How do I make Shotwell regenerate all the thumbnails?

This shell script will regenerate your thumbnails of sizes 128px and 360px so you'll see at least something in viewer.

sqlite3 ~/.local/share/shotwell/data/photo.db \
  "select id||' '||filename from PhotoTable order by timestamp desc" | 
  while read id filename; do
    for size in 128 360; do
      tf=$(printf ~/.cache/shotwell/thumbs/thumbs${size}/thumb%016x.jpg $id);
      test -e "$tf" ||  {
        echo -n "Generating thumb for $filename ($tf)";
        convert "$filename" -auto-orient -thumbnail ${size}x${size} $tf
        echo
      }
    done
  done

An easy way for Shotwell to regenerate thumbnails (this is a workaround):

1. Go to your Photos page.
2. Edit -> Select All
3. Ctrl+R (rotate)
4. When finished rotating, Ctrl+Z (undo).

This will force Shotwell to regenerate all photos and honor any transformations it has stored for them.

As phq mentioned, there is an outstanding bug to fix this so you won't need this workaround. Until then, this is the recommended way to solve this problem. The script posted above will not honor transformations, meaning it's possible for your thumbnails to not look like your photos in Shotwell.

Update With shotwell 0.18 thumbnails are now generated when manually viewed.

Don't remove .shotwell/thumbs!

According to this bug in launchpad, thumbnails are only generated on import.

Thumbnails are only generated at import. If the thumbnails are removed from disk they are not regenerated.

I just used the shell script given in this answer, but I have about 22000 thumbnails to generate.

So here is a bash version of this script, using ~/.shotwell instead of ~/.local/shotwell (which is what I have) and using as many cores as my processor have (8 times faster in my case!):

#!/bin/bash

# under linux, use this to launch as many convert as your processor core number
#MAX_PROCESSES=`cat /proc/cpuinfo |grep ^processor | wc -l`
# or use a static value
MAX_PROCESSES=4

sqlite3 ~/.shotwell/data/photo.db "select id||' '||filename from PhotoTable order by timestamp desc" | 
  while read id filename; do
      for size in 128 360; do
      tf=$(printf ~/.shotwell/thumbs/thumbs${size}/thumb%016x.jpg $id);
      test -e "$tf" ||  {
              echo "Generating thumb for $filename ($tf)";
              convert "$filename" -auto-orient -thumbnail ${size}x${size} $tf &
          RUNNING="`jobs -p |wc -l`"
          while [ "$RUNNING" -ge "$MAX_PROCESSES" ]
          do
          sleep 0.3
          RUNNING="`jobs -p |wc -l`"
          done
      }
      done
  done

Shotwell version 0.13.11 on Ubuntu 12.10 Thumbnails seem no longuer to be located in .shotwell/thumbs but now in .cache/shotwell You may check user has full rigths to read thumbs. Having access to pictures but not to thumbs may cause grey thumbs.

For version 0.26.3 (up-to-date as of Nov. 2017), with Shotwell closed, just delete the thumbnails in

• /home/user_name/.cache/shotwell/thumbs/thumbs128 and in
• /home/user_name/.cache/shotwell/thumbs/thumbs360

and fire up Shotwell, again.

Shotwell 0.28.4 (Braunschweig) will sometimes not create video thumbnails. The following script derived from the accepted answer fixes this:

#!/bin/bash
declare -A default_thumbnail_sizes
declare -A default_thumbnail_checksums
default_thumbnail_sizes[128]="3038"
default_thumbnail_checksums[128]="dc292dd5c9eacadee4fa02c2062d364d8a3a42cb4a58b99abb42dd1950803e4d"
default_thumbnail_sizes[360]="12333"
default_thumbnail_checksums[360]="1f5859761cbbc44f7444b5a61ffd7b2dfe560017d8376905df67db51a4641926"

sqlite3 ~/.local/share/shotwell/data/photo.db \
  "select id||' '||filename from VideoTable" |
  while read id filename
  do
    for size in 128 360
    do
      tf=$(printf ~/.cache/shotwell/thumbs/thumbs${size}/video-%016x.jpg $id);
      exit_code="$?"
      if [[ "$exit_code" != "0" ]]
      then
        echo $id
        continue
      fi

      if [[ -e "$tf" ]]
      then
        tfilelength=$(stat -c '%s' $tf)
        if [[ "$tfilelength" != ${default_thumbnail_sizes[$size]} ]]
        then
          continue
        fi

        tchecksum=$(sha256sum $tf)
        if [[ "$tchecksum" != ${default_thumbnail_checksums[$size]}* ]]
        then
          continue
        fi
      fi

      echo -n "$filename --> $tf";
      ttf=/tmp/$(basename "$tf")
      rm -f "$ttf"
      ffmpeg -i "$filename" -loglevel quiet -vf "thumbnail,scale=${size}:-1" -frames:v 1 "$ttf"
      exit_code="$?"
      if [[ -e "$ttf" && $exit_code == "0" ]]
      then
        echo ": SUCCESS"
        mv -f -v "$ttf" "$tf"
        echo
      else
        echo ": FAILED ($exit_code)"
        echo
      fi
    done
  done