Thanks for having a second look at this, Eric. I consider mixed-mode
drawing somewhat of an experiment at this point -- it's still an open
question whether it should be included in the next release, and
definitely needs more use cases. It is currently only used in the trunk
by quad meshes. In the common case where the mesh is of sufficient
resolution, the rasterized version results in a smaller, faster file.
However, it probably needs to be exposed as an option to the user, in
the case where the quads are large (or to do it adaptively based on dpi,
but that may be too smart).
John Hunter wrote:
> On Sat, May 3, 2008 at 11:44 PM, Eric Bruning <[EMAIL PROTECTED]> wrote:
>
>
>> The switch to/from raster mode was made in Axes.draw, where the artists for
>> each axes are looped over. In the artist loop, I check if the artist to be
>> rendered is listed in the draw_raster attribute on the renderer instance. If
>> so, the appropriate calls are made to start and stop rasterizing.
>>
>
> Hi Eric, thanks for the patch. There are a couple of aspects of the
> design here that I am not comfortable with, but I think with a few
> changes this will be useful (though Michael, who implemented the mixed
> mode renderer, will surely have important comments). The primary
> thing that bothers me is that one of the core aspects of the
> matplotlib backend design is that the renderers know nothing about
> artists -- artists know about renderers, but not the other way around.
> So I don't like using the renderer to store the rasterized artists.
> It makes more sense to me for the artist to have has a property set
> ("set_rasterized" which could be True|False|None where None means "do
> the default thing for the renderer"). Then you could do:
>
> if a.get_rasterized():
> renderer.start_rasterizing()
> a.draw(renderer)
> renderer.stop_rasterizing()
> else:
> a.draw(renderer)
>
This is where I was (implicitly) going with all this.
> Doing this in the axes.draw method may not be the most natural place
> to do this since it could be done in the artist.draw method, but it
> may be the most expedient. This is an area where having support for
> before_draw and after_draw hooks might be useful. One potential
> problem with either of these approached is it looks like the mixed
> mode renderer is set up to handle multiple rasterized draws before
> dumping the aggregate image into the backend on a stop_renderer, so
> doing the start/stop in any of the approaches above would obviate this
> efficiency.
> The axes could aggregate the rasterized artists before
> rendering and then do them all together, but making this play properly
> with zorder will be tricky.
That's right. To receive significant gains, you would generally want to
perform a number of drawing operations in a single raster buffer. Given
how mpl is currently designed, that generally implies a Collection
(which is fairly easy to wrap start/stop_rasterizing calls around) -- it
doesn't necessarily have to mean a whole bunch of separate Artist
objects (which would be much harder to do given the current way things
are drawn). The latter may be ultimately more optimal, but the former
is an easy win.
My concern with this patch is that it expects the user to know about
artists at all. Sure, there are many advanced techniques where the user
has to know what artists are etc., but for a lot of the pylab-level
usage, that's not a concept many users must be familiar with. I think
it makes more sense to just add a "rasterized" kwarg (and
get/set_rasterized) to calls such as "pcolormesh" so the user can choose
how it is drawn. The "draw_raster" list concept requires users to
create a "set" from something that IMHO is more like a flag on
individual objects. As an obtuse example, it would be like creating a
list of all blue objects, and a list of all red ones, rather than just
setting their colors appropriately.
As for the implementation, Eric's patch does appear to deal with the
z-order problem, (by interleaving rasterized and non-rasterized drawing
correctly base on zorder), but it doesn't combine adjacent rasterized
artists into a single buffer. The drawing loop in Axes.draw could
fairly easily be modified to do that. However, I think a better
solution that doesn't require an explicit "draw_raster" list, is to make
"stop_rasterizing" lazy. At the moment, when "stop_rasterizing" is
called, the buffer is immediately flushed and written out. If instead
it just set a flag, causing the buffer to be flushed when the next
vector object is written, then something like
start_rasterizing()
draw_line()
stop_rastering()
start_rastering()
draw_line()
stop_rasterizing()
draw_line()
would write out a single raster buffer with two lines, followed by a
vector line. Of course, and here is the tricky bit, if the two
rasterized objects are really far apart spatially, you waste a lot of
space on transparent pixels between them. We can let the user decide
whether to rasterize each individually with a nice clean API, but
letting the user decide whether to combine adjacent rasterizations gets
tricky and I think is asking too much of someone who doesn't know the
mpl internals. Perhaps that is an argument against trying to implicitly
combine adjacent rasterized draws -- it's trying to be too smart?
Anyway, somewhere in the midst of all this is the correct path...
Cheers,
Mike
--
Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA
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