text_vert_align

#!/usr/bin/python3
#+
# Test of calculations for vertically-aligning text: comparison of
# alignment based on overall font metrics, versus specific text metrics.
# For the derivation of the formulas used, see the IPython notebook
# “Beginning Text”
# <https://gitlab.com/ldo/qahirah_notebooks/blob/master/Beginning%20Text.ipynb>,
# <https://bitbucket.org/ldo17/qahirah_notebooks/src/master/Beginning%20Text.ipynb>.
#
# 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 qahirah as qah
from qahirah import \
    CAIRO, \
    Colour, \
    Rect, \
    Vector

box_dimensions = Vector(300, 100)
margins = Vector(20, 20)
texts = \
    (
        "gamey", # descenders only
        "ballot", # ascenders only
        "battery", # ascenders and descenders
        "woman", # neither
        "WXM", # full height
    )
bg_colour = Colour.grey(1)
main_colour = Colour.grey(0)
dim_colour = Colour.grey(0.9)
pix = qah.ImageSurface.create \
  (
    format = CAIRO.FORMAT_RGB24,
    dimensions = (box_dimensions + margins) * Vector(2, len(texts)) + margins
  )
ctx = \
    (qah.Context.create(pix)
        .set_operator(CAIRO.OPERATOR_SOURCE)
        .set_source_colour(bg_colour)
        .paint()
        .set_font_size(60)
    )
font_extents = ctx.font_extents
font_mid_y = (font_extents.ascent - font_extents.descent) / 2
for i, text in enumerate(texts) :
    topleft = (box_dimensions + margins) * Vector(0, i) + margins
    ctx.set_source_colour(dim_colour)
    ctx.move_to(topleft + Vector(0, box_dimensions.y / 2))
    ctx.rel_line_to((box_dimensions.x, 0))
    ctx.stroke()
    ctx.set_source_colour(main_colour)
    ctx.move_to(topleft + Vector(0, font_mid_y) + Vector(0, box_dimensions.y / 2))
    ctx.show_text(text)
    ctx.rectangle(Rect.from_dimensions(box_dimensions) + topleft)
    ctx.stroke()
    topleft = (box_dimensions + margins) * Vector(1, i) + margins
    ctx.set_source_colour(dim_colour)
    ctx.move_to(topleft + Vector(0, box_dimensions.y / 2))
    ctx.rel_line_to((box_dimensions.x, 0))
    ctx.stroke()
    ctx.set_source_colour(main_colour)
    text_extents = ctx.text_extents(text)
    text_mid_y = (- 2 * text_extents.y_bearing - text_extents.height) / 2
    ctx.move_to(topleft + Vector(0, text_mid_y) + Vector(0, box_dimensions.y / 2))
    ctx.show_text(text)
    ctx.rectangle(Rect.from_dimensions(box_dimensions) + topleft)
    ctx.stroke()
#end for

pix \
    .flush() \
    .write_to_png("%s.png" % os.path.basename(sys.argv[0]))