为什么matlab help 文件中找不到imshow.m

应该是安装不完整。不过你也可以down一个imshow函数文件，copy进去，试试看。

把以下存为imshow.m，就可以了。
function h=imshow(varargin)
%IMSHOW Display image in Handle Graphics figure.
% IMSHOW(I) displays the grayscale image I.
%
% IMSHOW(I,[LOW HIGH]) displays the grayscale image I, specifying the display
% range for I in [LOW HIGH]. The value LOW (and any value less than LOW)
% displays as black, the value HIGH (and any value greater than HIGH) displays
% as white. Values in between are displayed as intermediate shades of gray,
% using the default number of gray levels. If you use an empty matrix ([]) for
% [LOW HIGH], IMSHOW uses [min(I(:)) max(I(:))]; that is, the minimum value in
% I is displayed as black, and the maximum value is displayed as white.
%
% IMSHOW(RGB) displays the truecolor image RGB.
%
% IMSHOW(BW) displays the binary image BW. IMSHOW displays pixels with the
% value 0 (zero) as black and pixels with the value 1 as white.
%
% IMSHOW(X,MAP) displays the indexed image X with the colormap MAP.
%
% IMSHOW(FILENAME) displays the image stored in the graphics file FILENAME.
% The file must contain an image that can be read by IMREAD or
% DICOMREAD. IMSHOW calls IMREAD or DICOMREAD to read the image from the file,
% but does not store the image data in the MATLAB workspace. If the file
% contains multiple images, the first one will be displayed. The file must be
% in the current directory or on the MATLAB path.
%
% HIMAGE = IMSHOW(...) returns the handle to the image object created by
% IMSHOW.
%
% IMSHOW(...,PARAM1,VAL1,PARAM2,VAL2,...) displays the image, specifying
% parameters and corresponding values that control various aspects of the
% image display. Parameter names can be abbreviated, and case does not matter.
%
% Parameters include:
%
% 'DisplayRange' Two-element vector [LOW HIGH] that controls the
% display range of a grayscale image. See above
% for more details about how to set [LOW HIGH].
%
% Including the parameter name is optional, except
% when the image is specified by a filename.
% The syntax IMSHOW(I,[LOW HIGH]) is equivalent to
% IMSHOW(I,'DisplayRange',[LOW HIGH]).
% The parameter name must be specified when
% using IMSHOW with a filename, as in the syntax
% IMSHOW(FILENAME,'DisplayRange'[LOW HIGH]).
%
% 'InitialMagnification' A numeric scalar value, or the text string 'fit',
% that specifies the initial magnification used to
% display the image. When set to 100, the image is
% displayed at 100% magnification. When set to
% 'fit' IMSHOW scales the entire image to fit in
% the window.
%
% On initial display, the entire image is visible.
% If the magnification value would create an image
% that is too large to display on the screen,
% IMSHOW warns and displays the image at the
% largest magnification that fits on the screen.
%
% By default, the initial magnification is set to
% the value returned by
% IPTGETPREF('ImshowInitialMagnification').
%
% If the image is displayed in a figure with its
% 'WindowStyle' property set to 'docked', then
% IMSHOW warns and displays the image at the
% largest magnification that fits in the figure.
%
% 'XData' Two-element vector that establishes a
% nondefault spatial coordinate system by
% specifying the image XData. The value can
% have more than 2 elements, but only the first
% and last elements are actually used.
%
% 'YData' Two-element vector that establishes a
% nondefault spatial coordinate system by
% specifying the image YData. The value can
% have more than 2 elements, but only the first
% and last elements are actually used.
%
% Class Support
% -------------
% A truecolor image can be uint8, uint16, single, or double. An indexed
% image can be logical, uint8, single, or double. A grayscale image can
% be any numeric datatype. A binary image is of class logical.
%
% If your grayscale image is single or double, the default display range is
% [0 1]. If your image's data range is much larger or smaller than the default
% display range, you may need to experiment with setting the display range to
% see features in the image that would not be visible using the default
% display range. For all grayscale images having integer types, the default
% display range is [intmin(class(I)) intmax(class(I))].
%
% If your image is int8, int16, uint32, int32, or single, the CData in
% the resulting image object will be double. For all other classes, the
% CData matches the input image class.
%
% Related Toolbox Preferences
% ---------------------------
% You can use the IPTSETPREF function to set several toolbox preferences that
% modify the behavior of IMSHOW:
%
% - 'ImshowBorder' controls whether IMSHOW displays the image with a border
% around it.
%
% - 'ImshowAxesVisible' controls whether IMSHOW displays the image with the
% axes box and tick labels.
%
% - 'ImshowInitialMagnification' controls the initial magnification for
% image display, unless you override it in a particular call by
% specifying IMSHOW(...,'InitialMagnification',INITIAL_MAG).
%
% For more information about these preferences, see the reference entry for
% IPTSETPREF.
%
% Remarks
% -------
% IMSHOW is the toolbox's fundamental image display function, optimizing
% figure, axes, and image object property settings for image display. IMTOOL
% provides all the image display capabilities of IMSHOW but also provides
% access to several other tools for navigating and exploring images, such as
% the Pixel Region tool, Image Information tool, and the Adjust Contrast
% tool. IMTOOL presents an integrated environment for displaying images and
% performing some common image processing tasks.
%
% Examples
% --------
% % Display an image from a file
% imshow('board.tif')
%
% % Display an indexed image
% [X,map] = imread('trees.tif');
% imshow(X,map)
%
% % Display a grayscale image
% I = imread('cameraman.tif');
% imshow(I)
%
% % Display a grayscale image, adjust the display range
% h = imshow(I,[0 80]);
%
% See also IMREAD, IMTOOL, IPTGETPREF, IPTSETPREF, SUBIMAGE, TRUESIZE,
% WARP, IMAGE, IMAGESC.

