[Matplotlib-users] contourf looking ugly with transparent colors

[Remo Goetschi]

Hi,

Thanks for your response. Hm, are you saying there is probably no way to
work around this?
Are there other opinions?

Personally, I consider this a bug (or two bugs, since pcolormesh() has a
problem as well). It looks really ugly if you, e.g., use
semi-transparent plots as map overlays.

Cheers,
Remo


On 11.11.2015 15:53, Jens Nielsen wrote:
> The issue you are seeing is slightly different from the one the docs
> mention.
> I wrote the docs suggesting the work around and this is mainly relevant
> for vector backends (PDF and so on) The problem with PDF viewers is that
> many of them create visible gaps when two polygons are rendered next to
> each other with zero overlap. This is a viewer specific thing but lots
> of viewers suffers from this. 
> This effect is much more visible that the one you see. I think the one
> you see is due to the way the edge between the 2 colours is antialiased
> by the render.
> 
> The work around with adding  edges is only a workaround exactly as you
> remarked because it doesn't work well with non transparent surfaces.
> 
> best
> Jens
> 
> On Wed, 11 Nov 2015 at 13:44 Remo Goetschi <surf at libecciu.ch
> <mailto:surf at libecciu.ch>> wrote:
> 
>     Hi,
> 
>     Does somebody know how to produce a good-looking filled contour plot
>     with semi-transparent colors? If contourf() is passed a colormap with
>     semi-transparent colors, it produces small gaps between the filled
>     areas:
>     http://i.stack.imgur.com/eEQXI.png
> 
>     According to the docs, this is not a bug ("contourf() [...] does not
>     draw the polygon edges"). To draw the edges, it is suggested to "add
>     line contours with calls to contour()". But that doesn't look good
>     either as the edges become too opaque:
>     http://i.stack.imgur.com/s17F9.png
>     You can play with the linewidth argument of contour(), but that doesn't
>     help much. Any ideas?
> 
>     The code that reproduces the problem is attached below (I use the
>     object-oriented API, but the result is the same with pyplot).
> 
>     BTW, pcolormesh() suffers from a similar problem:
>     http://i.stack.imgur.com/Gbwcb.png
> 
>     Both problems do not seem to occur with the SVG backend.
> 
>     I asked the same question already on stackoverflow. Feel free to respond
>     there:
>     http://stackoverflow.com/questions/33547926/matplotlib-filled-contour-plot-with-transparent-colors
> 
>     Thanks,
>     Remo
> 
>     ---------
>     import matplotlib
>     import numpy as np
>     from matplotlib.figure import Figure
>     from matplotlib.backends.backend_agg import FigureCanvasAgg
> 
>     # generate some data
>     shape = (100, 100)
>     x_rng = np.linspace(-1, 1, shape[1])
>     y_rng = np.linspace(-1, 1, shape[0])
>     x, y = np.meshgrid(x_rng, y_rng)
>     z = np.sqrt(x**2 + y**2)
> 
>     # create figure
>     width_inch, height_inch = 5, 5  # results in 500x500px with dpi=100
>     fig = Figure()
>     fig.set_size_inches((width_inch, height_inch))
>     FigureCanvasAgg(fig)
>     ax = fig.add_axes([0., 0., 1., 1.])
>     ax.set_axis_off()
> 
>     # define some colors with alpha < 1
>     alpha = 0.9
>     colors = [
>         (0.1, 0.1, 0.5, alpha),  # dark blue
>         (0.0, 0.7, 0.3, alpha),  # green
>         (0.9, 0.2, 0.7, alpha),  # pink
>         (0.0, 0.0, 0.0, alpha),  # black
>         (0.1, 0.7, 0.7, alpha),  # light blue
>     ]
>     cmap = matplotlib.colors.ListedColormap(colors)
>     levels = np.array(np.linspace(0, z.max(), len(colors)))
>     norm = matplotlib.colors.BoundaryNorm(levels, ncolors=cmap.N)
> 
>     # contourf plot produces small gaps between filled areas
>     cnt = ax.contourf(x, y, z, levels, cmap=cmap, norm=norm,
>                       antialiased=True, linecolor='none')
> 
>     # this fills the gaps, but it makes them too opaque
>     # ax.contour(x, y, z, levels, cmap=cmap, norm=norm,
>     #            antialiased=True)
> 
>     # the same is true for this trick:
>     # for c in cnt.collections:
>     #     c.set_edgecolor("face")
> 
>     filename = "/tmp/contourf.png"
>     fig.savefig(filename, dpi=100, transparent=True, format="png")
>     print("Saved plot to {}.".format(filename))
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