Understanding the htmlDump option

[greybeard]

The htmlDump option is potentially one of the most helpful ways of digging into image file metadata and showing the details of the EXIF and JPEG metadata.

When a tag for which the actual value data does not fit within the original 12 byte tag block is selected (highlighted in blue or purple when displayed by a suitable browser) this shows the group, group sequence, tag name, tag ID (in hex), format, size, value and up to 4 offsets (in hex).

Can someone explain the exact meaning of the offsets and how they relate to the value location within the image file?

The offsets are named: Value Offset, Actual Offset, Offset Base and File Offset and the number of offsets included varies depending on which type of image file is being viewed (such as jpg, raf, cr3, arw etc.)

JPG files and Canon cr3 files have Value Offset and File Offset. The Value Offset appears to show the offset from the TIFF header and the File Offset shows the offset from the start of the file.

Sony arw and Leica dng files have a single offset (Value Offset) and this shows the offset from the TIFF header which is also the offset from the start of the file.

Nikon nef files have blue highlighted tags which match the standard used for Leica dng and Sony arw but also have purple tags with Value Offset, Actual Offset and Offset Base. The Actual Offset is the sum of the Value Offset and Offset Base fields and indicates the value location within the file.

FujiFilm RAF files have blue and purple tags and all four offsets: Value Offset, Actual Offset, Offset Base and File Offset but I can't figure out a consistent way of using them to calculate the offset of the value within the image file.

I have only looked at a single image file from each type and haven't yet looked at variations from other cameras.

Is there an algorithm for using these offsets to locate the value within the file regardless of image file format?

[StarGeek]

Phil will have to respond, but I believe this paragraph from the Problems with current Metadata Standards-Tiff 6.0 is part of it

A significant problem of the 1992 TIFF 6.0 specification is that there is no way to distinguish an IFD (image file directory) offset from a simple integer value. As a result, new IFD's may not be created without risking corruption of the files by unaware software. This is not only a problem for proprietary maker notes which commonly use a TIFF IFD structure, but is also a problem for extensibility of TIFF-based RAW image formats (as demonstrated by the DNG 1.3 specification -- see below).