Re: [Matplotlib-users] Feature request: nice grouping in postscript output

[Andrew S. <as...@in...>]

John Hunter wrote:

>     Andrew> I just tried a few things with the axes_demo and the
>     Andrew> errorbar_demo in the examples directory.  I liked that the
>     Andrew> points grouped together.  I didn't like that in the
>     Andrew> errorbar_demo that the points and the errorbars grouped
>     Andrew> together almost inseparably.
> 
> What do you mean by "grouped together".  I assume this has something
> to do with editing in Illustrator, but can you explain in more detail?

Basically, there are two select modes in Illustrator.  The first, 
"Selection tool", selects a whole group of paths.  The second, the 
"Direct selection tool" selects the path segment or otherwise smallest 
path portion possible.

Let's take the example of 5 circles which have been drawn and then 
grouped in Illustrator. Clicking on one with the selection tool will 
select all of them, because they are all grouped.  Clicking one with the 
direct select tool will only get one (or actually probably only a single 
path component between anchor points or the anchor point itself if it 
was clicked).

There is some way Illustrator extracts this information from all 
PostScript files, but it probably just makes intelligent guesses when 
it's dealing with "foreign" PS.  (I think it must embed 
Illustrator-specific comments or other directives when it saves an 
"Illustrator .eps".)

> Learning (a little bit of) postscript has been a mind opening
> experience.  I know a lot of programming languages, and postscript
> introduced me to several new ideas.  It is difficult to take a
> (somewhat) state independent OO representation of a graphical object
> and translate it into the postscript state machine efficiently,
> especially, when the postscript backend has to act like the other
> backends at the interface level.  

(Sounds like OpenGL!)  I know less about PS than you, but it seems one 
way to go about doing what you describe is to build a virtual PS engine 
and render to it, and have it spit out only the state-changing 
instructions it received.

 > A smart postscript backend keep track of this information so it wouldn't
> needlessly regenerate the information leading to file bloat.  I would
> like to make these improvements, but my first goal was to get
> something that works

Always a good first step! :) Seriously, I realized that things were 
probably not very baroque yet, so I thought I'd pipe up to let you know 
about what is, IMHO, an important feature of good PS rendering.

>     Andrew> With the demos tested, the primary curve or points grouped
>     Andrew> with a rectangle around the plotting region that had no
>     Andrew> fill or stroke but seemed to clip the contents to within
>     Andrew> that box.  I wonder if it would be nicer to produce
>     Andrew> postscript output where the clipping is done before
>     Andrew> rendering to a file, thus eliminating the need for this
>     Andrew> rather strangely behaved box?
> 
> Could you also give me some detail here?  Is the "box" the rectangular
> border of the axes?  

Yes.

> With regards to a specific demo, what is "the
> primary curve" and "rectangle"?

The "primary curve" consists of the main data points, either plotted as 
points/circles (in the case of the errorbar demo) or as line segments 
(in the axes_demo). "box" == "rectangle".

> I do use postscript clipping of lines and other objects etc so that
> they do not extend beyond the axes borders.

Yes, I see what you mean -- with this clipping box, the leftmost circle 
in the errorbars demo does not extend beyond the axes, and is therefore 
half cut-off.  I'm not sure if this is desirable or not, but at least 
with the current behavior I could just go in and remove the clipping 
box.  FYI, the circles, the errorbars (vertical lines), the "caps" on 
the bars, and the clipping rectangle all group together in Illustrator.

There is something that seems inconsistent to me with the current 
behavior -- the lower error "caps" that are completely beyond the 
clipping rectangle aren't present in the PS file at all.  However, the 
errorbar does extend below the clipping rectangle to the position where 
the cap would be. Would things be more consistent if, when a clipping 
rectangle is used to do the clipping, all primitives get rendered and 
only the clipping rectangle handles clipping?

>  Generally, I think this
> is *a good thing*.  The general organization of matplotlib is figures
> contain axes which contain lines, patches and text.  Normally, I don't
> want lines, patches and text spilling out of their axes containers.

Yes, I just wonder about the explicit-ness of a decision about whether 
it's matplotlib or PS that does clipping. I don't know enough to feel 
strongly, but if file-size is a factor, it should presumably be done by 
matplotlib.  On the other hand, I think optimizations (even for file 
size) should happen later and for now maybe rendering everything to PS 
and letting it handle clipping is best. On the third(!) hand, huge files 
are clearly undesirable and perhaps the best plan is what seems to 
already be done -- any primitives totally outside the clipping area 
aren't drawn, but otherwise, they are drawn with PS itself doing the 
clipping. This point is just food for thought.

> Can you explain a little more what you are trying to achieve in
> Illustrator so I can get a better idea of what is missing?  What
> exactly is the 'strangely behaved box'?

"strangely behaved" == if you remove one corner from a clipping 
rectangle, it then becomes a clipping triangle that leaves half of your 
plot normal and the other half disappears.  This happens up to some 
distance away from the corner you just deleted.  That's why I call it 
strangely behaved, but I think I do understand it.

>     Andrew> Also, the generated plots have some two boxes, one with a
>     Andrew> white stroke and one with a white fill, surrounding the
>     Andrew> figure.  These, too, seem unnecessary.
> 
> Yes, this is a holdover from the GUI.  In a GUI presentation, the
> plots look nicer with a boundary -- see eg,
> http://matplotlib.sourceforge.net/screenshots/subplot_demo_large.png
> where the gray border is the default figure background -- matlab does
> this.  So the figure (which contains the axes) renders a rectangular
> border with a fill color.  For the postscript backend, I simply made
> these white and when I print on white paper, I never see them.  They
> can easily be done away with by commenting out the line
> 
>    self._figurePatch.draw(drawable)
> 
> in backends/backend_ps.py.  I don't really have a problem removing it
> entirely as I don't see much need for it in the PS backend, unless
> someone wants to frame their plots with background rectangle.  I
> mainly left it in their for vestigial compatibility with the other
> backends.  But, so I can get a better understanding of the twisted
> mind of Illustrator, could you explain to me what kind of problem this
> is causing you?

No real problem, I'm just (mildly) against idea of invisible primitives 
in PS files.  (This probably stems from me dealing with PS output from 
matlab5 many years ago when I remember sorting through layer after layer 
after layer of "strangely behaved rectangles" just to manipulate my 
data.  It's quite funny to me that matplotlib produces the most 
matlab-like PS files I've seen in a while!  Still nowhere near the 
number of layers, though!)

Anyhow, I'd love to dive into the code and help you with the 
PS/Illustrator improvements, but I have no time at the moment...

Cheers!
Andrew