A short while ago, a StackOverflow user asked how to extract data from FIG files in Matlab. The answer to this question is easy, but I wish to use the opportunity to show how this can be useful in Matlab applications.
The need: load a FIG without displaying it
In data-processing applications, we often present data in graphs that can then be manipulated and saved for later use. In Matlab, saving is done interactively (via the figure’s File/Save menu option or via the figure’s Save toolbar button), or programmatically (via the hgsave function). In all cases, the data is stored in a *.FIG file (the FIG extension can actually be changed, but I really don’t see why).
The point is that in order to re-load the stored data, we need to use hgload, which has a nasty side-effect of actually displaying the stored figure.
This is bad in terms of performance and memory, especially for heavily-laden GUI figures, not to mention the fact that sometimes we do not want to display the GUI at all. We often just need to extract a sub-set of the stored information (for example, the plot values), without having to display the figure.
The answer: FIGs are simply MATs in disguise
Here’s where the undocumented stuff enters:
It seems that a FIG file is simply a hierarchical structure of all the non-default properties of the stored figure and its descendant handles, all this stored within a simple MAT file.
This is hinted in several places, for example in hgsave‘s doc page, and in the MAT-file preferences panel:
Knowing this, we can easily load the data, without displaying the figure, by using load(filename,’-mat’) rather than hgload. For example:
% Create and store a simple plot figure >> figure('Name','Test #1'); >> plot(1:9, sqrt(1:9)); >> hgsave(gcf,'testFig'); % Load the figure data >> d = load('testFig.fig','-mat') d = hgS_070000: [1x1 struct] % a Matlab 7 figure % Investigate the figure data >> d.hgS_070000 ans = type: 'figure' handle: 1 properties: [1x1 struct] children: [1x1 struct] special:  >> d.hgS_070000.children ans = type: 'axes' handle: 176.08251953125 properties: [1x1 struct] children: [1x1 struct] special: [4x1 double] >> d.hgS_070000.children.children ans = type: 'graph2d.lineseries' handle: 177.0830078125 properties: [1x1 struct] children:  special:  >> d.hgS_070000.children.children.properties ans = Color: [0 0 1] XData: [1 2 3 4 5 6 7 8 9] YData: [1 1.4142 1.7321 2 2.2361 2.4495 2.6458 2.8284 3] ApplicationData: [1x1 struct] XDataJitter: 0 XDataMode: 'manual' ObeyXDataMode: 'auto' ... % Extract plot data >> YData = d.hgS_070000.children.children.properties.YData YData = 1.0000 1.4142 1.7321 2.0000 2.2361 2.4495 2.6458 2.8284 3.0000
We see a couple of interesting things in this simple example:
First, note that the top-level structure node is called “hgS_070000″ – we’ll dig into this next week. Basically, it indicates that the structure holds a Matlab 7 figure.
Secondly, we see hidden/undocumented properties of the stored objects (for example, the lineseries’ XDataJitter and ObeyXDataMode properties). ApplicationData is such a hidden handle, which is widely used as a substitute for any handle’s UserData property (the built-in setappdata and getappdata functions are simply wrappers for accessing this property using standard set and get functions).
Have you used the FIG/MAT feature for any useful functionality? If so, please do share your experience in a comment.
Next week, I will explain how looking under hgsave‘s hood leads to useful functions and to an interesting insight about Matlab’s future. Merry Christmas everybody!
- Setting axes tick labels format Matlab plot axes ticks can be customized in a way that will automatically update whenever the tick values change. ...
- Reading non-Latin text files A workaround that enables reading non-Latin text files in Matlab is shown...
- Getting default HG property values Matlab has documented how to modify default property values, but not how to get the full list of current defaults. This article explains how to do this. ...
- Plot LimInclude properties The plot objects' XLimInclude, YLimInclude, ZLimInclude, ALimInclude and CLimInclude properties are an important feature, that has both functional and performance implications....
- Handle Graphics Behavior HG behaviors are an important aspect of Matlab graphics that enable custom control of handle functionality. ...
- Undocumented button highlighting Matlab button uicontrols can easily be highlighted by simply setting their Value property. ...