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Not too sure what you meant by ````. <br>
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<div class="moz-cite-prefix">On 05/09/15 19:25, Joe Kington wrote:<br>
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cite="mid:CACe1pJdxmbSckwbzTDXgn0jLnRE_GNGR+VgUv557wPRNR9g_oA@mail.gmail.com"
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<div><br>
First off, I don't intend this to come across as overly
critical! I think this is a very good discussion to have.<br>
</div>
<div>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.
:)<br>
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</blockquote>
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.<br>
<br>
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.<br>
<br>
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<div><br>
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.<br>
<br>
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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 <a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/Spherical_geometry">https://en.wikipedia.org/wiki/Spherical_geometry</a>. 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.<br>
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<blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px
solid rgb(204,204,204);padding-left:1ex" class="gmail_quote">
<p>First we define our coordinate transformation functions:
axes_to_base(self, *q)
base_to_axes(self, x, y)</p>
<p>The term <code>base</code> could get replaced with <code>screen</code>
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.</p>
</blockquote>
<div><br>
</div>
<div>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. <br>
<br>
</div>
<div>Or am I misunderstanding, and this is just a refactoring
of `_get_core_transform` and `_get_affine_transform` into
one method? <br>
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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).<br>
<br>
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).<br>
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<div><br>
My other main concern centers on map projections. The MEP
currently mentions:<br>
<br>
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<blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px
solid rgb(204,204,204);padding-left:1ex" class="gmail_quote">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.<br>
</blockquote>
<div><br>
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<div>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.<br>
<br>
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<div>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. <br>
</div>
<div>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. <br>
<br>
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.<br>
<br>
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.)<br>
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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.<br>
<br>
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?<br>
<br>
Best,<br>
OceanWolf<br>
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