Matlab数字图像亮度变换函数

摘自《数字图像处理》冈萨雷斯

一、处理可变数量的输入和输出

利用nargin和nargout进行检测

T = testhv(4, 5)

该函数体中使用nargin返回2，使用nargout返回1.

函数nargchk可用于一个M函数体中，以检测传递函数的参量数目是否正确。

msg = nargchk(low, high, number)

参量number介于low与high之间返回空矩阵，否则返回错误信息。通常可这样使用：

function G = testhv2(x, y, z)

...

error(nargchk(2, 3, nargin));

...

键入仅有一个输入变量的语句

testhv2(6);

将产生错误消息

Not enough input arguments

同时执行终止。

通常，写出具有可变数目的输入变量和输出变量的函数时十分有用的。这里，我们使用变量varargin和变量varargout。varargin和varargout鼻血使用小写形式。例如，

function [m, n] = testhv3(varargin)

将输入的变量数读取到函数testhv3中，而

function [varargout] = testhv4(m, n, p)

则通过函数testhv4返回输出的变量数。若函数testhv3有一个固定的输入变量x，后跟输入变量的可变数目，则调用

function [m, n] = testhv3(x, varargin)

函数时，会导致varargin由用户提供的第二个输入变量开始运行。varargout的情形与此类似。一个函数的输入变量和输出变量的个数是可变的。

当varargin用做一个函数的输入变量时，Matlab会将其置入一个单元数组中，该数组接受由用户输入的变量数。由于varargin是一个单元数组，所以此类配置的一个重要方面是对函数的调用可包括输入的混合集。例如，若我们要使用假设函数testhv3的代码来处理此项操作，则它能很好的接受输入的混合集，如

[m, n] = testhv3(f, [0 0.5 1.5], A, 'label');

其中，f是一幅图像，下一个变量时一个长度为3的行向量，A是一个矩阵，'label'是一个字符串。

二、亮度变换的另一个M函数

在这一节中，我们将开发一个计算如下变换功能的函数：负片变换、对数变换、gamma变换和对比度拉伸变换。选用这些变换是因为随后我们将用到它们。此外，我们将进一步说明编写亮度变换M函数所涉及的机理。在编写该函数时，我们将用到函数changeclass，其语法为：

g = changeclass(newclass, f)

function image = changeclass(class, varargin)

%CHANGECLASS changes the storage class of an image.

% I2 = CHANGECLASS(CLASS, I);

% RGB2 = CHANGECLASS(CLASS, RGB);

% BW2 = CHANGECLASS(CLASS, BW);

% X2 = CHANGECLASS(CLASS, X, 'indexed');

% $Revision: 1.2 $ $Date: 2003/02/19 22:09:58 $

switch class

case 'uint8'

image = im2uint8(varargin{:});

case 'uint16'

image = im2uint16(varargin{:});

case 'double'

image = im2double(varargin{:});

otherwise

error('Unsupported IPT data class.');

end

此函数将图象f转换成有参数newclass指定的类别，并输出图像g。newclass的有效值是'uint8'， 'uint16'，和'double'。

function g = intrans(f, varargin)

%INTRANS Performs intensity (gray-level) transformations.

% G = INTRANS(F, 'neg') computes the negative of input image F.

% G = INTRANS(F, 'log', C, CLASS) computes C*log(1 + F) and

% multiplies the result by (positive) constant C. If the last two

% parameters are omitted, C defaults to 1. Because the log is used

% frequently to display Fourier spectra, parameter CLASS offers the

% option to specify the class of the output as 'uint8' or

% 'uint16'. If parameter CLASS is omitted, the output is of the

% same class as the input.

% G = INTRANS(F, 'gamma', GAM) performs a gamma transformation on

% the input image using parameter GAM (a required input).

% G = INTRANS(F, 'stretch', M, E) computes a contrast-stretching

% transformation using the expression 1./(1 + (M./(F +

% eps)).^E). Parameter M must be in the range [0, 1]. The default

% value for M is mean2(im2double(F)), and the default value for E

% is 4.

% For the 'neg', 'gamma', and 'stretch' transformations, double

% input images whose maximum value is greater than 1 are scaled

% first using MAT2GRAY. Other images are converted to double first

% using IM2DOUBLE. For the 'log' transformation, double images are

% transformed without being scaled; other images are converted to

% double first using IM2DOUBLE.

% The output is of the same class as the input, except if a

% different class is specified for the 'log' option.

% Digital Image Processing Using MATLAB, Prentice-Hall, 2004

% $Revision: 1.7 $ $Date: 2003/10/13 00:45:53 $

% Verify the correct number of inputs.

error(nargchk(2, 4, nargin))

% Store the class of the input for use later.

classin = class(f);

% If the input is of class double, and it is outside the range

% [0, 1], and the specified transformation is not 'log', convert the

% input to the range [0, 1].

if strcmp(class(f), 'double') & max(f(:)) > 1 & ...

