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Reverse engineering of macOS dynamic wallpapers
Quentin Ligier edited this page Feb 27, 2023
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9 revisions
- Choosing the wallpaper depending on the light or dark theme that is enabled;
- choosing the wallpaper depending on the time of the day;
- choosing the wallpaper depending on the sun position.
The file used here for example is the updated Catalina.heic.
Passing the file in exiftool (exiftool -k -G Catalina.heic) gives us:
[ExifTool] ExifTool Version Number : 12.47
[File] File Name : Catalina.heic
[File] File Type : HEIC
[File] File Type Extension : heic
[File] MIME Type : image/heic
[File] Image Width : 6016
[File] Image Height : 6016
[QuickTime] Major Brand : High Efficiency Image Format HEVC still image (.HEIC)
[QuickTime] Handler Type : Picture
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[QuickTime] Image Spatial Extent : 6016x6016
[XMP] XMP Toolkit : XMP Core 5.4.0
[XMP] Solar : 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
[Composite] Image Size : 6016x6016
[Composite] Megapixels : 36.2
Dynamic wallpapers are only heif files (with the .heic extension).
This one is interestingly a square image (6016x6016).
The Image Spatial Extent metadata (and others) indicate there are 8 different images in this single file.
The Solar metadata is interesting, its value seems encoded in base64.
Decoding it gives a string that looks like bplist00, bplist being the magic number of a binary property list.
Converting the binary property list to xml gives the following:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>ap</key>
<dict>
<key>l</key>
<integer>0</integer>
<key>d</key>
<integer>1</integer>
</dict>
<key>si</key>
<array>
<dict>
<key>a</key>
<real>0.0</real>
<key>z</key>
<real>270.0</real>
<key>i</key>
<integer>0</integer>
</dict>
<dict>
<key>a</key>
<real>-25.0</real>
<key>z</key>
<real>70.0</real>
<key>i</key>
<integer>1</integer>
</dict>
<dict>
<key>a</key>
<real>-9.0</real>
<key>z</key>
<real>80.0</real>
<key>i</key>
<integer>2</integer>
</dict>
<dict>
<key>a</key>
<real>0.0</real>
<key>z</key>
<real>90.0</real>
<key>i</key>
<integer>3</integer>
</dict>
<dict>
<key>a</key>
<real>10.0</real>
<key>z</key>
<real>100.0</real>
<key>i</key>
<integer>4</integer>
</dict>
<dict>
<key>a</key>
<real>25.0</real>
<key>z</key>
<real>110.0</real>
<key>i</key>
<integer>5</integer>
</dict>
<dict>
<key>a</key>
<real>25.0</real>
<key>z</key>
<real>250.0</real>
<key>i</key>
<integer>5</integer>
</dict>
<dict>
<key>a</key>
<real>10.0</real>
<key>z</key>
<real>260.0</real>
<key>i</key>
<integer>6</integer>
</dict>
<dict>
<key>a</key>
<real>-9.0</real>
<key>z</key>
<real>280.0</real>
<key>i</key>
<integer>7</integer>
</dict>
<dict>
<key>a</key>
<real>-25.0</real>
<key>z</key>
<real>290.0</real>
<key>i</key>
<integer>1</integer>
</dict>
</array>
</dict>
</plist>The si array contains 10 dictionaries, more than there are images in the file. In those:
- i is the image index (images 1 and 5 are used twice each);
- a is the sun elevation;
- z is the sun azimuth.