MATLAB: Does axes() not return an Object but a graphics handle

graphicsgraphics objecthandle graphics

Hi, guys.

The graphics system in MATLAB puzzles me, here take axes for example.

What's the class name of axes object? Can users subclass it?

Why does axes() not return an axes object, but a handle?

Graphics handle is some like index/pointer/reference of a graphics object. For C Language, one can allocate a block of memory in a function and return the pointer. But I have not find that Matlab Language can implement such thing. If true, how does axes() allocate the memory for axes object and return a handle? I think the axes object should have disappeared when returned from axes

I think it is the function of MATLAB Software, not of the Matlab Language that makes axes object alive after returned from axes().

Any idea?

Best Answer

Maybe I put my words differently: the handle graphic objects are not true MATLAB objects in the sense that you have some class file with methods etc. They are somewhat built into MATLAB itself. You can see some of it (try the following:

h = plot([1 3 2]);
hh = handle(h)

But looking at methods or properties will still not be the same as for "usual" objects.

Does this help?

Titus

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MATLAB: How to keep a handle and use FINDOBJ to get a handle again

What Matt Fig said, plus: are you actually copying the number in (rather than saving it in a variable and passing the variable into the function)? Because that won't work for anything other than a figure handle. Figure handles are integers, but everything else is actually a hex reference (that appears as a double because, as the old joke goes, you can have any data type you want in MATLAB as long as it's a double -- no longer true, of course, but the remnants of that are seen in places, like HG). Anyway, the key point being that the handle isn't 123.0045 -- it's 123.0045blahblahblah. Any attempt to copy/paste the actual number will invariably get the floating point representation slightly off, and will therefore result in an incorrect hex reference, and that error message you love so much.

Bottom line, then: save handles as variables.

As Matt F alluded to in his answer, you can store handles as outputs from 99% of graphics function calls:

hf = figure;
ha = axes;
hp = plot(x,y,x,z);

etc. In this case, hf and ha will be scalar handles, hp will be a vector of 2 handles (one to each line object on the plot).

Use findobj to locate handles that you don't already have:

h = findobj('type','line')

And use the parent and children properties to navigate the hierarchy. For example, the above example could also be done:

hp = plot(x,y,x,z)
ha = get(hp(1),'parent')  % parent of a line is the axes
hf = get(ha,'parent')     % parent of the axes is the figure

MATLAB: Fzero of two functions subtracted

fzero(fun,1);

Best wishes

Torsten.

Best Answer

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