% Copyright 1993-2006 The MathWorks, Inc.
% $Revision: 1.1.8.7 $ $Date: 2006/05/24 03:32:40 $

varargin_translated = preParseInputs(varargin{:});

[cdata, cdatamapping, clim, map, xdata, ydata, ...
initial_mag, style] = ...
imageDisplayParseInputs(varargin_translated{:});

if isempty(initial_mag)
initial_mag = iptgetpref('ImshowInitialMagnification');
else
initial_mag = checkInitialMagnification(initial_mag,{'fit'},...
mfilename,'INITIAL_MAG', ...
[]);
end

new_figure = isempty(get(0,'CurrentFigure')) | ...
strcmp(get(get(0,'CurrentFigure'), 'NextPlot'), 'new');

if (new_figure)
fig_handle = figure('Visible', 'off');
ax_handle = axes('Parent', fig_handle);
else
ax_handle = newplot;
fig_handle = ancestor(ax_handle,'figure');
end

do_fit = strcmp(initial_mag,'fit');
if isempty(style)
style = get(fig_handle,'WindowStyle');
end
if ~do_fit && strcmp(style,'docked')
wid = sprintf('Images:%s:magnificationMustBeFitForDockedFigure',mfilename);
warning(wid,'%s%s','The initial magnification of the image is set to',...
' ''fit'' in a docked figure.');
do_fit = true;
end

hh = basicImageDisplay(fig_handle,ax_handle,...
cdata,cdatamapping,clim,map,xdata,ydata);
set(get(ax_handle,'Title'),'Visible','on');
set(get(ax_handle,'XLabel'),'Visible','on');
set(get(ax_handle,'YLabel'),'Visible','on');

single_image = isSingleImageDefaultPos(fig_handle, ax_handle);

if single_image && do_fit && isBorderTight
% Have the image fill the figure.
set(ax_handle, 'Units', 'normalized', 'Position', [0 0 1 1]);
axes_moved = true;

elseif single_image && ~do_fit
initSize(hh,initial_mag/100,isBorderTight)
axes_moved = true;

else
axes_moved = false;

end

if axes_moved
% The next line is so that a subsequent plot(1:10) goes back
% to the default axes position.
set(fig_handle, 'NextPlot', 'replacechildren');
end

if (nargout > 0)
% Only return handle if caller requested it.
h = hh;
end

if (new_figure)
set(fig_handle, 'Visible', 'on');
end

end % imshow

%------------------------------------------------------------------------------
function is_tight = isBorderTight

borderPref = iptgetpref('ImshowBorder');
if strcmp(borderPref, 'tight')
is_tight = true;
else
is_tight = false;
end

end

%----------------------------------------------------------------------
function varargin_translated = preParseInputs(varargin)
% Catch old style syntaxes and warn

