Extracting/reading PNG iTXt chunks with Exiv2

Added by Arnold Wiegert 11 months ago

Being a newb to Exiv2, I have been trying to find a way to check for and extract or read the various chunks, particularly the iTXt chunks, in a PNG, file using the current Exiv2 library.
Any comments or pointer will much appreciated.
TIA


Replies (4)

RE: Extracting/reading PNG iTXt chunks with Exiv2 - Added by Robin Mills 11 months ago

Arnold

Yes, you can do this with the -pS (printStructure in v0.25) and -pR (printRecursive in trunk/v0.26) options.

951 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 -pS test/data/imagemagick.png 
STRUCTURE OF PNG FILE: test/data/imagemagick.png
 address | index | chunk_type |  length | data
       8 |     0 |       IHDR |      13 | ...@........
      33 |     1 |       iCCP |    1404 | icc..x...i8.......af\...w_3...+
    1449 |     2 |       sBIT |       3 | ...
    1464 |     3 |       zTXt |      87 | Software..x...A.. ......B....}.N
    1563 |     4 |       tEXt |   24482 | Raw profile type exif..exif.   1
   26057 |     5 |       tEXt |     471 | Raw profile type iptc..iptc.    
   26540 |     6 |       IDAT |    8192 | x...Y.$Wv&v.{.{l.T.......[w.=m$G
   34744 |     7 |       IDAT |    8192 | .4X.y.AR...4....:Ue..U.|1..:..D.
   42948 |     8 |       IDAT |    8192 | 'g.!... ...n...s..Jdz.........6.
   51152 |     9 |       IDAT |    8192 | ........k....CY/75I..u;.. .z...}
   59356 |    10 |       IDAT |    8192 | .f>..]....UKqD2s.(.q....=x.l.\V.
   67560 |    11 |       IDAT |    8192 | .i.{!!B0...C!4.p..`D g`.........
   75764 |    12 |       IDAT |    8192 | .*.].4..Q..}(9...S0&.......T.9.+
   83968 |    13 |       IDAT |    8192 | ..k...6....g.1..}.].&.H........
   92172 |    14 |       IDAT |    8192 | .j..S.........z..!U.G0*.m%..09<,
  100376 |    15 |       IDAT |    8192 | .....t.>!.....6^.<..;..?$I..M.9
  108580 |    16 |       IDAT |    8192 | W.&5.5J........FW`....3.N.9Pk;.s
  116784 |    17 |       IDAT |    8192 | .....d.z".`...v=g-..-.c8...Z.5.x
  124988 |    18 |       IDAT |    8192 | .."...o<&."....1M....1&. ..5....
  133192 |    19 |       IDAT |    8192 | k........!..B*.....\*.(!..0.s..B
  141396 |    20 |       IDAT |    3346 | .Y.L@I$M.Z[.0A ...K#.t.0+.G(.jX.
  144754 |    21 |       IEND |       0 | 
952 rmills@rmillsmbp:~/gnu/exiv2/trunk $ 
I don't know for which purpose you are interested in the iTXt chunks. Some can be extracted by exiv2. For example, the following command will extract an ICC profile to stdout.
$ exiv2 -pC foo.png
The following command will extract XMP (xml) to stdout:
$ exiv2 -pX foo.png
If you have other chunks of interest, you can write a script to parse the output of $ exiv2 -pS foo.png to obtain the offset and length of a chunk which can then be extracted with dd. This is discussed in relation to TIFF files in this article: http://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_TIFF_files

Print Recursive -pR on the trunk and heading for v0.26 shows the same information and descends into data structures it recognises.

