Documentation update for Michal.
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clanmills
Dan Čermák
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WORK-IN-PROGRESS
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WORK-IN-PROGRESS
@ -28,6 +28,11 @@ T A B L E o f C O N T E N T S
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5 Refactoring the Tiff Code
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5.1 Background
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5.2 How does Exiv2 decode the ExifData in a JPEG?
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5.3 How is metadata organized in Exiv2
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5.4 Where are the tags defined?
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5.5 How do the MakerNotes get decoded?
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5.6 How do the encoders work?
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==========================================================================
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@ -572,7 +577,7 @@ Modified: 2017-09-24
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- because the code is so hard to understand, I can't simply browse and read it.
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4) It separates file navigation from data analysis.
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The code in image::printStructure was originally written to understand "what is a tiff"
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The code in image::printStructure was originally written to understand "what is a tiff?"
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It has problems:
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1) It was intended to be a single threaded debugging function and has security issues.
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2) It doesn't handle BigTiff
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@ -583,7 +588,228 @@ Modified: 2017-09-24
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2) Keep everything good in the code and address known deficiencies
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3) Establish a Team Exiv2 "Tiff Expert" who knows the code intimately.
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5.2 How does Exiv2 decode the ExifData in a JPEG?
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You can get my test file from http://clanmills.com/Stonehenge.jpg
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808 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/build $ exiv2 -pS ~/Stonehenge.jpg
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STRUCTURE OF JPEG FILE: /Users/rmills/Stonehenge.jpg
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address | marker | length | data
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0 | 0xffd8 SOI
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2 | 0xffe1 APP1 | 15288 | Exif..II*......................
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15292 | 0xffe1 APP1 | 2610 | http://ns.adobe.com/xap/1.0/.<?x
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17904 | 0xffed APP13 | 96 | Photoshop 3.0.8BIM.......'.....
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18002 | 0xffe2 APP2 | 4094 | MPF.II*...............0100.....
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22098 | 0xffdb DQT | 132
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22232 | 0xffc0 SOF0 | 17
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22251 | 0xffc4 DHT | 418
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22671 | 0xffda SOS
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809 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/build $
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Exiv2 calls JpegBase::readMetadata which locates the APP1/Exif segment.
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It invokes the ExifParser:
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ExifParser::decode(exifData_, rawExif.pData_, rawExif.size_);
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This is thin wrapper over:
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TiffParserWorker::decode(....) in tiffimage.cpp
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What happens then? I don't know. The metadata is decoded in:
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tiffvisitor.cpp TiffDecoder::visitEntry()
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The design of the TiffMumble classes is the "Visitor" pattern
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described in "Design Patterns" by Addison & Wesley. The aim of the pattern
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is to separate parsing from dealing with the data.
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The data is being stored in ExifData which is a vector.
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Order is important and preserved.
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As the data values are recovered they are stored as Exifdatum in the vector.
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How does the tiff visitor work? I think the reader and processor
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are connected by this line in TiffParser::
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rootDir->accept(reader);
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The class tree for the decoder is:
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class TiffDecoder : public TiffFinder {
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class TiffReader ,
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class TiffFinder : public TiffVisitor {
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class TiffVisitor {
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public:
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//! Events for the stop/go flag. See setGo().
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enum GoEvent {
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geTraverse = 0,
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geKnownMakernote = 1
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};
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void setGo(GoEvent event, bool go);
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virtual void visitEntry(TiffEntry* object) =0;
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virtual void visitDataEntry(TiffDataEntry* object) =0;
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virtual void visitImageEntry(TiffImageEntry* object) =0;
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virtual void visitSizeEntry(TiffSizeEntry* object) =0;
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virtual void visitDirectory(TiffDirectory* object) =0;
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virtual void visitSubIfd(TiffSubIfd* object) =0;
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virtual void visitMnEntry(TiffMnEntry* object) =0;
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virtual void visitIfdMakernote(TiffIfdMakernote* object) =0;
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virtual void visitIfdMakernoteEnd(TiffIfdMakernote* object);
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virtual void visitBinaryArray(TiffBinaryArray* object) =0;
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virtual void visitBinaryArrayEnd(TiffBinaryArray* object);
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//! Operation to perform for an element of a binary array
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virtual void visitBinaryElement(TiffBinaryElement* object) =0;
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//! Check if stop flag for \em event is clear, return true if it's clear.
