为double类型的，那么imshow认为输入矩阵的值为0-1.

很多时候需要将图像转换为double类型的， 但是转换以后直接使用imshow显示的是一片白色， 是因为当imshow显示图像的时候， 会认为double类型的图像矩阵的范围在0-1， 超过1的像素值当作1处理， 这样就是几乎所有的像素都是白色。

情况1：

% 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);
fprintf('Program paused. Press enter to continue.\n');
pause;

这个时候直接显示读入的图像是正确的， 因为这个时候图像的类型是uint8
显示结果：

情况2：

当把图像转换double数据类型后：

% 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);
fprintf('Program paused. Press enter to continue.\n');
pause;

这个时候， 因为imshow的参数为double类型， imshow认为参数的值为0-1， 但是实际还是0-255， 这样超过1的值被认为是1， 显示几乎全是白色

情况3： 如何正确的显示 double 类型的图像呢， 有两种方式

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

方式1 将图像转换为uint8类型， 方式2将图像像素值归约到0-1之间
显示结果：

这两个显示结果是一样的。

情况4： 有些时候，图像经过处理以后会存在值为负数的像素， 如何显示呢？

这需要将所有的负数处理掉， 可以将所有的像素减去这个图像的最小的像素值， 最小的像素值为负数， 那么图像的像素值就都为正数了。

%% 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));
fprintf('Program paused. Press enter to continue.\n');
pause;

显示结果：

下面是整个示例代码：

%% 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);
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);
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));
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);
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));
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.