953 rmills@rmillsmbp:~/gnu/exiv2/trunk $ exiv2 -pR test/data/imagemagick.pngSTRUCTURE OF PNG FILE: test/data/imagemagick.png
 address | index | chunk_type |  length | data
       8 |     0 |       IHDR |      13 | ...@........
      33 |     1 |       iCCP |    1404 | icc..x...i8.......af\...w_3...+
    1449 |     2 |       sBIT |       3 | ...
    1464 |     3 |       zTXt |      87 | Software..x...A.. ......B....}.N
Software: digiKam 0.9.0-svn ( libpng version 1.2.8 - December 3, 2004 (header) )
    1563 |     4 |       tEXt |   24482 | Raw profile type exif..exif.   1
  STRUCTURE OF TIFF FILE (MM): MemIo
   address |    tag                           |      type |    count |    offset | value
        10 | 0x0100 ImageWidth                |     SLONG |        1 |       320 | 320
        22 | 0x0101 ImageLength               |     SLONG |        1 |       211 | 211
        34 | 0x010f Make                      |     ASCII |       18 |       146 | NIKON CORPORATION
        46 | 0x0110 Model                     |     ASCII |       10 |       164 | NIKON D70
        58 | 0x0112 Orientation               |     SHORT |        1 |     65536 | 1
        70 | 0x011a XResolution               |  RATIONAL |        1 |       174 | 174/0
        82 | 0x011b YResolution               |  RATIONAL |        1 |       182 | 182/0
        94 | 0x0128 ResolutionUnit            |     SHORT |        1 |    131072 | 2
       106 | 0x0131 Software                  |     ASCII |       18 |       190 | digiKam-0.9.0-svn
       118 | 0x0132 DateTime                  |     ASCII |       20 |       208 | 2006:02:04 16:09:30
       130 | 0x8769 ExifTag                   |      LONG |        1 |       228 | 228
    STRUCTURE OF TIFF FILE (MM): MemIo
     address |    tag                           |      type |    count |    offset | value
         230 | 0x829a ExposureTime              |  RATIONAL |        1 |       546 | 546/0
         242 | 0x829d FNumber                   |  RATIONAL |        1 |       554 | 554/0
         254 | 0x8822 ExposureProgram           |     SHORT |        1 |    262144 | 4
         266 | 0x8827 ISOSpeedRatings           |     SHORT |        1 |  13107200 | 200
         278 | 0x9003 DateTimeOriginal          |     ASCII |       20 |       562 | 2006:02:04 16:09:30
         290 | 0x9004 DateTimeDigitized         |     ASCII |       20 |       582 | 2006:02:04 16:09:30
         302 | 0x9201 ShutterSpeedValue         |  RATIONAL |        1 |       602 | 602/0
         314 | 0x9202 ApertureValue             |  RATIONAL |        1 |       610 | 610/0
         326 | 0x9204 ExposureBiasValue         |  RATIONAL |        1 |       618 | 618/0
         338 | 0x9205 MaxApertureValue          |  RATIONAL |        1 |       626 | 626/0
         350 | 0x9208 LightSource               |     SHORT |        1 |         0 | 0
         362 | 0x9209 Flash                     |     SHORT |        1 |         0 | 0
         374 | 0x920a FocalLength               |  RATIONAL |        1 |       634 | 634/0
         386 | 0x927c MakerNote                 | UNDEFINED |     6989 |       642 | Nikon.....MM.*.....+........0210 ...
      STRUCTURE OF TIFF FILE (MM): MemIo
       address |    tag                           |      type |    count |    offset | value
            10 | 0x0001 Version                   | UNDEFINED |        4 | 808595760 | 0210
            22 | 0x0002 ISOSpeed                  |     SHORT |        2 |       200 | 0 200
            34 | 0x0004 Quality                   |     ASCII |        8 |       530 | RAW    
            46 | 0x0005 WhiteBalance              |     ASCII |       13 |       538 | AUTO        
            58 | 0x0006 Sharpening                |     ASCII |        7 |       551 | NORMAL
