In Aperture 3, Why need option ‘detect missing faces’? why not recognise them all in the first scan

Here's how I eventually solved this problem. It took a few steps, and requires some scripting and command line hackery, but it did the trick.

1. Export the pictures you want to update from Aperture to a folder, say ~/Pictures/Updates
2. Open the album in iPhoto

3. Use this AppleScript to get a list of filenames for the master images

   set output_filename to "/Users/user/Pictures/album.txt"
   
   tell application "iPhoto"
       set pics to photos in current album
       do shell script "echo \"# Current album contents\" > " & output_filename
       repeat with pic in pics
           set picpath to image path of pic
           do shell script "echo " & picpath & " >> " & output_filename
       end repeat
   end tell
   

4. Using any text editor, save the script below to update_pics.sh and mark it executable (chmod 755 update_pics.sh on the command line)

   #! /bin/bash
   # a quick script to parse in a series of filenames, and update them
   
   input_folder="$1"
   backup_folder="$2"
   
   if [ "$#" -ne 2 ]; then
           echo "Usage: $0 [folder with new images] [folder for backups]"
           exit 0
   fi
   
   if [ ! -d "$input_folder" ]; then
           echo "Can't read input follder $input_folder"
           exit -1
   fi
   
   if [ ! -d "$backup_folder" ]; then
           mkdir -p "$backup_folder"
   fi
   
   if [ ! -d "$backup_folder" -o ! -w "$backup_folder" ]; then
           echo "Can't write to backup folder $backup_folder"
           exit -1
   fi
   
   while read line; do
   
           # skip empty lines or comments
           [ -z "$line" -o "${line:0:1}" = "#" ] && continue
   
           if [ -f "$line" ]; then
                   filename=`basename "$line"`
                   input_file="$input_folder/$filename"
                   cp "$line" "$backup_folder"
                   if [ -f "$input_file" ]; then
                           echo "Replacing $filename"
                           cp "$input_file" "$line"
                   else
                           echo "Could not find input file $input_file"
                   fi
           else
                   echo "$line does not exist"
           fi
   done
   

5. To use the script, give it the location of the updated pictures, a folder to put backups (I'm paranoid of losing data) and then pipe in the file generated before. For example:

   update_pics.sh Updates/ Backups/ < album.txt
   

6. If you haven't already, close iPhoto. Then restart it while holding down Option and Command. That should give you a dialog to rebuild the iPhoto. You should only need to rebuild the thumbnails, though you may want to rebuild everything else. More info on rebuilding the iPhoto Library from Apple

iPhoto will probably take a long time to rebuild the thumbnails, since it will redo all of them. You have a number of albums to do, it's probably best to update the pictures first, album-by-album, and then rebuild the library. That's what I did, and it worked pretty well.

Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)

This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.

Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.