Re: [matplotlib-devel] "Flash" rendering in the Agg backend

[Michael D. <md...@st...>]

I'm trying to compile your examples, but it seems perhaps you forget to 
include a file -- pixel_formats.hpp?  It's not in the agg24 source tree.

Mike

On 03/06/2013 12:06 PM, Phil Elson wrote:
> Smart rendering of adjacent, anti-aliased patches is a question which 
> has come up a couple of times in various guises in the past.
> It is my understanding that the lack of this functionality led us to 
> disable anti-aliasing for contouring and is the reason the following 
> image has a white stripe around the circle where there should be just 
> a nice blend of the two colors:
>
>
> import matplotlib.pyplot as plt
> import numpy as np
> import matplotlib.patches as mpatches
> import matplotlib.path as mpath
> import matplotlib.collections as mcol
>
>
> # create two paths. One a circle, the other
> # a square with the same circle cut out.
> x = np.linspace(0, np.pi * 2, 1000)
>
> circle_coords = np.array(zip(*[np.sin(x) * 0.8, np.cos(x) * 0.8]))
> pth_circle = mpath.Path(circle_coords)
>
> sqr_codes = np.repeat(mpath.Path.MOVETO, len(circle_coords) + 5)
> sqr_codes[1:5] = mpath.Path.LINETO
> sqr_codes[6:] = mpath.Path.LINETO
> sqr_coords = np.concatenate([[[-1, -1], [-1, 1], [1, 1], [1, -1], [-1, 
> -1]],
> circle_coords[::-1]], axis=0)
> sqr_path = mpath.Path(sqr_coords, sqr_codes)
>
>
> ax = plt.axes()
> patches = [mpatches.PathPatch(pth_circle), mpatches.PathPatch(sqr_path)]
> col = mcol.PatchCollection(patches,
> antialiaseds=True,
> edgecolors='none',
> facecolors=[(0, 0.0, 0.0, 0.9), (0.1, 0.1, 0.02, 0.9)])
> ax.add_collection(col)
> ax.set_xlim([-1, 1])
> ax.set_ylim([-1, 1])
> plt.show()
>
>
>
> I know of lots of the workarounds for this (turn off AA, turn on 
> lines, extend the path slightly, set a dark background color) all of 
> which have down-sides, so I'm keen to find a final solution to the 
> problem.
>
> When the two patches marry up perfectly with full anti-aliasing, the 
> antigrain (AGG) community call this "flash" or compound rendering, and 
> this capability was added to Agg 2.4 (which we already ship with mpl).
>
> In order to make full use of the compound rendering technique I 
> believe the drawing pipeline in "_backend_agg.cpp" would need to 
> change, which could be problematic. A less wide-impacting alternative 
> would be to draw all "patches" of a single Collection in the same 
> rasterization step (i.e. just change _draw_path_collection_generic), 
> though this does mean that, as it stands, the result of plt.contourf 
> would not be able to make use of this new functionality - a MEP which 
> changes the return type of plt.contourf to a single Collection might 
> be able to fix that.
>
> I've put together a simple example similar to this in C++ using agg 
> (no mpl changes yet), showing the differences in the code needed 
> between the old technique vs the "new" compound renderer (attached).
>
>
> Ok, so the question to those that have knowledge of the 
> _backend_agg.cpp code (Mike, Eric, JJ + others?):
>
>   * Have you already looked at doing this and determined that this is
>     a non-starter?
>   * Do you support adding the ability for the agg backend to draw
>     compound artists (i.e. Collections) in this way rather than
>     treating them as individual primitives in the canvas?
>   * Since many of the other backends can't do flash rendering, would
>     we even want to make this change?
>       o SVG in Firefox 10.0.2 has the same problem, it is discussed
>         slightly more in
>         http://www.svgopen.org/2002/papers/sorotokin__svg_secrets/
>       o Acroread has the same problem with PDFs, only to a much lesser
>         extent than in the PNG attached
>
>
> Thoughts?
>
>
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>
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