6.4 Advanced Features of Plotting

6.4.1 Using Plot Handles

The function fplot returns the x and y coordinates used on the figure. On the other hand plot give handles to each of the curves shown on the plot. Thus we won't try to deal with handles associated with plots from fplot, but use an our example a case where we start with:

>> t=-2*pi:pi/8:2*pi; 
>> hplot=plot(t,sin(t),t,cos(t).*exp(-.5*abs(t))) 
hplot = 
       2.0001 
       5.0001

Each of the handles: hplot(1) and hplot(2) is associated with one of the curves that is shown. Note that if you have forgotten to save the handle to an object, you can find its value with the gco command. This is very handy if you have a curve (or any other object) you wish to remove from a figure, just click on it and issue the command:

>> delete(gco)

Each of the annotation commands: xlabel, ylabel, title, text, gtext and legend also returns one or more handles.

>> hxl=xlabel('time im ms') 
hxl = 
    6.0001 
>> hyl=ylabel('sin(t) and cos(t)e^{-\alpha*|t|}') 
hyl = 
    7.0001 
>> ht=title('Experimenting with Handles') 
ht = 
    8.0001

Note the very limited use of TeX characters in the label on the Y axis. This feature was used to make the Greek character for alpha and to make the exponential a superscript. It has more extensive possibilities in annotating graphs. You may also see that the exponetial part of the Y label is so small that it is unreadable in its default size and font. This is where the handles become very useful. The numeric values for the handles may be used to "fine tune" your plots. Here are a couple of examples. We may see that the font used for labeling both axes are too small. We can correct that by:

>> set(hxl,'FontSize',12) 
>> set(hyl,'FontSize',12)

We can make the title even larger:

>> set(ht,'FontSize',14)

We may not like the color used to plot the first curve. To find which color was used the get command is appropriate:

>> get(hplot(1),'Color') 
ans = 
     0 0 1

Those three numbers give the amount of Red, Green and Blue in the color. To change to red, we will use: [1 0 0], but note that the numbers are not limited to being 0 or 1. They may also be fractions. Here is red:

>> set(hplot(1),'Color',[1 0 0])

We may even see after we use the larger font that we had a typo in the x label and need to correct it:

>> set(hxl,'String','Time in ms')

We can add text to label on of the curves in the color associated with the curve:

>> a=get(hplot(2)); 
>> text(-3,.8,'Color of cos*exp(-|t|/2)','Color',a.Color)

Note that a is a structure and a.color is the color associated with the second curve. Our final plot looked like:




Figure 6.10 Use of Handles















6.4.2 Printing and Saving

   
   
Graphics Files



There are a variety of ways to save the graphics results that you
produce in Matlab. You can of course print the window directly to a
connected printer. This is easily done with the Print... Command
under the File Menu at the top of the Figure Window. This
File menu is shown in Figure 6.2. The selection of
Print...brings up a window with lots of options.














The default settings should produce a copy of the figure on your
default printer. You can chose a variety of other options such
as:






    
   
  • print on other paper type if your printer supports them
    • 
   
   
  • print in Landscape mode rather than portrait
    • 
   
   
  • print with a large number of possible Device Options
    • 
   
   
  • produce a file rather than printing immediately
    • 
   
   
  • open a window to do a Page Setup
    • 




You will find that after you print your figure or even if you send
an image of it to a file, you will not have something that you can
work with in Matlab in a different session. Matlab now allows you to
recreate your figure by using the Save As... option under the
File Menu. If you save your figure this way Matlab creates an
m file together with a mat file. The m file can
then be executed to recreate the figure. I saved Figure 6.10 calling
it figure1. Here is a listing of the first part of the m file it
produced:



whale% cat figure1.m 
function figure1() 
% This is the machine-generated representation of a MATLAB object 
% and its children. Note that handle values may change when these 
% objects are re-created. This may cause problems with some callbacks. 
% The command syntax may be supported in the future, but is currently 
% incomplete and subject to change. 
% % To re-open this system, just type the name of the m-file 
%   at the MATLAB prompt. 
%The M-file and its associtated MAT-file must be on your path. 
 
load figure1 
a = figure('Color',[0.8 0.8 0.8], ... 
'Colormap',mat0, ... 'Position',[517 351 560 420]); 
b = axes('Parent',a, ... 
'Box','on', ... 
'CameraUpVector',[0 1 0], ... 
'Color',[1 1 1], ... 
............... 




