This article provides technical information about how ThumbnailCity works.

For usual video files such as mp4 and FLV, there is no specification for storing thumbnails in the file unlike mp3 audio. Therefor, filers like Explorer extracts one image from the video frames and create a thumbnail for display. In Windows, such a software is called “Thumbnail provider”. Therefore, “changing the thumbnail of a video file” means “changing the way the thumbnail provider chooses the scene”. It is impossible that “Rewriting thumbnail in video file data”.

Thumbnail providers can be replaced for each extension. These for mp4 and wmv are provided as part of the OS, but FLV and others are provided from 3rd party.

Apps such as Explorer requests a thumbnail to the thumbnail provider, but the request has not contain a positional information which frame to be a thumbnail. Therefore, typical thumbnail providers makes unsuitable thumbnails, such as fixed position image in the video.

ThumbnailCity records the thumbnail position for each video file. And then, choose the original thumbnail provider, DirectShow or MediaFoundation to generate thumbnail according to the position information and the type of extension.

With these mechanism, ThumbnailCity realizes the following features.

• The specified thumbnail is displayed again even if the thumbnail cache cleared.
• The original thumbnail is displayed when user not specify a thumbnail yet.
• Even if you move or copy the video file, the position information is also moved or copied. (Shareware version only)

In other hands, there are some restrictions as follows.

• For movies that OS does not support as standard such as FLV, Splitter and Decoder for it are required.
• The thumbnail position information saved for each movie file can be recorded only on NTFS or ReFS.

ThumbnailCity Version 2 and later adds the ability to embed thumbnail position information and cover art images into MP4 container movies.

This makes it possible to do the following, although it is limited to MP4 containers.

• Embedded thumbnail images will be displayed even on Windows 10 or later PCs that do not have ThumbnailCity installed.
• It will also be displayed on Apple devices such as macOS and iOS.
• Thumbnail and bookmarks can be specified even for movie files stored on media without ADS feature such as FAT.
• Even if the specified movie is written to optical media such as CDFS, thumbnail and bookmarks will be displayed.