Re: [matplotlib-devel] How to decouple non-GUI stuff from the GUI?

[Michael D. <md...@st...>]

On 11/17/2010 03:41 PM, Kynn Jones wrote:
> Hi.  I am finding that matplotlib does not allow me to decouple 
> certain actions from the GUI as much as I'd like.
>
> For example, I have not been able to do any of the following tasks. 
>  Some of these tasks are certainly artificial, but they all succinctly 
> illustrate some of the problems I've been running into when doing more 
> realistic stuff.
>
> 1. /without bringing up any GUI window/,
Set the backend to a non-GUI backend, such as agg, pdf, ps, svg or cairo 
by setting the 'backend' rcParam in matplotlibrc, or doing:

   import matplotlib
   matplotlib.use("Agg") # Must be the first call after importing matplotlib
>  generate a list of all the filetypes available to a specific 
> matplotlib installation for saving graphics (e.g. .ps, .svg, .pdf, 
> .png, etc.);
 From a figure object:

    figure.canvas.get_supported_filetypes()

returns a dictionary of supported file extensions, with mapping to more 
detailed descriptions of each.
>
> 2. switch back-and-forth between sending graphics objects for display 
> to the GUI and sending them for saving to any of the available 
> filetypes (as defined in (1));
Once set to any of the GUI backends, you can use "show()" and 
"savefig()" interchangeably.
>
> 3. (by far the most important one) /without activating any 
> backend/, create a graphics object (I explain what I mean by this below).
>
> To clarify what I mean by (3), the best I can do is to give an example 
> from Mathematica.  In Mathematica, a Graphics object does not need to 
> be displayed.  It can be defined as a collection of Graphics 
> primitives, without ever displaying it in the GUI.  One such 
> definition would be something like
>
> In[1]:= g = Graphics[Line[{{1., 2.}, {2., 3.}, {3., 5.}}]];
>
> This is what I mean in (3) by creating a graphics object without 
> displaying it (in the GUI).  From a design perspective, this makes a 
> lot of sense, since a Graphics object, as an idea, is entirely 
> independent of whatever mean one chooses to display it.  I want my 
> code to reflect this conceptual independence.
>
> Regarding task (2), to make it more concrete, consider this (also 
> artificial) exercise: write a script that iterates over all filetypes 
> identified in (1); at each iteration, generate some random graphic 
> (e.g. of the sort shown here 
> http://matplotlib.sourceforge.net/examples/api/patch_collection.html), 
> display it on the GUI, and save it as a file of the type corresponding 
> to the iteration.
>
> I would very much appreciate some hints/guidance on how to solve them.
>
> As I noted, by far the most important of these tasks is (3).  From 
> looking into the matplotlib source code, it looks to me impossible to 
> do this with standard matplotlib functions.  But I am very much of a 
> noob with mpl, so I still hope there's a way.
>
> In the worst case scenario (i.e. (3) can't be done directly using 
> standard mpl functions), then the only solution that I can think of 
> would involve implementing a separate layer of abstract graphics 
> objects, distinct from mpl's.  This layer would then delegate to mpl 
> the task of displaying the graphics objects whenever the user requests 
> it.  Any suggestions on how best to do this would be much appreciated. 
>  In particular, I'd like to know what would be the right place to 
> insert this new graphics object layer into the matplotlib objects 
> stack.  I think it would be best to make this connection at a level 
> deeper than the axes object, but I'm not quite sure how to do this.
I think you'll find matplotlib works exactly as you want.  I think you 
might find this part of the documentation useful:

http://matplotlib.sourceforge.net/faq/installing_faq.html#backends

Mike
>
> Thanks in advance!
>
> kj
>
>
>
>
>
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-- 
Michael Droettboom
Science Software Branch
Space Telescope Science Institute
Baltimore, Maryland, USA