            70 | 0x0007 Focus                     |     ASCII |        7 |       558 | AF-S  
            82 | 0x0008 FlashSetting              |     ASCII |       13 |       565 | NORMAL      
            94 | 0x0009 GPSStatus                 |     ASCII |       13 |       578 |             
           106 | 0x000b ProcessingSoftware        |    SSHORT |        1 |         0 | 0
           118 | 0x000d GPSSpeed                  | UNDEFINED |        4 |     67072 | ...
           130 | 0x000e GPSTrackRef               | UNDEFINED |        4 |     68608 | ...
           142 | 0x0011 GPSImgDirection           |      LONG |        1 |      7006 | 7006
           154 | 0x0012 GPSMapDatum               | UNDEFINED |        4 |     67072 | ...
           166 | 0x0013 GPSDestLatitudeRef        |     SHORT |        2 |       200 | 0 200
           178 | 0x0017 GPSDestBearingRef         | UNDEFINED |        4 |     67072 | ...
           190 | 0x0018 GPSDestBearing            | UNDEFINED |        4 |     67072 | ...
           202 | 0x0019 GPSDestDistanceRef        | SRATIONAL |        1 |       591 | 591/0
           214 | 0x0081                           |     ASCII |        9 |       599 | NORMAL  
           226 | 0x0083                           |      BYTE |        1 | 100663296 | .
           238 | 0x0084                           |  RATIONAL |        4 |       608 | 180/0 10/0 500/0 10/0
           250 | 0x0087                           |      BYTE |        1 |         0 | 
           262 | 0x0088 AFFocusPos                | UNDEFINED |        4 |  16777217 | ....
           274 | 0x0089                           |     SHORT |        1 |         0 | 0
           286 | 0x008b                           | UNDEFINED |        4 |1208028160 | H..
           298 | 0x008c                           | UNDEFINED |     4160 |       640 | I.............................. ...
           310 | 0x008d                           |     ASCII |        9 |      4800 | MODE2   
           322 | 0x0090                           |     ASCII |       12 |      4809 | NATURAL    
           334 | 0x0091                           | UNDEFINED |      465 |      4821 | 0103........................... ...
           346 | 0x0092                           |    SSHORT |        1 |         0 | 0
           358 | 0x0093                           |     SHORT |        1 |     65536 | 1
           370 | 0x0095                           |     ASCII |        5 |      5286 | OFF 
           382 | 0x0096                           | UNDEFINED |     1412 |      5291 | D..H.H.H.H...................... ...
           394 | 0x0097                           | UNDEFINED |      140 |      6703 | 0103.................:...z...... ...
           406 | 0x0098                           | UNDEFINED |       31 |      6843 | ...............................
           418 | 0x0099                           |     SHORT |        2 |  99615728 | 1520 1008
           430 | 0x009a                           |  RATIONAL |        2 |      6874 | 3/296 1/0
           442 | 0x00a0                           |     ASCII |       21 |      6890 | .........2...........
           454 | 0x00a4                           | UNDEFINED |        4 | 808595504 | 0200
           466 | 0x00a7                           |      LONG |        1 |      5670 | 5670
           478 | 0x00a8                           | UNDEFINED |       20 |      6911 | ..........a~NIKON CO
           490 | 0x00a9                           |     ASCII |       16 |      6931 | RPORATION.NIKON 
           502 | 0x00aa                           |     ASCII |       16 |      6947 | D70....,.......,
           514 | 0x00ab                           |     ASCII |       16 |      6963 | ....Bibble 4.5.