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bool go(GoEvent event) const;
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}
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}
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}
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The reader works by stepping along the Tiff directory and calls the visitor's
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"callbacks" as it reads.
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There are 2000 lines of code in tiffcomposite.cpp and, to be honest,
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I don't know what most of it does!
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Set a breakpoint in src/exif.cpp#571.
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That’s where he adds the key/value to the exifData vector.
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Exactly how did he get here? That’s a puzzle.
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void ExifData::add(const ExifKey& key, const Value* pValue)
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{
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add(Exifdatum(key, pValue));
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}
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5.3 How is metadata organized in Exiv2
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section.group.tag
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section: Exif | IPTC | Xmp
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group: Photo | Image | MakerNote | Nikon3 ....
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tag: YResolution etc ...
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820 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ exiv2 -pa ~/Stonehenge.jpg | cut -d' ' -f 1 | cut -d. -f 1 | sort | uniq
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Exif
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Iptc
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Xmp
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821 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ exiv2 -pa --grep Exif ~/Stonehenge.jpg | cut -d'.' -f 2 | sort | uniq
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GPSInfo
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Image
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Iop
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MakerNote
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Nikon3
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NikonAf2
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NikonCb2b
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NikonFi
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NikonIi
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NikonLd3
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NikonMe
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NikonPc
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NikonVr
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NikonWt
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Photo
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Thumbnail
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822 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ 533 rmills@rmillsmbp:~/Downloads $ exiv2 -pa --grep Exif ~/Stonehenge.jpg | cut -d'.' -f 3 | cut -d' ' -f 1 | sort | uniq
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AFAperture
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AFAreaHeight
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AFAreaMode
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...
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XResolution
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YCbCrPositioning
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YResolution
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534 rmills@rmillsmbp:~/Downloads $
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823 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $
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The data in IFD0 of is Exiv2.Image:
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826 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ exiv2 -pR ~/Stonehenge.jpg | head -20
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STRUCTURE OF JPEG FILE: /Users/rmills/Stonehenge.jpg
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address | marker | length | data
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0 | 0xffd8 SOI
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2 | 0xffe1 APP1 | 15288 | Exif..II*......................
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STRUCTURE OF TIFF FILE (II): MemIo
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address | tag | type | count | offset | value
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10 | 0x010f Make | ASCII | 18 | 146 | NIKON CORPORATION
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22 | 0x0110 Model | ASCII | 12 | 164 | NIKON D5300
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34 | 0x0112 Orientation | SHORT | 1 | | 1
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46 | 0x011a XResolution | RATIONAL | 1 | 176 | 300/1
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58 | 0x011b YResolution | RATIONAL | 1 | 184 | 300/1
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70 | 0x0128 ResolutionUnit | SHORT | 1 | | 2
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82 | 0x0131 Software | ASCII | 10 | 192 | Ver.1.00
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94 | 0x0132 DateTime | ASCII | 20 | 202 | 2015:07:16 20:25:28
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106 | 0x0213 YCbCrPositioning | SHORT | 1 | | 1
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118 | 0x8769 ExifTag | LONG | 1 | | 222
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STRUCTURE OF TIFF FILE (II): MemIo
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address | tag | type | count | offset | value
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224 | 0x829a ExposureTime | RATIONAL | 1 | 732 | 10/4000
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236 | 0x829d FNumber | RATIONAL | 1 | 740 | 100/10
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827 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ exiv2 -pa --grep Image ~/Stonehenge.jpg
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Exif.Image.Make Ascii 18 NIKON CORPORATION
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Exif.Image.Model Ascii 12 NIKON D5300
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Exif.Image.Orientation Short 1 top, left
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Exif.Image.XResolution Rational 1 300
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Exif.Image.YResolution Rational 1 300
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Exif.Image.ResolutionUnit Short 1 inch
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Exif.Image.Software Ascii 10 Ver.1.00
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Exif.Image.DateTime Ascii 20 2015:07:16 20:25:28
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Exif.Image.YCbCrPositioning Short 1 Centered
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Exif.Image.ExifTag Long 1 222
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Exif.Nikon3.ImageBoundary Short 4 0 0 6000 4000
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Exif.Nikon3.ImageDataSize Long 1 6173648
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Exif.NikonAf2.AFImageWidth Short 1 0
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Exif.NikonAf2.AFImageHeight Short 1 0
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Exif.Photo.ImageUniqueID Ascii 33 090caaf2c085f3e102513b24750041aa
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Exif.Image.GPSTag Long 1 4060
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828 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $
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The data in IFD1 is Exiv2.Photo
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The data in the MakerNote is another embedded TIFF (which more embedded tiffs)
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829 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $ exiv2 -pa --grep MakerNote ~/Stonehenge.jpg
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Exif.Photo.MakerNote Undefined 3152 (Binary value suppressed)
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Exif.MakerNote.Offset Long 1 914
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Exif.MakerNote.ByteOrder Ascii 3 II
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830 rmills@rmillsmbp:~/gnu/github/exiv2/exiv2/src $
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The MakerNote decodes them into:
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Exif.Nikon1, Exiv2.NikonAf2 and so on. I don't know exactly it achieves this.
