Re: [PyX-user] Gradient according to external data file

[André W. <wo...@us...>]

Am 27.10.2010 um 21:10 schrieb Stefan Schenk:
> Am Mittwoch, 27. Oktober 2010, 16:34:56 schrieb Mico Filós:
>> I have an array containing N points (x, y, r), where x is an
>> abscissa, y is an ordinate, and r is a real between 0 and 1. I would
>> like to plot the the array {(x, y)} as a continuous path, using the
>> sequence of r's as the normalized gray value at each particular
>> point.  This is not hard if data points are plotted with symbols,
>> but I want to use the line style for data points, taking the
>> sequence of N r's as the (externally defined) color gradient  for
>> the curve defined by {(x, y)}.
>> It sounds more complicated than it really is. Any ideas?
> 
> I don't see an easy way to do this. What makes this even more complex is 
> that with lines your third dimension (r) is not well defined. For points 
> you can always assign a color to the point, but a line lives between two 
> points. What color do you assign to the line?
> 
> Of course there are complicated ways to do something like this:
> The most straightforward way I can think of at the moment is to create 
> the paths between the points by hand, then splitting these paths and 
> then drawing each of these subpaths with colors according to the colors 
> at your points.


You are right, Stefan. I'm not aware of a possibility to alter the color of a line either. However, we can go along the line of splitting the each line manually and alter the color as needed. And, this is PyX, where you can introduce such a graph style by just a couple of lines. Here we go:

--------------------

from pyx import *

class gradientline(graph.style.line):

    def __init__(self, colorcolumnname="color", colorprecision=100, gradient=color.gradient.Gray, **kwargs):
        self.colorcolumnname = colorcolumnname
        self.colorprecision = colorprecision
        self.gradient = gradient
        graph.style.line.__init__(self, **kwargs)

    def columnnames(self, privatedata, sharedata, agraph, columnnames):
        if self.colorcolumnname not in columnnames:
            raise ValueError("column '%s' missing" % self.colorcolumnname)
        return ([self.colorcolumnname] +
                graph.style.line.columnnames(self, privatedata,
                                             sharedata, agraph, columnnames))

    def initdrawpoints(self, privatedata, sharedata, graph):
        privatedata.color = None

    def drawpoint(self, privatedata, sharedata, graph, point):
        if privatedata.color is not None:
            self.addpoint(privatedata, graph.vpos_pt, sharedata.vposavailable, sharedata.vposvalid, sharedata.vpos)
            path = self.donepointstopath(privatedata)
            if privatedata.lineattrs is not None and len(path):
                splits = int(abs(point[self.colorcolumnname] - privatedata.color)*self.colorprecision)
                if splits:
                    path = path.normpath()
                    arclen_pt = path.arclen_pt()
                    segments = path.split_pt([(i+1)*arclen_pt/(splits+1) for i in range(splits)])
                    for i, segment in enumerate(segments):
                        graph.stroke(segment, privatedata.lineattrs + [self.gradient.getcolor(privatedata.color + (point[self.colorcolumnname]-privatedata.color)*(i+0.5)/splits)])
                else:
                    graph.stroke(path, privatedata.lineattrs + [self.gradient.getcolor(point[self.colorcolumnname])])
        self.initpointstopath(privatedata)
        self.addpoint(privatedata, graph.vpos_pt, sharedata.vposavailable, sharedata.vposvalid, sharedata.vpos)
        privatedata.color = point[self.colorcolumnname]

    def donedrawpoints(self, privatedata, sharedata, graph):
        pass

g = graph.graphxy(width=10)
g.plot(graph.data.points([[random.random() for i in range(3)]
                          for i in range(10)],
                         x=1, y=2, color=3),
       [gradientline(gradient=color.gradient.Rainbow)])
g.writePDFfile()

--------------------


André

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