      END MemIo
         398 | 0xa002 PixelXDimension           |     SLONG |        1 |       320 | 320
         410 | 0xa003 PixelYDimension           |     SLONG |        1 |       211 | 211
         422 | 0xa217 SensingMethod             |      BYTE |        1 |  33554432 | .
         434 | 0xa301 SceneType                 |      BYTE |        1 |  16777216 | .
         446 | 0xa402 ExposureMode              |     SHORT |        1 |         0 | 0
         458 | 0xa403 WhiteBalance              |     SHORT |        1 |         0 | 0
         470 | 0xa405 FocalLengthIn35mmFilm     |     SHORT |        1 |   4915200 | 75
         482 | 0xa406 SceneCaptureType          |     SHORT |        1 |         0 | 0
         494 | 0xa408 Contrast                  |     SHORT |        1 |         0 | 0
         506 | 0xa409 Saturation                |     SHORT |        1 |         0 | 0
         518 | 0xa40a Sharpness                 |     SHORT |        1 |         0 | 0
         530 | 0xa40c SubjectDistanceRange      |     SHORT |        1 |         0 | 0
    END MemIo
      7633 | 0x0103 Compression               |     SHORT |        1 |    393216 | 6
      7645 | 0x0201 JPEGInterchangeFormat     |      LONG |        1 |      7673 | 7673
      7657 | 0x0202 JPEGInterchangeFormatLeng |      LONG |        1 |      4376 | 4376
  END MemIo
   26057 |     5 |       tEXt |     471 | Raw profile type iptc..iptc.    
  Record | DataSet | Name                     | Length | Data
       2 |       5 | ObjectName               |      4 | ovni
       2 |      10 | Urgency                  |      1 | 0
       2 |      15 | Category                 |      3 | I -
       2 |      20 | SuppCategory             |     13 | SPCL Specials
       2 |     116 | Copyright                |      4 | E.T.
       2 |     110 | Credit                   |      9 | Spielberg
       2 |     122 | Writer                   |     21 | L'ovni de la Bastille
       2 |      90 | City                     |      5 | Paris
       2 |      95 | ProvinceState            |     13 | ..le de Franc
       2 |     101 | CountryName              |     12 | FRA - France
       2 |     100 | CountryCode              |      3 | 750
       2 |      40 | SpecialInstructions      |     10 | Y'en a pas
       2 |     103 | TransmissionReference    |      2 | 36
       2 |     115 | Source                   |      8 | Chez moi
       2 |      55 | DateCreated              |      8 | 20060204
       2 |      60 | TimeCreated              |     11 | 160930+0000
       2 |       0 | RecordVersion            |      2 | ..
   26540 |     6 |       IDAT |    8192 | x...Y.$Wv&v.{.{l.T.......[w.=m$G
   34744 |     7 |       IDAT |    8192 | .4X.y.AR...4....:Ue..U.|1..:..D.
   42948 |     8 |       IDAT |    8192 | 'g.!... ...n...s..Jdz.........6.
   51152 |     9 |       IDAT |    8192 | ........k....CY/75I..u;.. .z...}
   59356 |    10 |       IDAT |    8192 | .f>..]....UKqD2s.(.q....=x.l.\V.
   67560 |    11 |       IDAT |    8192 | .i.{!!B0...C!4.p..`D g`.........
   75764 |    12 |       IDAT |    8192 | .*.].4..Q..}(9...S0&.......T.9.+
   83968 |    13 |       IDAT |    8192 | ..k...6....g.1..}.].&.H........
   92172 |    14 |       IDAT |    8192 | .j..S.........z..!U.G0*.m%..09<,
  100376 |    15 |       IDAT |    8192 | .....t.>!.....6^.<..;..?$I..M.9
  108580 |    16 |       IDAT |    8192 | W.&5.5J........FW`....3.N.9Pk;.s
  116784 |    17 |       IDAT |    8192 | .....d.z".`...v=g-..-.c8...Z.5.x
  124988 |    18 |       IDAT |    8192 | .."...o<&."....1M....1&. ..5....
  133192 |    19 |       IDAT |    8192 | k........!..B*.....\*.(!..0.s..B
  141396 |    20 |       IDAT |    3346 | .Y.L@I$M.Z[.0A ...K#.t.0+.G(.jX.
  144754 |    21 |       IEND |       0 | 

