Parses Exif metadata from TIFF-like files. Tested on Canon's CR2 and Olympus' ORF files.
This library provides a low-level API to parse IFD entries from TIFF files: low-level here means that clients of this library need to provide the IFD Entry ID of any entry they would like to retrieve and they also need to know what is its datatype, so that they can later read it into the appropriate type using the provided parser.Read* methods.
This is because there are many manufacturer-specific exceptions to how IFD entries are written, even for basic entries such as imageWidth (uint16 in CR2, uint32 in ORF).
ExifTool - Exif Tags provides a very extensive compendium of all known IFD entries.
See examples/main.go
TIFF header: 8 bytes
CR2 header: 8 bytes
IFD#0:
Number of entries: 2 bytes, ushort
Entry#1: 12 bytes per entry
...
Entry_Exif (pointer to the Exif sub-IFD)
Entry_GPSInfo (pointer to the GPS Info sub-IFD)
...
Entry#N
Pointer to IFD#1: 4 bytes, ulong
Exif sub-IFD:
Number of entries: 2 bytes, ushort
Entry#1: 12 bytes per entry
...
Entry_MakerNote (pointer to the MakerNote sub-IFD)
...
Entry#N
MakerNote sub-IFD:
sequence of bytes containing manufacturer-specific entries, structure depends on manufacturer
GPSInfo sub-IFD: (it may be before or after Exif, it all depends on their respective pointers' values)
Number of entries: 2 bytes
Entry#1 ... Entry#N: 12 bytes per entry
IFD#0 Data Area: variable size
IFD#1:
...
IFD#2:
...
IFD#3:
...
Image#0:
...
Image#1:
...
Image#2:
...
Image#3:
...
The biggest challenge in parsing Exif metadata is that its structure is poorly documented and there are tons of manufacturer-specific exceptions to take into account.
I was able to write this library only thanks to the excellent work created, over the years, by people like Phil Harvey (ExifTool) and Laurent Clévy (Understanding what is stored in a Canon RAW .CR2 file, how and why).