[Matplotlib-devel] MEP 29 Axes Refactor

OceanWolf juichenieder-nabb at yahoo.co.uk
Sat Sep 5 22:10:51 CEST 2015

Not too sure what you meant by ````.

On 05/09/15 19:25, Joe Kington wrote:
> First off, I don't intend this to come across as overly critical!  I 
> think this is a very good discussion to have.
> Also, I tend to have a bad "knee-jerk" reaction to change and tend to 
> come around over time, so keep that in mind too. :)
No worries, I experience the same, and yes I wanted to open this up for 
just this kind of interrogation, especially as we have a lot of axes 
related code, and I have only touched a fraction of it to date.

To explain where I come from, I should say that I like to work 
bottom-up.  I find designing good code starts with asking probing 
questions about what you want to model, in this case we have an 
``_AxesBase`` class and so by definition it should model an abstract 
Axes,  because of this in the "Detailed Description" of this MEP I begin 
by asking the question probing the definition of an Axes.  I believe 
that if we model the world intutitively as we see it, everything else 
will fall into place.  I find the most direct route in code usually 
contains lots of inflexibility, like building a road through the 
mountain, you might go the direct route, but it becomes very difficult 
to maintain and expand upon.  Hence the focus lies in the journey.

> However, while I agree that `Axes` is quite a beast, I'm not sure this 
> proposal simplifies things.  From my perspective, it adds complexity. 
> If I'm understanding correctly, this would effectively tie the 
> Transform stack to the Axes, instead of having the Axes generate a 
> Transform object that may or may not be used by the artists in the Axes.
If I understand you correctly you don't like the idea of forcing Artists 
to use the transform.  I don't see this as a problem (at the moment).  
As far as I see it, all coordinates supplied to an Artist will come in 
the form of Axes coordinates, i.e. Axes space, and thus we need to 
transform those coordinates to screen coordinates... at least at some 
point, probably when it comes to drawing... I especially think of 
drawing a triangle onto a spherical geometry, see 
https://en.wikipedia.org/wiki/Spherical_geometry.  We create a Polygon 
patch and supply the three vertices that define our Triangle... however 
on a spherical geometry, these "straight" lines do not conform to the 
Euclid definition of straight, we need to draw them curved.  Because of 
this the transform will need to come very late in the drawing process.
>     First we define our coordinate transformation functions:
>     axes_to_base(self, *q) base_to_axes(self, x, y)
>     The term |base| could get replaced with |screen| but for now we
>     will keep it simple to reflect another transformation from base
>     coords to screen coords, e.g. perhaps to differentiate between
>     window and screen coords.
> This is my main concern.  We have a (i.m.o.) very flexible and 
> actually quite clean Transform system to handle this.  Why shift away 
> from it? `ax.transData` may be non-PEP8 naming, but it's a good way to 
> do this.  The concept of having Transform objects that handle this but 
> are separate from the Axes gives a lot of flexibility.  In my opinion, 
> the core concept of having this transformation handled by a Transform 
> object that's separate from the Axes is one of the best things about 
> matplotlib's design.
> Or am I misunderstanding, and this is just a refactoring of 
> `_get_core_transform` and `_get_affine_transform` into one method?
As far as I know, I want to keep the transform system.  I think I do 
just mean refactoring that into one method.  I say think as I still 
don't feel fully understand how it all works, the Transform system, 
brilliant, but very mind-boggling.  I had to delve into it to find a bug 
reported by a user on github, and went through around 50 (perhaps more) 
Transform operations before I got to the problem.  If you need to debug 
part of it, like i had to, it becomes a tangled mess, luckily for most 
people they don't have to, and the usage works quite simply.  When I 
tracked down the bug I also spent quite some time trying to figure out 
the Transform classes, prior to the bug I only knew of Rotation, Shear 
and Reflection Transforms. Anyway my point here comes that while great, 
it can become quite the head-ache for the average user developer, 
especially for those who know even less then I do about transforms, and 
so I want to blackbox the transforms in the Axes with simple names such 
as axes_to_***_coords(self, *q), and ***_to_axes_coords(self, x, y).

So I want to make it easy for people to write their own axes with their 
own transform methods without having to worry about how the rest of the 
Artist code and plot methods work (unless it works really bizarrely); 
and I want people to work on Artist code, and creating their own tools 
and user interaction stuff without having to worry about learning about 
transforms (they just need to know that these two methods will do the 
conversion for them from data coordinates, which they understand, to the 
location on the screen or whatever, which they will also understand, 
start talking about AffineTransforms and I think we will scare people off).
> ---------------------
> My other main concern centers on map projections. The MEP currently 
> mentions:
>     an anticipated structure of a base mapping class with a coordinate
>     system in lat/lon coordinates, but with different mapping
>     projections available for the conversion between the Axes
>     coordinate system and the screen.
> However, this is a bad approach for cartographic data. Geographic is 
> not the base for a projected coordinate system. There are several 
> reasons for that.
> 1. Map data is usually _in the projected coordinate system_.  Lat, 
> long data is actually not terribly common unless you're working with 
> global datasets.
> 2. Raster data (i.e. anything displayed with imshow) is typically 
> going to be gridded on a regular grid in the projected coordinate 
> system.  Forcing a transformation back to a non-uniform grid in lat, 
> long space then back onto a different uniform grid than the original 
> in display space is unnecessarily expensive.
> One of the great things about Cartopy is that it leaves the 
> fundamental Cartesian projected space unchanged, and let's you specify 
> the transform if you want to use geographic coordinates.  Basemap 
> handles it a bit differently but has the same core concept.  Latitudes 
> and longitudes aren't the data coordinate system.  The projected 
> coordinate system is.
> There's a reason for that approach.  Forcing people to convert their 
> data into a geographic coordinate system before plotting it is a bad 
> idea. It's good to have plotting methods that allow geographic 
> coordinates, but bad to require that transformation.  (I'll skip the 
> very important datum part for the moment.  Just be aware that a lat, 
> long only gets you to within ~1km of a location without more information.)
Hmm, when I have used Basemap, the data files I work with I always get 
in lat/lon format.  One of my biggest annoyances with Basemap comes from 
having to work projection coordinates.  I move the mouse over the map 
and statusbar shows me useless projection coordinate information; I want 
to rotate the globe (in 'ortho' projection), but I can't, it becomes 
very difficult to use from a user interface point of view.

I think the solution here comes from using a dual approach.  With the 
functions above I used the term base, as in the ``axes_to_base`` and 
``base_to_axes`` functions.  Here ``base`` defines the projected 
coordinates.  We can then leave it up to the user to decide whether to 
plot in axes coordinates or base coordinates.  We can start of leaving 
the axes side unimplemented, and perhaps we will never implement the 
axes side for some Axes classes... what do you think?

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