Last week I explained that FIG files are simply MAT files in disguise ^[1]. Today, we look under the hood of Matlab’s hgsave function, which is used to save FIG files. We shall see that this is both useful and illuminating vis-a-vis Matlab’s future.

handle2struct

Under the hood, hgsave uses the semi-documented ^[2] built-in handle2struct function to convert the figure handle into a Matlab struct that is then stored with a simple save (the same function that saves MAT files) function call.
The fact that handle2struct is semi-documented means that the function is explained in a help comment (which can be seen via the help command), that is nonetheless not part of the official doc sections. It is an unsupported feature originally intended only for internal Matlab use (which of course doesn’t mean we can’t use it).
handle2struct merits a dedicated mention, since I can envision several use-cases for storing only a specific GUI handle (for example, a uipanel, a specific graph, or a set of GUI controls’ state). In this case, all we need to do is to call handle2struct with the requested parent handle, then save the returned structure. So simple, so powerful. handle2struct automatically returns all the non-default property information, recursively in all the handle’s children.
Note that features that are not properties of displayed handles (camera position, 3D rotation/pan/zoom states, annotations, axes-linking etc.) are not processed by handle2struct. For storing the states of these features, you need to use some specific handling – see the code within %matlabroot%\toolbox\matlab\graphics\private\hgsaveStructDbl.m for details. Basically, hgsaveStructDbl.m reads the state of all these features and temporarily stores them in the base handle’s ApplicationData property; handle2struct then reads them as any other regular handle property data, and then hgsaveStructDbl.m clears the temporary data from the handle’s ApplicationData. We can use the same trick for any other application state, of course.

struct2handle

handle2struct has a reverse function – the semi-documented struct2handle. I use it for creating dynamic preference panels: As in Matlab’s Preferences window, I have a list of preference topics and a set of corresponding options panels. In my case, it was easy to design each panel as a separate FIG file using GUIDE. In run-time, I simply load the relevant panel from its FIG file as described above, and place it onscreen in a dedicated uipanel using struct2handle. This enables very easy maintenance of preference panels, without sacrificing any functionality.

Matlab 8: Boldly going where no FIG has gone before…

Remember my post earlier this year about the new HG2 mechanism ^[3]? I speculated that when MathWorks decides to release HG2, it will define this as a major Matlab release and label it Matlab 8.0.
The source code in hgsave.m appears to confirm my speculation. Here is the relevant code section (slightly edited for clarity), which speaks for itself:

% Decide which save code path to use
if ~feature('HGUsingMatlabClasses')   % <== existing HG
    % Warn if user passed in 'all' flag
    if SaveAll
        warning( 'MATLAB:hgsave:DeprecatedOption', ...
            'The ''all'' option to hgsave will be removed in a future release.');
    end
    hgS = hgsaveStructDbl(h, SaveAll);
    SaveVer = '070000';
    SaveOldFig = true;
else   % <== HG2
    % Warn if user passed in 'all' flag
    if SaveAll
        warning( 'MATLAB:hgsave:DeprecatedOption', ...
            'The ''all'' option to hgsave has been removed.');
    end
    if SaveOldFig
        hgS = hgsaveStructClass(h);
        SaveVer = '080000';
    else
        hgO = hgsaveObject(h);
        SaveVer = '080000';
    end
end
% Revision encoded as 2 digits for major revision,
% 2 digits for minor revision, and 2 digits for
% patch revision.  This is the minimum revision
% required to fully support the file format.
% e.g. 070000 means 7.0.0

As can be seen, when Matlab starts using HG2 (perhaps in 2011?), the top-level structure node will be called "hgS_080000", indicating Matlab 8.0. QED.
As a side-note, note that in HG2/Matlab8, although the comment about using 'all' indicates that it has been removed, in practice it is still accepted (although not being used). This will enable your code to be backward-compatible whenever HG2 launches, and future-compatible today.
Have you used handle2struct or struct2handle? If so, please share your experience in a comment ^[4].
Let the upcoming 2011 be a year filled with revelations, announcements and fulfillment! Happy New Year everybody!

11 Comments To "handle2struct, struct2handle & Matlab 8.0"

#1 Comment By Jason On January 3, 2011 @ 07:29

#2 Comment By matt dash On January 11, 2011 @ 14:17

#3 Comment By matt dash On January 11, 2011 @ 14:23

#4 Comment By matt dash On January 11, 2011 @ 15:18

#5 Comment By Nievinski On May 19, 2011 @ 09:13

#6 Comment By Yair Altman On May 19, 2011 @ 09:45

#7 Comment By Cameron Carroll On May 12, 2012 @ 15:07

#8 Comment By Augusto Medeiros On June 9, 2014 @ 11:03

struct2handle(s.hgS_070000,0,'convert');

#9 Comment By Manoj On October 29, 2014 @ 07:11

#10 Pingback By Matlab installation take 2 | Undocumented Matlab On September 12, 2012 @ 06:08

#11 Pingback By Handle Graphics Behavior | Undocumented Matlab On March 6, 2013 @ 14:25