P: Invalid XMP metadata written to exported JPEGs, tripping up reading software

Lightroom 6 often writes invalid XMP metadata in exported JPEGs, including for photos containing lots of brush strokes made with the adjustment brush. This trips up Google Photos, preventing it from showing any of the EXIF metadata. It may well trip up other software.

To reproduce:

1. Start with any image.

2. Use a small adjustment brush with Exposure = 100.

3. Make lots and lots of brush strokes (see the example pic below).

4. Export the image as a JPEG, including all metadata.

5. Load the image to Google Photos and observe that it doesn't show any EXIF metadata.

6. Delete all of the XMP develop settings with:

exiftool -xmp-crs:all= file.jpg

7. Upload that modified file to Google Photos and observe that it now shows the EXIF metadata.

Here's an example pic exported from step 4:

https://dl.dropboxusercontent.com/u/2...

If you extract the XMP metadata with:

exiftool -a -m -b -xmp file.jpg

you'll see that LR has recorded all of the develop settings twice, including all the brush strokes.

Worse, if you examine the file layout with:

exiftool -m -htmldump file.jpg > file.htm

you'll see that the APP1 Extended XMP segments recording the second copy of the develop settings have incorrect segment offsets, with the first segment of the duplicate settings having offset 0, and the following segments with offsets based on that. That's not allowed by the XMP standard, which requires all the segments to have linearly increasing offsets with no gaps. It's easy to imagine how this might trip up a program trying to read the metadata.