「コマ大数学科」GIFアニメ作成

cairo_sample1.rb

require 'cairo'
require_relative 'gifanime'
require 'matrix'
include Math

Dir.chdir("picture")    #作業用ディレクトリ

Width = 300
Surface = Cairo::ImageSurface.new(Cairo::FORMAT_ARGB32, Width, Width)
C = Cairo::Context.new(Surface)

class Vector
  def scrn() [Width / 2 + self[0], Width / 2 - self[1]] end
end

R = Width / 2 - 20
Steps = 200.0
agl_step = 2 * PI / Steps
rad_step = R * 2 / Steps

pa = ->(n) {Vector[cos(agl_step * n), sin(agl_step * n)] * R}
pb = ->(n) {Vector[cos(PI + agl_step * n), sin(PI + agl_step * n)] * R}
pp = ->(n) {
  a, b = pa.(n), pb.(n)
  a + (b - a) / (2 * R) * rad_step * n
}

(Steps.to_i + 1).times do |i|
  #背景色
  C.set_source_color(Cairo::Color.parse("#ffe7d6"))
  C.rectangle(0, 0, Width, Width)
  C.fill
  
  #円
  C.set_source_rgb(1, 1, 1)
  C.arc(Width / 2, Width / 2, R, 0, 2 * PI)
  C.stroke
  
  #線分AB
  a, b = pa.(i).scrn, pb.(i).scrn
  C.move_to(*a)
  C.line_to(*b)
  C.stroke
  
  #点A
  C.set_source_rgb(1, 0, 0)
  C.arc(*a, 3, 0, 2 * PI)
  C.fill
  
  #点B
  C.set_source_rgb(1, 0, 0)
  C.arc(*b, 3, 0, 2 * PI)
  C.fill
  
  #点P
  C.set_source_rgb(0, 0.5, 0)
  (0..i).each do |j|
    C.arc(*pp.(j).scrn, 3, 0, 2 * PI)
    C.fill
  end
  
  Surface.write_to_png("%04d.png" % i)
end

40.times {|j| Surface.write_to_png("%04d.png" % (Steps.to_i + j + 1))}    #2秒のwait
gifanime(5)    #50ミリ秒間隔でGIF画像生成

# "gifanime.rb" -- http://obelisk.hatenablog.com/entry/2016/05/26/170134
# ブログ記事 -- https://marginalia.hatenablog.com/entry/2018/10/17/000843#20171003_3