text_along_path

#!/usr/bin/python3
#+
# Qahirah example--draw text along a path.
#
# This is based on an example from the old “PostScript Language Tutorial
# and Cookbook” (a.k.a. the PostScript Blue Book), with some refinements
# of my own.
#
# Copyright 2015 by Lawrence D'Oliveiro <ldo@geek-central.gen.nz>. This
# script is licensed CC0
# <https://creativecommons.org/publicdomain/zero/1.0/>; do with it
# what you will.
#-

import sys
import os
import math
import qahirah as qah
from qahirah import \
    CAIRO, \
    Colour, \
    Matrix, \
    Vector

def pathtext(g, path, txt, offset) :
    "draws the characters of txt along the specified path in the Context g, using its" \
    " current font and other rendering settings. offset is the initial character placement" \
    " offset from the start of the path."
    path = path.flatten() # ensure all straight-line segments
    curch = 0 # index into txt
    setdist = offset # distance at which to place next char
    pathdist = 0
    for seg in path.segments :
        curpos = None
        ovr = 0
        for pt in tuple(seg.points) + ((), (seg.points[0],))[seg.closed] :
            assert not pt.off
            prevpos = curpos
            curpos = pt.pt
            if prevpos != None :
                delta = curpos - prevpos
                dist = abs(delta) # length of line segment
                if dist != 0 :
                    ds = delta / dist * ovr
                    cp = g.user_to_device(prevpos + ds)
                    pathdist += dist # accumulate length of path
                    while True :
                        if setdist > pathdist :
                            # no more room to place a character
                            ovr = setdist - pathdist
                              # deduct off placement of first char on next line segment
                            break
                        #end if
                        if curch == len(txt) :
                            # no more characters to place
                            break
                        # place another character along this line segment
                        ch = txt[curch] # FIXME: should not split off trailing diacritics
                        curch += 1
                        text_extents = g.text_extents(ch)
                        charbounds = Vector(text_extents.x_advance, text_extents.y_bearing)
                        g.save()
                        g.transform \
                          (
                                Matrix.translate
                                  (
                                    g.device_to_user(cp) + delta * charbounds / 2 / dist
                                  ) # midpoint of character back to character position
                            *
                                Matrix.rotate(delta.angle())
                                  # rotate about midpoint of character
                            *
                                Matrix.translate(- charbounds / 2)
                                  # midpoint of character to origin
                          )
                        g.show_text(ch)
                        cp = g.user_to_device(g.current_point)
                        g.restore()
                        setdist += charbounds.x # update distance travelled along path
                    #end while
                #end if
            #end if
        #end for
    #end for
#end pathtext

#+
# Mainline
#-

pix = qah.ImageSurface.create \
  (
    format = CAIRO.FORMAT_RGB24,
    dimensions = (400, 400)
  )
g = \
    (qah.Context.create(pix)
        .set_source_colour(Colour.grey(1))
        .paint()
        .set_source_colour(Colour.grey(0))
        .set_font_face
          (
            qah.FontFace.create_for_pattern("Helvetica")
          )
        .set_font_size(16)
    )
txt = "If my film makes one more person feel miserable I’ll feel I’ve done my job. — WOODY ALLEN"
path = (qah.Context.create_for_dummy()
    .new_path()
    .arc
      (
        centre = (130, 130),
        radius = 70,
        angle1 = 0 * qah.deg,
        angle2 = 90 * qah.deg,
        negative = True
      )
    .arc
      (
        centre = (240, 130),
        radius = 70,
        angle1 = 90 * qah.deg,
        angle2 = 180 * qah.deg,
        negative = True
      )
    .copy_path()
)

pathtext(g, path, txt, 55)
if False : # debug
    # just to compare placement of text with original path
    (g
        .append_path(path)
        .stroke()
        .move_to((-130, 330))
        .show_text(txt)
    )
#end if

(g
    .new_path()
    .move_to((80, 320))
    .line_to((290, 320))
    .line_to((290, 230))
    .line_to((80, 230))
    .close_path()
    .move_to((290, 283))
    .line_to((340, 300))
    .line_to((340, 250))
    .line_to((290, 267))
    .set_line_width(2)
    .stroke()
)

pix.flush().write_to_png("{}.png".format(os.path.basename(sys.argv[0])))