STRUCTURE OF PNG FILE: test/data/imagemagick.png
 address | index | chunk_type |  length | data
       8 |     0 |       IHDR |      13 | ...@........
      33 |     1 |       iCCP |    1404 | icc..x...i8.......af\...w_3...+
    1449 |     2 |       sBIT |       3 | ...
    1464 |     3 |       zTXt |      87 | Software..x...A.. ......B....}.N
Software: digiKam 0.9.0-svn ( libpng version 1.2.8 - December 3, 2004 (header) )
    1563 |     4 |       tEXt |   24482 | Raw profile type exif..exif.   1
  STRUCTURE OF TIFF FILE (MM): MemIo
   address |    tag                           |      type |    count |    offset | value
        10 | 0x0100 ImageWidth                |     SLONG |        1 |       320 | 320
        22 | 0x0101 ImageLength               |     SLONG |        1 |       211 | 211
        34 | 0x010f Make                      |     ASCII |       18 |       146 | NIKON CORPORATION
        46 | 0x0110 Model                     |     ASCII |       10 |       164 | NIKON D70
        58 | 0x0112 Orientation               |     SHORT |        1 |     65536 | 1
        70 | 0x011a XResolution               |  RATIONAL |        1 |       174 | 174/0
        82 | 0x011b YResolution               |  RATIONAL |        1 |       182 | 182/0
        94 | 0x0128 ResolutionUnit            |     SHORT |        1 |    131072 | 2
       106 | 0x0131 Software                  |     ASCII |       18 |       190 | digiKam-0.9.0-svn
       118 | 0x0132 DateTime                  |     ASCII |       20 |       208 | 2006:02:04 16:09:30
       130 | 0x8769 ExifTag                   |      LONG |        1 |       228 | 228
    STRUCTURE OF TIFF FILE (MM): MemIo
     address |    tag                           |      type |    count |    offset | value
         230 | 0x829a ExposureTime              |  RATIONAL |        1 |       546 | 546/0
         242 | 0x829d FNumber                   |  RATIONAL |        1 |       554 | 554/0
         254 | 0x8822 ExposureProgram           |     SHORT |        1 |    262144 | 4
         266 | 0x8827 ISOSpeedRatings           |     SHORT |        1 |  13107200 | 200
         278 | 0x9003 DateTimeOriginal          |     ASCII |       20 |       562 | 2006:02:04 16:09:30
         290 | 0x9004 DateTimeDigitized         |     ASCII |       20 |       582 | 2006:02:04 16:09:30
         302 | 0x9201 ShutterSpeedValue         |  RATIONAL |        1 |       602 | 602/0
         314 | 0x9202 ApertureValue             |  RATIONAL |        1 |       610 | 610/0
         326 | 0x9204 ExposureBiasValue         |  RATIONAL |        1 |       618 | 618/0
         338 | 0x9205 MaxApertureValue          |  RATIONAL |        1 |       626 | 626/0
         350 | 0x9208 LightSource               |     SHORT |        1 |         0 | 0
         362 | 0x9209 Flash                     |     SHORT |        1 |         0 | 0
         374 | 0x920a FocalLength               |  RATIONAL |        1 |       634 | 634/0
         386 | 0x927c MakerNote                 | UNDEFINED |     6989 |       642 | Nikon.....MM.*.....+........0210 ...
      STRUCTURE OF TIFF FILE (MM): MemIo
       address |    tag                           |      type |    count |    offset | value
            10 | 0x0001 Version                   | UNDEFINED |        4 | 808595760 | 0210
            22 | 0x0002 ISOSpeed                  |     SHORT |        2 |       200 | 0 200
            34 | 0x0004 Quality                   |     ASCII |        8 |       530 | RAW    
            46 | 0x0005 WhiteBalance              |     ASCII |       13 |       538 | AUTO        
            58 | 0x0006 Sharpening                |     ASCII |        7 |       551 | NORMAL
            70 | 0x0007 Focus                     |     ASCII |        7 |       558 | AF-S  
            82 | 0x0008 FlashSetting              |     ASCII |       13 |       565 | NORMAL      
            94 | 0x0009 GPSStatus                 |     ASCII |       13 |       578 |             
           106 | 0x000b ProcessingSoftware        |    SSHORT |        1 |         0 | 0
           118 | 0x000d GPSSpeed                  | UNDEFINED |        4 |     67072 | ...
           130 | 0x000e GPSTrackRef               | UNDEFINED |        4 |     68608 | ...
           142 | 0x0011 GPSImgDirection           |      LONG |        1 |      7006 | 7006
           154 | 0x0012 GPSMapDatum               | UNDEFINED |        4 |     67072 | ...
           166 | 0x0013 GPSDestLatitudeRef        |     SHORT |        2 |       200 | 0 200
           178 | 0x0017 GPSDestBearingRef         | UNDEFINED |        4 |     67072 | ...
           190 | 0x0018 GPSDestBearing            | UNDEFINED |        4 |     67072 | ...
           202 | 0x0019 GPSDestDistanceRef        | SRATIONAL |        1 |       591 | 591/0
           214 | 0x0081                           |     ASCII |        9 |       599 | NORMAL  
           226 | 0x0083                           |      BYTE |        1 | 100663296 | .
           238 | 0x0084                           |  RATIONAL |        4 |       608 | 180/0 10/0 500/0 10/0
           250 | 0x0087                           |      BYTE |        1 |         0 | 
           262 | 0x0088 AFFocusPos                | UNDEFINED |        4 |  16777217 | ....
           274 | 0x0089                           |     SHORT |        1 |         0 | 0
           286 | 0x008b                           | UNDEFINED |        4 |1208028160 | H..
           298 | 0x008c                           | UNDEFINED |     4160 |       640 | I.............................. ...
