On 2013/03/06 7:06 AM, 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?
Phil, Would this greatly slow down the rendering? Does it work with alpha < 1? I'm initially not enthusiastic about having contourf return a single Collection, but maybe in practice it would not make much difference. The drawback, apart from code brakeage, is that it would remove the ability to pick out a level for additional customization. Could this be handled at a subsequent level, by having the renderer able to treat an arbitrary collection of artists as a group? It seems that contourf is where this "flash" capability would be most important; if it can't be made to work there, I think it might not be worth the trouble to add. Eric ------------------------------------------------------------------------------ Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the endpoint security space. For insight on selecting the right partner to tackle endpoint security challenges, access the full report. http://p.sf.net/sfu/symantec-dev2dev _______________________________________________ Matplotlib-devel mailing list Matplotlib-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-devel
