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%! viewjpeg.ps   Copyright (C) 1994 Thomas Merz <tm@pdflib.com>
%
% This software is provided AS-IS with no warranty, either express or
% implied.
%
% This software is distributed under license and may not be copied,
% modified or distributed except as expressly authorized under the terms
% of the license contained in the file LICENSE in this distribution.
%
% For more information about licensing, please refer to
% http://www.ghostscript.com/licensing/. For information on
% commercial licensing, go to http://www.artifex.com/licensing/ or
% contact Artifex Software, Inc., 1305 Grant Avenue - Suite 200,
% Novato, CA 94945, U.S.A., +1(415)492-9861.

% View JPEG files with Ghostscript
%
% This PostScript code relies on level 2 features.
%
% Only JPEG baseline, extended sequential, and progressive files
% are supported.  Note that Adobe PostScript level 2 does not include
% progressive-JPEG support.  Ghostscript with IJG JPEG v6 or later
% will decode progressive JPEG, but only if you edit gsjmorec.h to
% enable that feature.
%
% Author's address:
% ------------------------------+
% {(pstack exec quit) = flush } |    Thomas Merz, Munich
% pstack exec quit              |    voice +49/89/29160728
% ------------------------------+    tm@muc.de  http://www.muc.de/~tm/
%
% Updated by L. Peter Deutsch 20-May-1997:
%   move the usage example to the beginning
% Updates by Tom Lane 6-Sep-1995

% Usage example:
%	(jpeg-6/testimg.jpg) viewJPEG

/languagelevel where {pop languagelevel 2 lt}{true} ifelse {
  (JPEG needs PostScript Level 2!\n) print flush stop
} if

/JPEGdict 20 dict def
JPEGdict begin

/NoParamMarkers [	% JPEG markers without additional parameters
    16#D0 16#D1 16#D2 16#D3 16#D4 16#D5 16#D6 16#D7 16#D8 16#01
] def

/NotSupportedMarkers [ 	% JPEG markers not supported by PostScript level 2
    16#C3 16#C5 16#C6 16#C7 16#C8 16#C9 16#CA 16#CB 16#CD 16#CE 16#CF
] def

% Names of color spaces
/ColorSpaceNames << /1 /DeviceGray /3 /DeviceRGB /4 /DeviceCMYK >> def

% read one byte from file F
% - ==> int --or-- stop context
/NextByte {
    F read not { (Read error in ViewJPEG!\n) print flush stop } if
} bind def

/SkipSegment {	% read two bytes and skip that much data
    NextByte 8 bitshift NextByte add 2 sub { NextByte pop } repeat
} bind def

% read width, height, and # of components from JPEG markers
% and store in dict
/readJPEGmarkers {	% - ==> dict --or-- stop context
    5 dict begin

{ % loop: read JPEG marker segments until we find SOFn marker or EOF
        NextByte
        16#FF eq {				% found marker
            /markertype NextByte def
            % Is it S0F0=baseline, SOF1=extended sequential, SOF2=progressive ?
            markertype dup 16#C0 ge exch 16#C2 le and {
                NextByte pop NextByte pop	% segment length
                % Ghostscript and Adobe PS accept only data precision 8
                NextByte 8 ne {
                    (Error: not 8 bits per component!\n) print flush stop
                } if

% Read crucial image parameters
                /height NextByte 8 bitshift NextByte add def
                /width NextByte 8 bitshift NextByte add def
                /colors NextByte def

VJPGDEBUG { currentdict { exch == == } forall flush } if
                exit
            } if

% detect several segment types which are not compatible with PS
            NotSupportedMarkers {
                markertype eq {
                    (Marker ) print markertype ==
                    (not supported!\n) print flush stop
                } if
            } forall

% Skip segment if marker has parameters associated with it
            true NoParamMarkers { markertype eq {pop false exit} if } forall
            { SkipSegment } if
        } if
    } loop

currentdict dup /markertype undef
    end
} bind def

end	% JPEGdict

% read image parameters from JPEG file and display the image
/viewJPEG {		% <file|string> ==> -
    save
    JPEGdict begin
    /saved exch def
    /scratch 1 string def
    dup type /stringtype eq { (r) file } if
    /F exch def

readJPEGmarkers begin
    F 0 setfileposition		% reset file pointer

% We use the whole clipping area for the image (at least in one dimension)
    gsave clippath pathbbox grestore
    /ury exch def /urx exch def
    /lly exch def /llx exch def

llx lly translate
    width height scale

% use whole width or height, whichever is appropriate
    urx llx sub width div ury lly sub height div
    2 copy gt { exch } if pop		% min
    dup scale
    ColorSpaceNames colors scratch cvs get setcolorspace

% prepare image dictionary
    << /ImageType 1
       /Width width
       /Height height
       /ImageMatrix [ width 0 0 height neg 0 height ]
       /BitsPerComponent 8
       % If 4-component (CMYK), assume data is inverted per Adobe Photoshop
       colors 4 eq {
         /Decode [ colors { 1 0 } repeat ]
       } {
         /Decode [ colors { 0 1 } repeat ]
       } ifelse
       /DataSource F /DCTDecode filter
    >> image

end		% image parameter dictionary

saved end restore
} bind def

% This lets you do stuff on the command line like:
% gs -sDEVICE=pdfwrite -o stuff%03d.pdf viewjpeg.ps -c "(image.jpg) << /PageSize 2 index viewJPEGgetsize 2 array astore  >> setpagedevice viewJPEG"
% so the output size matches the original image.
/viewJPEGgetsize {		% <file|string> ==> width height
    save
    JPEGdict begin
    /saved exch def
    /scratch 1 string def
    dup type /stringtype eq { (r) file } if
    /F exch def
    readJPEGmarkers begin
    F 0 setfileposition		% reset file pointer
    width height
    saved end restore
} bind def

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✏️ viewjpeg.ps