get-dpi.el

(require 'cl-lib)

(defun get-dpi ()
  (or (get-dpi-connected) 0))

(defun get-dpi-connected ()
  (with-temp-buffer
    
    (insert (shell-command-to-string "xrandr"))
    (keep-lines " connected" (point-min) (point-max))
    (when (< 0 (cl-first (wc-count)))
      (gd--compute-dpi))))

;; (defun gd--compute-dpi ()
;;   (let* ((line (buffer-substring-no-properties (point-min)
;;                                                (point-max)))
;;          (words (split-string line " "))
;;          (n (max (or (cl-position "connected" words :test #'equal) 0)
;;                  (or (cl-position "primary" words :test #'equal) 0)))
;;          (sz (nth (1+ n) words))
;;          (sx (first (split-string sz "[x+]" )))
;;          (sy (second (split-string sz "[x+]" )))
;;          (lx (caddr (reverse words)))
;;          (ly (first (reverse words))))
;;     (pcase-let ((`(,sx ,sy ,lx ,ly) (mapcar #'string-to-number 
;;                                             `(,sx ,sy ,lx ,ly))))
;;       (* 0.5 25.4 (+ (/ sx lx 1.0) (/ sy ly 1.0))))
;;     ))

(defun gd--compute-dpi ()
  (pcase-let
      ((`(x0 y0 ,W ,H) (frame-monitor-attribute 'geometry))
       (`(,Wmm ,Hmm) (frame-monitor-attribute 'mm-size)))
    (* 0.5 25.4
       (+ (/ W Wmm 1.0)
          (/ H Hmm 1.0)))))

(defun insert-dpi ()
  (interactive)
  (insert (number-to-string (get-dpi))))

(provide 'get-dpi)