RMagick - autoscale text within a box, like iOS UILabel

annotate_box.rb

class Magick::Draw
  # Autoscale text within a box. The text will be scaled to fit if the
  # specified pointsize is too big. If minscale is set and the text still
  # won't fit at minscale * pointsize, it gets ellipsized. halign/valign can
  # be used to set horizontal and vertical alignment. This is similar to the
  # behavior of iOS UILabel.
  def annotate_box(image:, text:, x:, y:, w:, h:, pointsize:, minscale: nil, halign: 0, valign: 0)
    draw = self.clone
    draw.pointsize = pointsize
    draw.gravity = Magick::NorthWestGravity

    metrics = draw.get_type_metrics(text)
    tw, th = metrics[:width], metrics[:height]

    #
    # does the text exceed the box? scale/ellipsize if necessary
    #

    if tw > w || th > h
      scale = [ w / tw, h / th ].min
      if !minscale || scale > minscale
        # the new scale is valid
        draw.pointsize = pointsize * scale
      else
        # ellipsize at minscale - how much can we fit?
        draw.pointsize = pointsize * minscale
        truncated = ""
        0.upto(text.length) do |len|
          might_fit = "#{text[0, len].strip}…"
          break if draw.get_type_metrics(might_fit)[:width] > w
          truncated = might_fit
        end
        text = truncated
      end

      metrics = draw.get_type_metrics(text)
      tw, th = metrics[:width], metrics[:height]
    end

    #
    # alignment
    #

    dx = case halign
    when -1 then 0
    when 1 then w - tw
    else (w - tw) / 2
    end

    dy = case valign
    when -1 then 0
    when 1 then h - th
    else (h - th) / 2
    end

    draw.annotate(image, 0, 0, x + dx, y + dy, text)
  end
end