           310 | 0x008d                           |     ASCII |        9 |      4800 | MODE2   
           322 | 0x0090                           |     ASCII |       12 |      4809 | NATURAL    
           334 | 0x0091                           | UNDEFINED |      465 |      4821 | 0103........................... ...
           346 | 0x0092                           |    SSHORT |        1 |         0 | 0
           358 | 0x0093                           |     SHORT |        1 |     65536 | 1
           370 | 0x0095                           |     ASCII |        5 |      5286 | OFF 
           382 | 0x0096                           | UNDEFINED |     1412 |      5291 | D..H.H.H.H...................... ...
           394 | 0x0097                           | UNDEFINED |      140 |      6703 | 0103.................:...z...... ...
           406 | 0x0098                           | UNDEFINED |       31 |      6843 | ...............................
           418 | 0x0099                           |     SHORT |        2 |  99615728 | 1520 1008
           430 | 0x009a                           |  RATIONAL |        2 |      6874 | 3/296 1/0
           442 | 0x00a0                           |     ASCII |       21 |      6890 | .........2...........
           454 | 0x00a4                           | UNDEFINED |        4 | 808595504 | 0200
           466 | 0x00a7                           |      LONG |        1 |      5670 | 5670
           478 | 0x00a8                           | UNDEFINED |       20 |      6911 | ..........a~NIKON CO
           490 | 0x00a9                           |     ASCII |       16 |      6931 | RPORATION.NIKON 
           502 | 0x00aa                           |     ASCII |       16 |      6947 | D70....,.......,
           514 | 0x00ab                           |     ASCII |       16 |      6963 | ....Bibble 4.5.
      END MemIo
         398 | 0xa002 PixelXDimension           |     SLONG |        1 |       320 | 320
         410 | 0xa003 PixelYDimension           |     SLONG |        1 |       211 | 211
         422 | 0xa217 SensingMethod             |      BYTE |        1 |  33554432 | .
         434 | 0xa301 SceneType                 |      BYTE |        1 |  16777216 | .
         446 | 0xa402 ExposureMode              |     SHORT |        1 |         0 | 0
         458 | 0xa403 WhiteBalance              |     SHORT |        1 |         0 | 0
         470 | 0xa405 FocalLengthIn35mmFilm     |     SHORT |        1 |   4915200 | 75
         482 | 0xa406 SceneCaptureType          |     SHORT |        1 |         0 | 0
         494 | 0xa408 Contrast                  |     SHORT |        1 |         0 | 0
         506 | 0xa409 Saturation                |     SHORT |        1 |         0 | 0
         518 | 0xa40a Sharpness                 |     SHORT |        1 |         0 | 0
         530 | 0xa40c SubjectDistanceRange      |     SHORT |        1 |         0 | 0
    END MemIo
      7633 | 0x0103 Compression               |     SHORT |        1 |    393216 | 6
      7645 | 0x0201 JPEGInterchangeFormat     |      LONG |        1 |      7673 | 7673
      7657 | 0x0202 JPEGInterchangeFormatLeng |      LONG |        1 |      4376 | 4376
  END MemIo
   26057 |     5 |       tEXt |     471 | Raw profile type iptc..iptc.    
  Record | DataSet | Name                     | Length | Data
       2 |       5 | ObjectName               |      4 | ovni
       2 |      10 | Urgency                  |      1 | 0
       2 |      15 | Category                 |      3 | I -
       2 |      20 | SuppCategory             |     13 | SPCL Specials
       2 |     116 | Copyright                |      4 | E.T.
       2 |     110 | Credit                   |      9 | Spielberg
       2 |     122 | Writer                   |     21 | L'ovni de la Bastille
       2 |      90 | City                     |      5 | Paris
       2 |      95 | ProvinceState            |     13 | ..le de Franc
       2 |     101 | CountryName              |     12 | FRA - France
       2 |     100 | CountryCode              |      3 | 750
       2 |      40 | SpecialInstructions      |     10 | Y'en a pas
       2 |     103 | TransmissionReference    |      2 | 36
       2 |     115 | Source                   |      8 | Chez moi
       2 |      55 | DateCreated              |      8 | 20060204
       2 |      60 | TimeCreated              |     11 | 160930+0000
       2 |       0 | RecordVersion            |      2 | ..
   26540 |     6 |       IDAT |    8192 | x...Y.$Wv&v.{.{l.T.......[w.=m$G
   34744 |     7 |       IDAT |    8192 | .4X.y.AR...4....:Ue..U.|1..:..D.
   42948 |     8 |       IDAT |    8192 | 'g.!... ...n...s..Jdz.........6.
   51152 |     9 |       IDAT |    8192 | ........k....CY/75I..u;.. .z...}
   59356 |    10 |       IDAT |    8192 | .f>..]....UKqD2s.(.q....=x.l.\V.
   67560 |    11 |       IDAT |    8192 | .i.{!!B0...C!4.p..`D g`.........
   75764 |    12 |       IDAT |    8192 | .*.].4..Q..}(9...S0&.......T.9.+
   83968 |    13 |       IDAT |    8192 | ..k...6....g.1..}.].&.H........
   92172 |    14 |       IDAT |    8192 | .j..S.........z..!U.G0*.m%..09<,
  100376 |    15 |       IDAT |    8192 | .....t.>!.....6^.<..;..?$I..M.9
  108580 |    16 |       IDAT |    8192 | W.&5.5J........FW`....3.N.9Pk;.s
  116784 |    17 |       IDAT |    8192 | .....d.z".`...v=g-..-.c8...Z.5.x
  124988 |    18 |       IDAT |    8192 | .."...o<&."....1M....1&. ..5....
  133192 |    19 |       IDAT |    8192 | k........!..B*.....\*.(!..0.s..B
  141396 |    20 |       IDAT |    3346 | .Y.L@I$M.Z[.0A ...K#.t.0+.G(.jX.
  144754 |    21 |       IEND |       0 | 

