Microsoft Photo Info Tool Warning

Started Jan 24, 2007 | Discussions thread
Boardhead Senior Member • Posts: 1,122
"SchemaOffset" tag blunder


I just took a look at the new "SchemaOffset" tag written by Microsoft PhotoInfo. There is a fatal flaw in the design of this tag (aside for the annoyance I already mentioned about the poor choice of tag number).

From the document you referenced:

Any time the “Maker Note” is relocated by Windows, the Exif MakerNote tag (37500) is updated automatically to reference the new location. In addition, Windows records the offset (or difference) between the old and new locations in the Exif OffsetSchema tag (59933). If the “Maker Note” contains relative references, the developer can add the value in OffsetSchema to the original references to find the correct information.

My recommendation is for other developers to ignore this tag because the information it contains is unreliable. It will be wrong if the image has been subsequently edited by another application that doesn't recognize the new Microsoft tag.

The new tag unfortunately only gives the difference between the new maker note offset and the original offset. Instead, it should have been designed to store the original offset. The new offset may change if the image is edited, which will invalidate the tag as currently written. If instead the original offset had been stored, the new difference could be easily calculated because the new maker note offset is known.

What a mess.

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