#!/usr/bin/env gimp-script-fu-interpreter-3.0
; Works best if the following preference is set 1:1
; Edit->Preferences->Image Windows->initial zoom ratio 1:1
; Otherwise ensure image is set to 100% zoom, keyboard "1"

(define debug #f)

(define (script-fu-paper-scale img drawables)
  (let*
    (
      (placeOnPaper #t)
      (dpiX (car(gimp-image-get-resolution img))) 
      (dpiY (cadr(gimp-image-get-resolution img)))
      (scaleX (/ (car(gimp-get-monitor-resolution)) dpiX))
      (scaleY (/ (cadr(gimp-get-monitor-resolution)) dpiY))
      (paperX (* scaleX (* 2480 (/ dpiX 300)))); A4 at 300DPI
      (paperY (* scaleY (* 3508 (/ dpiY 300)))); A4 at 300DPI
      (dstImg 0)(dstL 0)
      (origW (car (gimp-image-get-width img)))
      (width (* scaleX origW))
      (origH (car (gimp-image-get-height img)))
      (height (* scaleY origH))
    )

    (gimp-context-push)
    (gimp-edit-copy-visible img)
    (set! dstImg (car(gimp-edit-paste-as-new-image)))

    (gimp-image-scale dstImg width height)

    (when placeOnPaper
      (gimp-image-resize dstImg paperX paperY (/ (- paperX width) 2)
                                            (/ (- paperY height) 4)
      )
      (gimp-context-set-background (list 225 225 225))
      (gimp-image-flatten dstImg)
    )
    (set! dstL (vector-ref (cadr(gimp-image-get-selected-layers dstImg))0))
    (gimp-item-set-name dstL "paper-scaled")
    (gimp-layer-set-composite-space dstL LAYER-COLOR-SPACE-RGB-PERCEPTUAL)

    (gimp-display-new dstImg)

    (gimp-image-set-file dstImg (string-append "*** Display DPI is "
                                        (number->string (trunc 
                                        (car(gimp-get-monitor-resolution))))
                                        ":  If printed at " 
                                        (number->string (trunc dpiX)) " DPI"
                                        " and if the display zoom is at 100% "
                                        " the image should look this size on "
                                        " paper ***   " ".xcf"
                                )
    )

    (gimp-image-clean-all dstImg)
    (gimp-context-pop)

  )
)


(script-fu-register-filter "script-fu-paper-scale"
 "Paper Scale" 
 "Creates a new image showing how big the printed image would look on paper"
 "Mark Sweeney"
 "Under GNU GENERAL PUBLIC LICENSE Version 3"
 "2023"
 "*"
 SF-ONE-OR-MORE-DRAWABLE
)
(script-fu-menu-register "script-fu-paper-scale" "<Image>/Image")

; copyright 2023, Mark Sweeney, Under GNU GENERAL PUBLIC LICENSE Version 3

; utility functions
(define (boolean->string bool) (if bool "#t" "#f"))

(define (exit msg)
  (gimp-message-set-handler 0)
  (gimp-message (string-append " >>> " msg " <<<"))
  (gimp-message-set-handler 2)
  (quit)
)

(define (here x)(gimp-message(string-append " >>> " (number->string x) " <<<")))