Looking at my home directory, I find there is a mat file
produced as well as the m file.



whale% ls figure1* 
figure1.m    figure1.mat 




If I quit my Matlab session and come back another time, I can
restore the same graph by simply typing.:



 >> figure1 




However, when you use the command who, you find all of the
handle variables have not been reproduced. This may limit what you
can do if you want to change details about the figure. If you need to
make such changes, you will have to become acquainted with the
commands:  findobj, gca, gcf,  and even more complicated
set and get. I found for example that the all the
handles could be determined with:



>>  Hs=findobj 
Hs =
         0
    1.0000
    3.0001
    9.0001
    5.0001
    4.0001
 



You can see that these are not the same numeric values set when
the objects were first formed.



A second topic in this section is concerned with the production of
gif files for use in web pages. This section might be applied
to copying a number of windows besides Matlab figures, but currently
it is quite important in Matlab since there is no direct way to
produce gifs or jpeg files from Figures. Here is the
procedure we have used to produce most of the figures in the ceng301
and 303 web pages. If you have a whole or part of a window you want
to make into a gif file, then:



    
   
  • type xv in an xterm: wsname%
   xv This will create a colorful window
   that you should click on with the RIGHT button to bring up an xv
   controls window.
    • 
   
   
  • Move all windows that you are using so they do not cover any
   part of the window you want to copy.
    • 
   
   
  • Click on Grab in the xv controls window. This brings up an xv
   grab window.
    • 
   
   
  • Change the delay to 5 seconds
    • 
   
   
  • Click the Grab button and move the cursor to the window to be
   copied.
    • 
   
   
  • Wait 5 seconds until you hear a beep.
    • 
   
   
  • Click on the window and a copy of that window should
   appear.
    • 
   
   
  • Bring the copy to the front and select the part you want to
   make into a gif file. Do this with the left mouse button by
   holding it down starting at one corner and wiping across to the
   other corner of the rectangle you want.
    • 
   
   
  • Hit the Crop button on the xv controls.
    • 
   
   
  • Save the file as GIF in Full Color.
    • 
   
   
  • Give the file a name with the appendage gif.
    • 
   
   
  • Quit xv unless you have another window to copy.
   
   
     
    
   
   
    6.4.3 Images and Graphs
    • 




Matlab can produce and manipulate two different types of figures:
graphs from programs like plot and images that might have originated
as scanned bit maps. The figure at the top of the main pages in these
notes is an image. A copy of it is stored in



/home/ceng301/public_html/images/089-sm.jpg. 



We can load that file in a Matlab session with the imread command:




>> Symb=imread('/home/ceng301/public_html/images/089-sm.jpg'); 



It loads as a three dimensional object with a large number of
elements.



>> size(Symb) ans = 115 118 3 



If you look at a few of the elements, you see that they are
numeric:



>> Symb(1:3,1:3,1) 
ans = 
     132 139 129 
     122 128 118 
     121 125 119 



To see the images, we use:



>> image(Symb) 



This produces a larger version of the figure that you have been
seeing. It looks better if it is made square:



 >> axis image 











You can manipulate such images in a number of ways such as by
changing the colors used to map the number in the array and changing
the location of the virtual light source for the image. If you
produce an image that you want to save and store in a web page, you
can do so with the imwrite command. It requires three
arguments: the name of the image, the finame where you are going to
store the data and the format you want to use. One of the allowable
formats is 'jpg' (Joint Photographic Experts Group) and can be used
in web pages.






Continue on to Chapter 7: More
Useful MATLAB Arithmetic Functions






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Contents