RE: Extracting/reading PNG iTXt chunks with Exiv2 - Added by Arnold Wiegert 11 months ago

Thank you, Robin,
but I should have been clearer, I want to do this from within C_++ code, not the command line.

RE: Extracting/reading PNG iTXt chumnks with Exiv2 - Added by Robin Mills 11 months ago

The API that performs this work in PngImage::printStructure(). The output of -pS is captured in a stream which you can easily parse. If you wish to process the chunks in some complex way, you may wish to copy the code from printStructure() to your application.

   void PngImage::printStructure(std::ostream& out, PrintStructureOption option, int depth)
   {
        if (io_->open() != 0) {
            throw Error(9, io_->path(), strError());
        }
        IoCloser closer(*io_);
        // Ensure that this is the correct image type
        if (!isPngType(*io_, true)) {
            if (io_->error() || io_->eof()) throw Error(14);
            throw Error(3, "PNG");
        }

        char    chType[5];
        chType[0]=0;
        chType[4]=0;

        if ( option == kpsBasic || option == kpsXMP || option == kpsIccProfile || option == kpsRecursive ) {

            const std::string xmpKey  = "XML:com.adobe.xmp";
            const std::string exifKey = "Raw profile type exif";
            const std::string iptcKey = "Raw profile type iptc";
            const std::string iccKey  = "icc";
            const std::string softKey = "Software";

            if ( option == kpsBasic || option == kpsRecursive ) {
                out << "STRUCTURE OF PNG FILE: " << io_->path() << std::endl;
                out << " address | index | chunk_type |  length | data" << std::endl;
            }

            long       index   = 0;
            const long imgSize = io_->size();
            DataBuf    cheaderBuf(8);

            while( !io_->eof() && ::strcmp(chType,"IEND") ) {
                size_t address = io_->tell();

                std::memset(cheaderBuf.pData_, 0x0, cheaderBuf.size_);
                long bufRead = io_->read(cheaderBuf.pData_, cheaderBuf.size_);
                if (io_->error()) throw Error(14);
                if (bufRead != cheaderBuf.size_) throw Error(20);

                // Decode chunk data length.
                uint32_t dataOffset = Exiv2::getULong(cheaderBuf.pData_, Exiv2::bigEndian);
                for (int i = 4; i < 8; i++) {
                    chType[i-4]=cheaderBuf.pData_[i];
                }

                // test that we haven't hit EOF, or wanting to read excessive data
                long restore = io_->tell();
                if(  restore == -1
                ||  dataOffset > uint32_t(0x7FFFFFFF)
                ||  static_cast<long>(dataOffset) > imgSize - restore
                ){
                    throw Exiv2::Error(14);
                }

