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;;; exif.el --- parsing Exif data in JPEG images -*- lexical-binding: t -*-
;; Copyright (C) 2019 Free Software Foundation, Inc.
;; Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
;; Keywords: images
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
;;; Commentary:
;; Specification at:
;; https://www.media.mit.edu/pia/Research/deepview/exif.html
;; but it's kinda er not very easy to read.
;; The JPEG format is:
;;
;; FFD8 and then any number of chunks on the format: FFxx SSSS ...,
;; where FFxx is the ID, and SSSS is the length of the chunk plus 2.
;; When you get to ID FFDA, the image itself is over and you can stop
;; parsing.
;;
;; The Exif data is in the TIFF format. It starts off with the six
;; bytes "Exif^0^0".
;;
;; Then either "II" or "MM", where "II" means little-endian and "MM"
;; means big-endian. All subsequent numbers should be read in
;; according to this.
;;
;; Next follows two bytes that should always represent 0x2a, and then
;; four bytes that's the offset to where the IFD "image file
;; directory" starts. (It's an offset from the start of this chunk;
;; i.e., where "II"/"MM" is; all offsets in the TIFF format are from
;; this point.)
;;
;; The IFD starts with two bytes that says how many entries there are
;; in the directory, and then that number of entries follows, and then
;; an offset to the next IFD.
;; Usage: (exif-parse "test.jpg") =>
;; ((:tag 274 :tag-name orientation :format 3 :format-type short :value 1)
;; (:tag 282 :tag-name x-resolution :format 5 :format-type rational :value
;; (180 . 1))
;; (:tag 306 :tag-name date-time :format 2 :format-type ascii
;; :value "2019:09:21 16:22:13")
;; ...)
;;; Code:
(require 'cl-lib)
(defvar exif-tag-alist
'((11 processing-software)
(271 make)
(272 model)
(274 orientation)
(282 x-resolution)
(283 y-resolution)
(296 resolution-unit)
(305 software)
(306 date-time))
"Alist of tag values and their names.")
(defun exif-parse (file)
"Parse FILE (a JPEG file) and return the Exif data, if any.
The return value is a list of Exif items."
(when-let ((app1 (cdr (assq #xffe1 (exif--parse-jpeg file)))))
(exif--parse-exif-chunk app1)))
(defun exif--parse-jpeg (file)
(with-temp-buffer
(set-buffer-multibyte nil)
(insert-file-contents-literally file)
(unless (= (exif--read-number-be 2) #xffd8) ; SOI (start of image)
(error "Not a valid JPEG file"))
(cl-loop for segment = (exif--read-number-be 2)
for size = (exif--read-number-be 2)
;; Stop parsing when we get to SOS (start of stream);
;; this is when the image itself starts, and there will
;; be no more chunks of interest after that.
while (not (= segment #xffda))
collect (cons segment (exif--read-chunk (- size 2))))))
(defun exif--parse-exif-chunk (data)
(with-temp-buffer
(set-buffer-multibyte nil)
(insert data)
(goto-char (point-min))
;; The Exif data is in the APP1 JPEG chunk and starts with
;; "Exif\0\0".
(unless (equal (exif--read-chunk 6) (string ?E ?x ?i ?f ?\0 ?\0))
(error "Not a valid Exif chunk"))
(delete-region (point-min) (point))
(let* ((endian-marker (exif--read-chunk 2))
(le (cond
;; "Motorola" is big-endian.
((equal endian-marker "MM")
nil)
;; "Intel" is little-endian.
((equal endian-marker "II")
t)
(t
(error "Invalid endian-ness %s" endian-marker)))))
;; Another magical number.
(unless (= (exif--read-number 2 le) #x002a)
(error "Invalid TIFF header length"))
(let ((offset (exif--read-number 2 le)))
;; Jump to where the IFD (directory) starts and parse it.
(goto-char (1+ offset))
(exif--parse-directory le)))))
(defun exif--field-format (number)
(cl-case number
(1 (cons 'byte 1))
(2 (cons 'ascii 1))
(3 (cons 'short 2))
(4 (cons 'long 4))
(5 (cons 'rational 8))
(otherwise (cons 'unknown 1))))
(defun exif--parse-directory (le)
(let ((dir
(cl-loop repeat (exif--read-number 2 le)
for tag = (exif--read-number 2 le)
for format = (exif--read-number 2 le)
for field-format = (exif--field-format format)
;; The actual length is the number in this field
;; times the "inherent" length of the field format
;; (i.e., "long integer" (4 bytes) or "ascii" (1
;; byte)).
for length = (* (exif--read-number 4 le)
(cdr field-format))
for value = (exif--read-number 4 le)
collect (list :tag tag
:tag-name (cadr (assq tag exif-tag-alist))
:format format
:format-type (car field-format)
:value (exif--process-value
(if (> length 4)
;; If the length of the data
;; is more than 4 bytes, then
;; it's actually stored after
;; this directory, and the
;; value here is just the
;; offset to use to find the
;; data.
(buffer-substring
(1+ value) (+ (1+ value) length))
;; The value is stored
;; directly in the directory.
value)
(car field-format)
le)))))
(let ((next (exif--read-number 4 le)))
(if (> next 0)
;; There's more than one directory; if so, jump to it and
;; keep parsing.
(progn
(goto-char (1+ next))
(nconc dir (exif--parse-directory le)))
;; We've reached the end of the directories.
dir))))
(defun exif--process-value (value type le)
"Do type-based post-processing of the value."
(cl-case type
;; Chop off trailing zero byte.
('ascii (substring value 0 (1- (length value))))
('rational (with-temp-buffer
(set-buffer-multibyte nil)
(insert value)
(goto-char (point-min))
(cons (exif--read-number 4 le)
(exif--read-number 4 le))))
(otherwise value)))
(defun exif--read-chunk (bytes)
"Return BYTES octets from the buffer and advance point that much."
(prog1
(buffer-substring (point) (+ (point) bytes))
(forward-char bytes)))
(defun exif--read-number-be (bytes)
"Read BYTES octets from the buffer as a chunk of big-endian bytes.
Advance point to after the read bytes."
(let ((sum 0))
(dotimes (_ bytes)
(setq sum (+ (* sum 256) (following-char)))
(forward-char 1))
sum))
(defun exif--read-number-le (bytes)
"Read BYTES octets from the buffer as a chunk of low-endian bytes.
Advance point to after the read bytes."
(let ((sum 0))
(dotimes (i bytes)
(setq sum (+ (* (following-char) (expt 256 i)) sum))
(forward-char 1))
sum))
(defun exif--read-number (bytes lower-endian)
"Read BYTES octets from the buffer with endianness determined by LOWER-ENDIAN.
Advance point to after the read bytes."
(if lower-endian
(exif--read-number-le bytes)
(exif--read-number-be bytes)))
(provide 'exif)
;;; exif.el ends here
|