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However it means that tag-numbers can be reused in different IFDs.
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Tag 0x0016 = Nikon GPSSpeed and can mean something different elsewhere.
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5.4 Where are the tags defined?
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There's an array of "TagInfo" data structures in each of the makernote decoders.
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These define the tag (a number) and the tag name, the groupID (eg canonId) and the default type.
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There's also a callback to print the value of the tag. This does the "interpretation"
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that is performed by the -pt in the exiv2 command-line program.
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TagInfo(0x4001, "ColorData", N_("Color Data"), N_("Color data"), canonId, makerTags, unsignedShort, -1, printValue),
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5.5 How do the MakerNotes get decoded?
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I don't know. It has something to do with this code in tiffcomposite.cpp#936
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TiffMnEntry::doAccept(TiffVisitor& visitor) { ... }
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Most makernotes are TiffStructures. So the TiffXXX classes are invoked recursively to decode the maker note.
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#0 0x000000010058b4b0 in Exiv2::Internal::TiffDirectory::doAccept(Exiv2::Internal::TiffVisitor&) at /Users/rmills/gnu/github/exiv2/exiv2/src/tiffcomposite.cpp:916
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This function iterated the array of entries
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#1 0x000000010058b3c6 in Exiv2::Internal::TiffComponent::accept(Exiv2::Internal::TiffVisitor&) at /Users/rmills/gnu/github/exiv2/exiv2/src/tiffcomposite.cpp:891
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#2 0x00000001005b5357 in Exiv2::Internal::TiffParserWorker::parse(unsigned char const*, unsigned int, unsigned int, Exiv2::Internal::TiffHeaderBase*) at /Users/rmills/gnu/github/exiv2/exiv2/src/tiffimage.cpp:2006
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This function creates an array of TiffEntries
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#3 0x00000001005a2a60 in Exiv2::Internal::TiffParserWorker::decode(Exiv2::ExifData&, Exiv2::IptcData&, Exiv2::XmpData&, unsigned char const*, unsigned int, unsigned int, void (Exiv2::Internal::TiffDecoder::* (*)(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, unsigned int, Exiv2::Internal::IfdId))(Exiv2::Internal::TiffEntryBase const*), Exiv2::Internal::TiffHeaderBase*) at /Users/rmills/gnu/github/exiv2/exiv2/src/tiffimage.cpp:1900
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#4 0x00000001005a1ae9 in Exiv2::TiffParser::decode(Exiv2::ExifData&, Exiv2::IptcData&, Exiv2::XmpData&, unsigned char const*, unsigned int) at /Users/rmills/gnu/github/exiv2/exiv2/src/tiffimage.cpp:260
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#5 0x000000010044d956 in Exiv2::ExifParser::decode(Exiv2::ExifData&, unsigned char const*, unsigned int) at /Users/rmills/gnu/github/exiv2/exiv2/src/exif.cpp:625
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#6 0x0000000100498fd7 in Exiv2::JpegBase::readMetadata() at /Users/rmills/gnu/github/exiv2/exiv2/src/jpgimage.cpp:386
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#7 0x000000010000bc59 in Action::Print::printList() at /Users/rmills/gnu/github/exiv2/exiv2/src/actions.cpp:530
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#8 0x0000000100005835 in Action::Print::run(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) at /Users/rmills/gnu/github/exiv2/exiv2/src/actions.cpp:245
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5.6 How do the encoders work?
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I understand writeMetadata() and will document that soon.
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I still have to study how the TiffVisitor writes metadata.
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# That's all Folks!
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==========================================================================
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