[FEATURE] Chords

[ggerman]

What problem are you trying to solve?

Draw chord just calling chord("C"), chord("c"), chord("F") and options on the fret.

How would you like the API to look?

require "gd"

img = GD::Image.new(520, 640)

white = GD::Color.rgb(255,255,255)
black = GD::Color.rgb(0,0,0)
gray  = GD::Color.rgb(80,80,80)

img.fill(white)

left   = 60
right  = 460
top    = 140
bottom = 580

strings = 6
frets   = 5

string_gap = (right - left) / (strings - 1).to_f
fret_gap   = (bottom - top) / frets.to_f

# ----------------------
# Title
# ----------------------
img.text("C", {
                x: 230,
                y: 50,
                size: 40,
                color: black,
                font: "./DejaVuSans.ttf"
                })
# ----------------------
# Strings (vertical)
# ----------------------
strings.times do |i|
  x = (left + i * string_gap).to_i
  img.line(x, top, x, bottom, gray)
end

# ----------------------
# Frets (horizontal)
# ----------------------
(0..frets).each do |f|
  y = (top + f * fret_gap).to_i
  if f == 0
    img.filled_rectangle(left - 3, y - 3, right + 3, y + 3, black)
  else
    img.line(left, y, right, y, gray)
  end
end

# ----------------------
# X / O labels  x32010
# ----------------------
labels = ["X", "3", "2", "0", "1", "0"]

labels.each_with_index do |lab, i|
  x = (left + i * string_gap).to_i - 6
  img.text(lab, {
                    x: x,
                    y: top - 55,
                    size: 30,
                    color: black,
                    font: "./DejaVuSans.ttf"
                })  if ["X", "0"].include?(lab)
end

# ----------------------
# Finger dots
# ----------------------
dots = [
  { string: 1, fret: 3 }, # A string
  { string: 2, fret: 2 }, # D
  { string: 4, fret: 1 }  # B
]

dots.each do |d|
  x = (left + d[:string] * string_gap).to_i
  y = (top + (d[:fret] - 0.5) * fret_gap).to_i
  img.filled_ellipse(x, y, 28, 28, black)
end

# ----------------------
# Fret numbers
# ----------------------
(1..frets).each do |f|
  y = (top + (f - 0.5) * fret_gap).to_i - 6
  img.text(f.to_s, {
                    x: left - 35,
                    y: y,
                    size: 20,
                    color: black,
                    font: "./DejaVuSans.ttf"
                })
end

img.save("chord_C.png")

Use case