Fitting a string into a predetermined width

scratch.lisp

;;;; scratch.lisp

(defpackage #:scratch
  (:use #:cl #:vecto)
  (:import-from #:zpb-ttf
                #:xmin
                #:xmax))

(in-package #:scratch)

(defun target-size (string loader target-width)
  (let* ((bbox (string-bounding-box string 100 loader))
         (width (- (xmax bbox) (xmin bbox))))
    (* 100 (/ target-width width))))

(defun example (&key (width 640) (height 480)
                (font-file #p "~/times.ttf")
                output-file string)
  (with-canvas (:width width :height height)
    (let* ((loader (get-font font-file))
           (target-size (target-size string loader (* width 0.75))))
      (set-rgb-fill 0 0 0)
      (clear-canvas)
      (set-rgb-fill 1 1 1)
      (set-font loader target-size)
      (draw-centered-string (/ width 2) (/ height 2) string)
      (save-png output-file))))