text_line.py

from matplotlib import pyplot as plt
from matplotlib import patches
from matplotlib import text as mtext
import numpy as np
import math

class CurvedText(mtext.Text):
    """
    A text object that follows an arbitrary curve.
    """
    def __init__(self, x, y, text, axes, alpha=1.0, **kwargs):
        super(CurvedText, self).__init__(x[0],y[0],' ', **kwargs)

        axes.add_artist(self)

        ##saving the curve:
        self.__x = x
        self.__y = y
        self.__zorder = self.get_zorder()
        self.__alpha = alpha

        ##creating the text objects
        self.__Characters = []
        for c in text:
            if c == ' ':
                ##make this an invisible 'a':
                t = mtext.Text(0,0,'a')
                t.set_alpha(0.0)
            else:
                t = mtext.Text(0,0,c, **kwargs)

            #resetting unnecessary arguments
            t.set_ha('center')
            t.set_rotation(0)
            t.set_zorder(self.__zorder +1)

            self.__Characters.append((c,t))
            axes.add_artist(t)


    ##overloading some member functions, to assure correct functionality
    ##on update
    def set_zorder(self, zorder):
        super(CurvedText, self).set_zorder(zorder)
        self.__zorder = self.get_zorder()
        for c,t in self.__Characters:
            t.set_zorder(self.__zorder+1)

    def draw(self, renderer, *args, **kwargs):
        """
        Overload of the Text.draw() function. Do not do
        do any drawing, but update the positions and rotation
        angles of self.__Characters.
        """
        self.update_positions(renderer)

    def update_positions(self,renderer):
        """
        Update positions and rotations of the individual text elements.
        """

        #preparations

        ##determining the aspect ratio:
        ##from https://stackoverflow.com/a/42014041/2454357

        ##data limits
        xlim = self.axes.get_xlim()
        ylim = self.axes.get_ylim()
        ## Axis size on figure
        figW, figH = self.axes.get_figure().get_size_inches()
        ## Ratio of display units
        _, _, w, h = self.axes.get_position().bounds
        ##final aspect ratio
        aspect = ((figW * w)/(figH * h))*(ylim[1]-ylim[0])/(xlim[1]-xlim[0])

        #points of the curve in figure coordinates:
        x_fig,y_fig = (
            np.array(l) for l in zip(*self.axes.transData.transform([
            (i,j) for i,j in zip(self.__x,self.__y)
            ]))
        )

        #point distances in figure coordinates
        x_fig_dist = (x_fig[1:]-x_fig[:-1])
        y_fig_dist = (y_fig[1:]-y_fig[:-1])
        r_fig_dist = np.sqrt(x_fig_dist**2+y_fig_dist**2)

        #arc length in figure coordinates
        l_fig = np.insert(np.cumsum(r_fig_dist),0,0)

        #angles in figure coordinates
        rads = np.arctan2((y_fig[1:] - y_fig[:-1]),(x_fig[1:] - x_fig[:-1]))
        degs = np.rad2deg(rads)


        rel_pos = 10
        for c,t in self.__Characters:
            #finding the width of c:
            t.set_rotation(0)
            t.set_va('center')
            bbox1  = t.get_window_extent(renderer=renderer)
            w = bbox1.width
            h = bbox1.height

            #ignore all letters that don't fit:
            if rel_pos+w/2 > l_fig[-1]:
                t.set_alpha(0.0)
                rel_pos += w
                continue

            elif c != ' ':
                t.set_alpha(self.__alpha)

            #finding the two data points between which the horizontal
            #center point of the character will be situated
            #left and right indices:
            il = np.where(rel_pos+w/2 >= l_fig)[0][-1]
            ir = np.where(rel_pos+w/2 <= l_fig)[0][0]

            #if we exactly hit a data point:
            if ir == il:
                ir += 1

            #how much of the letter width was needed to find il:
            used = l_fig[il]-rel_pos
            rel_pos = l_fig[il]

            #relative distance between il and ir where the center
            #of the character will be
            fraction = (w/2-used)/r_fig_dist[il]

            ##setting the character position in data coordinates:
            ##interpolate between the two points:
            x = self.__x[il]+fraction*(self.__x[ir]-self.__x[il])
            y = self.__y[il]+fraction*(self.__y[ir]-self.__y[il])

            #getting the offset when setting correct vertical alignment
            #in data coordinates
            t.set_va(self.get_va())
            bbox2  = t.get_window_extent(renderer=renderer)

            bbox1d = self.axes.transData.inverted().transform(bbox1)
            bbox2d = self.axes.transData.inverted().transform(bbox2)
            dr = np.array(bbox2d[0]-bbox1d[0])

            #the rotation/stretch matrix
            rad = rads[il]
            rot_mat = np.array([
                [math.cos(rad), math.sin(rad)*aspect],
                [-math.sin(rad)/aspect, math.cos(rad)]
            ])

            ##computing the offset vector of the rotated character
            drp = np.dot(dr,rot_mat)

            #setting final position and rotation:
            t.set_position(np.array([x,y])+drp)
            t.set_rotation(degs[il])

            t.set_va('center')
            t.set_ha('center')

            #updating rel_pos to right edge of character
            rel_pos += w-used




if __name__ == '__main__':
    Figure, Axes = plt.subplots(2,2, figsize=(7,7), dpi=100)


    N = 100

    curves = [
        [
            np.linspace(0,1,N),
            np.linspace(0,1,N),
        ],
        [
            np.linspace(0,2*np.pi,N),
            np.sin(np.linspace(0,2*np.pi,N)),
        ],
        [
            -np.cos(np.linspace(0,2*np.pi,N)),
            np.sin(np.linspace(0,2*np.pi,N)),
        ],
        [
            np.cos(np.linspace(0,2*np.pi,N)),
            np.sin(np.linspace(0,2*np.pi,N)),
        ],
    ]

    texts = [
        'straight lines work the same as rotated text',
        'wavy curves work well on the convex side',
        'you even can annotate parametric curves',
        'changing the plotting direction also changes text orientation',
    ]

    for ax, curve, text in zip(Axes.reshape(-1), curves, texts):
        #plotting the curve
        ax.plot(*curve, color='b')

        #adjusting plot limits
        stretch = 0.2
        xlim = ax.get_xlim()
        w = xlim[1] - xlim[0]
        ax.set_xlim([xlim[0]-stretch*w, xlim[1]+stretch*w])
        ylim = ax.get_ylim()
        h = ylim[1] - ylim[0]
        ax.set_ylim([ylim[0]-stretch*h, ylim[1]+stretch*h])

        #adding the text
        text = CurvedText(
            x = curve[0],
            y = curve[1],
            text=text,#'this this is a very, very long text',
            va = 'bottom',
            axes = ax, ##calls ax.add_artist in __init__
        )

    plt.show()