~strcmp(varargin{1}, 'log')

f = mat2gray(f);

else % Convert to double, regardless of class(f).

f = im2double(f);

end

% Determine the type of transformation specified.

method = varargin{1};

% Perform the intensity transformation specified.

switch method

case 'neg'

g = imcomplement(f);

case 'log'

if length(varargin) == 1

c = 1;

elseif length(varargin) == 2

c = varargin{2};

elseif length(varargin) == 3

c = varargin{2};

classin = varargin{3};

else

error('Incorrect number of inputs for the log option.')

end

g = c*(log(1 + double(f)));

case 'gamma'

if length(varargin) < 2

error('Not enough inputs for the gamma option.')

end

gam = varargin{2};

g = imadjust(f, [ ], [ ], gam);

case 'stretch'

if length(varargin) == 1

% Use defaults.

m = mean2(f);

E = 4.0;

elseif length(varargin) == 3

m = varargin{2};

E = varargin{3};

else error('Incorrect number of inputs for the stretch option.')

end

g = 1./(1 + (m./(f + eps)).^E);

otherwise

error('Unknown enhancement method.')

end

% Convert to the class of the input image.

g = changeclass(classin, g);

例3.3 函数intrans的说明

要说明函数intrans，我们可以利用下面的例子，左边为原始图，右边为经对比度拉伸后的图像。

>> f = imread('Fig0306(a)(bone-scan-GE).tif');

>> figure(1)

>> imshow(f)

>> g = intrans(f, 'stretch', mean2(im2double(f)), 0.9);

>> figure(2)

>> imshow(g)

三、亮度标度的M函数

当处理图像是，象素值域由负到正的现象是很普遍的。尽管在中间计算过程中没有问题，但当我们想利用8bit或16bit格式保存或查看一幅图像时，就会出现问题。在这种情况下，我们通常希望吧图像标度在全尺度，即最大范围[0, 255]或[0, 65535]。下列名为gscale的M函数可以实现此功能。此外，此函数能将输出映射到一个特定的范围。

function g = gscale(f, varargin)

%GSCALE Scales the intensity of the input image.

% G = GSCALE(F, 'full8') scales the intensities of F to the full

% 8-bit intensity range [0, 255]. This is the default if there is

% only one input argument.

% G = GSCALE(F, 'full16') scales the intensities of F to the full

% 16-bit intensity range [0, 65535].

% G = GSCALE(F, 'minmax', LOW, HIGH) scales the intensities of F to

% the range [LOW, HIGH]. These values must be provided, and they

% must be in the range [0, 1], independently of the class of the

% input. GSCALE performs any necessary scaling. If the input is of

% class double, and its values are not in the range [0, 1], then

% GSCALE scales it to this range before processing.

% The class of the output is the same as the class of the input.

% Digital Image Processing Using MATLAB, Prentice-Hall, 2004

% $Revision: 1.5 $ $Date: 2003/11/21 14:36:09 $

if length(varargin) == 0 % If only one argument it must be f.

method = 'full8';

else

method = varargin{1};

end

if strcmp(class(f), 'double') & (max(f(:)) > 1 | min(f(:)) < 0)

f = mat2gray(f);

end

% Perform the specified scaling.

switch method

case 'full8'

g = im2uint8(mat2gray(double(f)));

case 'full16'

g = im2uint16(mat2gray(double(f)));

case 'minmax'

low = varargin{2}; high = varargin{3};

if low > 1 | low < 0 | high > 1 | high < 0

error('Parameters low and high must be in the range [0, 1].')

end

if strcmp(class(f), 'double')

low_in = min(f(:));

high_in = max(f(:));

elseif strcmp(class(f), 'uint8')

low_in = double(min(f(:)))./255;

high_in = double(max(f(:)))./255;

elseif strcmp(class(f), 'uint16')

low_in = double(min(f(:)))./65535;

high_in = double(max(f(:)))./65535;

end

% imadjust automatically matches the class of the input.

g = imadjust(f, [low_in high_in], [low high]);

otherwise

error('Unknown method.')

end

调用语法为：

g = gscale(f, method, low, high)

其中，f是将被标度的图像，method的有效值为'full8'和'full16'，前者将输出标度为全范围[0, 255]，后者将输出标度为全范围[0, 65535]。若使用这两个有效值之一，则可在两种变换中省略参数low与high。method的第三个有效值为'minmax'，此时我们必须给出low与high在范围[0, 1]内的值。若选用的是'minmax'，则映射的结果

Matlab数字图像亮度变换函数

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