This page is a work in progress, and will be used to explain some details of ExifTool's inner workings.
Below is a diagram showing the flow of information for the exiftool application. Indicated outside the boxes on the diagram are some command-line options associated with the various stages of processing. All of these options are directly associated with options or function calls available via the API (Application Programming Interface), with the exception of the output text formatting which is handled at the application level.
The information flow is separated into two distinct modes: 1)
Reading or extracting information, and 2)
Writing or editing. The application runs in
read mode by default, but switches to write mode
if a new value is assigned to any tag (via "
-json=" on the command line).
When ExifTool reads or writes the value of a tag, there are 3 separate conversions applied to each value, resulting in 4 different levels for the value of each tag. By default, users interact only with the human-readable ("PrintConv") value, but other levels are exposed through various exiftool options:
-lang(Lang) option is used to specify the language for this conversion, and the
-d(CoordFormat and DateFormat) options specify this formatting for GPS coordinates and date/time values.
-noption is used, or for individual tags by suffixing the tag name with a
-v3option, or by using the
-htmlDumpfeature. Note that this value is not related to the
-binary) option, which actually returns the "ValueConv" value and is used for tags where this value can not be presented in a simple text format. The Writable column in the Tag Name documentation gives the format of this binary data for writable tags.
Below are some examples of these different values for a few tags:
Tag 3. PrintConv 2. ValueConv 1. Raw 0. Binary EXIF:Orientation Horizontal (normal) 1 1 00 01 EXIF:GPSLatitude 45 deg 20' 11.00" 45.3363888888889 45 20 11
(45/1 20/1 11/1)
00 00 00 2d 00 00 00 01
00 00 00 14 00 00 00 01
00 00 00 0b 00 00 00 01
XMP:GPSLatitude 45 deg 20' 11.00" 45.3363888888889 45,20.183333N "45,20.183333N" EXIF:ExposureTime 1/30 0.03333333333 0.03333333333
00 00 00 01 00 00 00 1e EXIF:ShutterSpeedValue 1/30 0.0333333334629176 4.90689059
00 00 4d 9c 00 00 0f d1 EXIF:ModifyDate (set by
2016:11:25 11:56:39 2016:11:25 11:56:39 "2016:11:25 11:56:39\0" XMP:ModifyDate (set by
2016:11:25 11:56:39.00-05:00 2016-11-25T11:56:39.00-05:00 "2016-11-25T11:56:39.00-05:00"
You have the right to know about the metadata contained in your images. A main goal of the Exiftool project is to make this information freely available, both to the general public and as a resource for other developers.
In the design of exiftool, there have been a number of underlying philosophies which have helped to influence the overall development:
At the start of ExifTool development, Perl, Python and C++ were all considered as possible languages for the project. It was recognized that the project would require considerable effort, and the choice of language could heavily influence the amount of work necessary. Python was a strong contender, but was discounted due to a personal preference for C-like syntax. Perl was chosen over C++ for the main reason that it would be less work to develop and support the project. Looking back, this was definitely the right choice, and there was the added benefit of a strong infrastructure in support of testing and distributing Perl software.
Perl 5 is very mature and extremely stable, so there is almost zero time wasted dealing with compilation issues. Compare this to C++, where a majority of development time for a large project may be spent in this area. Also, Perl's built-in regular expressions are fantastically useful for all of the string manipulations necessary for a metadata library. The biggest problem with Perl is its lack of support for Windows Unicode file names.
The bottom line is that most of the development time is spent dealing with the mechanics of metadata, with the result that ExifTool is full-featured metadata library. With one main developer (Phil Harvey) and 250 thousand lines of code (as of Jan 2019), this is a real accomplishment.
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