Re: [matplotlib-devel] distorted patches

Jordan Dawe wrote:
> 
>>> This is probably what you want to do.  You want to define your arrow
>>> in something like points, then do a rotation, and then apply one of
>>> the transformation offsets to place your arrow at an x,y location.
>>> Note there is a bug in some version of matplotlib in the affine code
>>> which is fixes in SVN -- this arrow should have it's base at 0.5, 0.5
>>> and be pointing NW and measure 2 inches from base to tip.  The arrow
>>> size is independent of zoom and figure window size, which may or may
>>> not be desirable....
> 
> Ok, I've done a bunch of work on making quiver work without distorting 
> the arrows.  I have a lot of questions.  I've been playing with the 
> transforms functions all weekend and I still don't understand the 
> different transforms between figure, axes, and absolute space.  Or how 
> to get access to the 'width' and 'height' Values that are mentioned in 
> the comments of transform.py.
> 
> The arrow size in quiver() should not be independent of zoom; it should 
> scale with the figure and axis size.  Ideally by default quiver() should 
> ensure that the largest arrow doesn't overlap the 'gridbox' of the 
> arrows next to it.  The problem is that in order to ensure the arrow is 
> rotated properly, the rotation must take place in absolute coordinates, 
> but the length scaling should take place in data coordinates.  
> Furthermore, the scaling should be a conditional: it should see if the x 
> or the y axis is most 'in danger' of having an arrow larger than its box 
> in data space, and then scale both the x and y coordinates equally in 
> absolute space to ensure the arrow isn't distorted.  I'm not sure this 
> is even possible with Values; I think it would require at least a 
> greater_than BinOp, but I don't really understand all the transform 
> functionality or how to get access to it.  Is there a way to do this?
> 
> After looking at all the files this weekend, I'm still not even sure 
> what matplotlib considers to be 'first-class' primitives.  Is it 
> patches, lines, etc?  What should a call to quiver() return?  A list of 
> polygons?  It's returning a collection in svn, but I don't think you can 
> set each object in a collection to a different transform.  When should a 
> collection be used?

I think your questions here have two aspects.  The first is, exactly 
what functionality should quiver have?  I have always thought Matlab's 
quiver was ugly and fairly useless for my own purposes, so I don't 
advocate copying it.  Have you written out a docstring or other 
specification of what quiver should do and what its API should be? The 
second part is, what should Axes plotting methods return?  There are two 
styles: contour returns a whole ContourSet object; most (all?) other 
methods return only the Artists (e.g. a Line or a Collection.)  When I 
changed contour to return the ContourSet, and more recently when I did a 
little reorganization of axes.py, John mentioned that it might be 
desirable to make other methods behave similarly, but I don't think 
there has been any more recent discussion, and I have not gotten back to 
it myself.  Returning higher-level objects does fit in with moving more 
complicated functionality out of axes.py, which I would still like to 
do--eventually.

> 
> I don't know if I'm making any sense here; I'm fairly frustrated after a 
> weekend with little progress.  Is there any hidden documentation that 
> could help me?

I made some scratch-paper notes for myself, but never considered my 
understanding complete, and it fades fast.  Parts are covered quite well 
in the transforms.py docstring, but a good overview, clearly laying out 
the various coordinate systems (with a good naming convention) and where 
and how transformations are made would be a big help.  This is something 
I might be able to work on, but it might not be very efficient for me to 
  do it.  And I might not be able to do it fast enough to help with your 
present quivering.

Eric