                // format output
                uint32_t    blen = dataOffset > 32 ? 32 : dataOffset ;
                std::string dataString ;
                DataBuf buff(blen);
                io_->read(buff.pData_,blen);
                io_->seek(restore, BasicIo::beg);
                dataString  = Internal::binaryToString(buff, blen);

                if ( option == kpsBasic || option == kpsRecursive )
                    out << Internal::stringFormat("%8d | %5d | %10s |%8d | " 
                              ,(uint32_t)address, index++,chType,dataOffset)
                                    << dataString << std::endl;

                // chunk type
                bool tEXt  = std::strcmp(chType,"tEXt")== 0;
                bool zTXt  = std::strcmp(chType,"zTXt")== 0;
                bool iCCP  = std::strcmp(chType,"iCCP")== 0;
                bool iTXt  = std::strcmp(chType,"iTXt")== 0;

                // for XMP, ICC etc: read and format data
                bool bXMP  = option == kpsXMP        && findi(dataString,xmpKey)==0;
                bool bICC  = option == kpsIccProfile && findi(dataString,iccKey)==0;
                bool bExif = option == kpsRecursive  && findi(dataString,exifKey)==0;
                bool bIptc = option == kpsRecursive  && findi(dataString,iptcKey)==0;
                bool bSoft = option == kpsRecursive  && findi(dataString,softKey)==0;
                bool bDump = bXMP || bICC || bExif || bIptc || bSoft ;

                if( bDump ) {
                    DataBuf   dataBuf;
                    byte*     data     = new byte[dataOffset+1];
                    data[dataOffset]=0;
                    io_->read(data,dataOffset);
                    io_->seek(restore, BasicIo::beg);
                    uint32_t    name_l = (uint32_t) std::strlen((const char*)data)+1; // leading string length
                    uint32_t     start = name_l;

                    // decode the chunk
                    bool bGood = false;
                    if ( tEXt ) {
                        bGood = tEXtToDataBuf(data+name_l,dataOffset-name_l,dataBuf);
                    }
                    if ( zTXt || iCCP ) {
                        name_l++ ; // +1 = 'compressed' flag
                        bGood = zlibToDataBuf(data+name_l,dataOffset-name_l,dataBuf);
                    }
                    if ( iTXt ) {
                        while ( data[start] == 0 && start < dataOffset ) start++; // crawl over the '\0' bytes between XML:....\0\0<xml stuff
                        data[dataOffset]=0;                 // ensure the XML is nul terminated
                        bGood = (start+3) < dataOffset ;    // good if not a nul chunk
                    }

                    // format is content dependent
                    if ( bGood ) {
                        if ( bXMP ) {
                            out <<  data+start;             // output the xml
                        }
                        if ( bExif ) {
                            const char* bytes = (const char*) dataBuf.pData_;
                            uint32_t    l     = (uint32_t) std::strlen(bytes)+2;
                            // create a copy on write memio object with the data, then print the structure
                            BasicIo::AutoPtr p = BasicIo::AutoPtr(new MemIo(dataBuf.pData_+l,dataBuf.size_-l));
                            TiffImage::printTiffStructure(*p,out,option,depth);
                        }

                        if ( bSoft ) {
                            const char* bytes = (const char*) dataBuf.pData_;
                            uint32_t    l     = (uint32_t) std::strlen(bytes)+2;
                            // create a copy on write memio object with the data, then print the structure
                            BasicIo::AutoPtr p = BasicIo::AutoPtr(new MemIo(dataBuf.pData_+l,dataBuf.size_-l));
                            out << Internal::indent(depth) << (const char*) buff.pData_ << ": " << (const char*) dataBuf.pData_ << std::endl;
                        }

                        if ( bICC ) {
                            out.write((const char*) dataBuf.pData_,dataBuf.size_);
                        }

                        if ( bIptc ) {
                            IptcData::printStructure(out,dataBuf.pData_,dataBuf.size_,depth);
                        }
                    }
                    delete[] data;
                }
                io_->seek(dataOffset + 4 , BasicIo::cur);
                if (io_->error()) throw Error(14);
            }
        }
    }

RE: Extracting/reading PNG iTXt chunks with Exiv2 - Added by Arnold Wiegert 11 months ago

Again, many thanks for this clarification.
For some reason I had expected to find this metadata under metadata in the Exiv2 code :-)
I'll give this sample code a try.

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