Re: [igraph] Plotting graph on cairo surface in Python

[Tamás Nepusz]

Hi Stephen,

The first catch is that igraph.plot() _constructs_ the plot itself (i.e. it 
returns a Plot instance) but it does not draw it yet on the surface you 
specified. This is probably an oversight on my side because igraph.plot() would 
draw the graph if the target you specify is a string (i.e. a filename) or None 
(meaning to plot the graph into a temporary file and show that). The workaround 
for the time being is to do this:

plot = igraph.plot(g, surface, bbox=(width, height), layout=layout)
plot.redraw()

The next thing you should do is to ensure that the plot igraph will draw also 
has a transparent background (it is white by default):
 
plot = igraph.plot(g, surface, bbox=(width, height), layout=layout)

plot.background = None
plot.redraw()

This should do the trick. As igraph 0.6 has just been released, I cannot fix 
the bug with igraph.plot in the 0.6 branch, but I will make sure that 0.7 does 
not have this bug.

Best,
Tamas

> I'm using the Python igraph bindings and would like to plot my graph
> with a transparent background. I think I need to use
> igraph.Graph.plot's support for a Cairo surface target. Can anyone
> point me at an example of plotting a graph to a cairo surface, or else
> is there a better way to do this? The below code snippet shows the
> approach I'm trying to get to work. The output image is transparent,
> but there's no graph.
> 
> Thanks for the library. I've gotten a lot of miles out of it on a wide
> variety of network analysis and visualization projects.
> 
> Steve
> 
> import igraph,cairo
> g = igraph.Graph([(0,1), (0,2), (2,3), (3,4), (4,2), (2,5), (5,0),
> (6,3), (5,6)])
> layout = g.layout('kk')
> width,height = 900,600
> surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
> ctx = cairo.Context(surface)
> ctx.scale(width,height)
> ctx.rectangle(0, 0, 1, 1)
> ctx.set_source_rgba(0,0,0,0)
> ctx.fill()
> igraph.plot(g,surface,bbox=(width,height),layout=layout)
> surface.write_to_png('example.png')
> 
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