% Obsolete syntaxes:
% IMSHOW(I,N)
% N is ignored, gray(256) is always used for viewing grayscale images.
%
% IMSHOW(...,DISPLAY_OPTION)
% DISPLAY_OPTION is translated as follows:
% 'truesize' -> 'InitialMagnification', 100
% 'notruesize' -> 'InitialMagnification', 'fit'
%
% IMSHOW(x,y,A,...)
% x,y are translated to 'XData',x,'YData',y

new_args = {};
num_args = nargin;

if (num_args == 0)
eid = sprintf('Images:%s:tooFewArgs',mfilename);
error(eid,'%s\n%s','IMSHOW expected at least 1 input argument',...
'but was called instead with 0 input arguments.')
end

if (num_args > 1) && ischar(varargin{end})
% IMSHOW(...,DISPLAY_OPTION)

str = varargin{end};
strs = {'truesize', 'notruesize'};
try
% If trailing string is not 'truesize' or 'notruesize' jump to
% catch block and pass trailing string argument to regular input
% parsing so error will come from that parsing code.
option = iptcheckstrs(str, strs, mfilename,'DISPLAY_OPTION', nargin);

% Remove old argument
varargin(end) = [];
num_args = num_args - 1;

% Translate to InitialMagnification
new_args{1} = 'InitialMagnification';
if strncmp(option,'truesize',length(option))
new_args{2} = 100;
msg1 = 'IMSHOW(...,''truesize'') is an obsolete syntax. ';
msg2 = 'Use IMSHOW(...,''InitialMagnification'',100) instead.';

else
new_args{2} = 'fit';
msg1 = 'IMSHOW(...,''notruesize'') is an obsolete syntax. ';
msg2 = 'Use IMSHOW(...,''InitialMagnification'',''fit'') instead.';

end

wid = sprintf('Images:%s:obsoleteSyntaxDISPLAY_OPTION',mfilename);
warning(wid,'%s\n%s',msg1,msg2)
catch
% Trailing string did not match 'truesize' or 'notruesize' let regular
% parsing deal with it.

% Reset lasterr since we are ignoring the error from iptcheckstrs if
% a valid syntax was used.
lasterr('');
end
end

if (num_args==3 || num_args==4) && ...
isvector(varargin{1}) && isvector(varargin{2}) && ...
isnumeric(varargin{1}) && isnumeric(varargin{2})
% IMSHOW(x,y,...)

% Translate to IMSHOW(...,'XData',x,'YData',y)
p = length(new_args);
new_args{p+1} = 'XData';
new_args{p+2} = varargin{1};
new_args{p+3} = 'YData';
new_args{p+4} = varargin{2};

% Remove old arguments
varargin(1:2) = [];
num_args = num_args - 2;

wid = sprintf('Images:%s:obsoleteSyntaxXY',mfilename);
msg1 = 'IMSHOW(x,y,...) is an obsolete syntax. ';
msg2 = 'Use IMSHOW(...,''XData'',x,''YData'',y) instead.';
warning(wid,'%s%s',msg1,msg2)
end

if num_args == 2 && (numel(varargin{2}) == 1)
% IMSHOW(I,N)

wid = sprintf('Images:%s:obsoleteSyntaxN',mfilename);
msg1 = 'IMSHOW(I,N) is an obsolete syntax. Your grayscale ';
msg2 = 'image will be displayed using 256 shades of gray.';
warning(wid,'%s%s',msg1,msg2)

% Remove old argument
varargin(2) = [];
end

varargin_translated = {varargin{:}, new_args{:}};

end