Windows Thumbnail Cache
The thumbnail cache ( English for preview image buffer ) is a database in which preview images are stored. This enables the images to be displayed more quickly because these preview images do not have to be recalculated every time the user opens the directory. On the Microsoft Windows - operating systems there is the possibility, in Windows Explorer to display thumbnails.
Use of thumbnails
Since Microsoft Windows 2000 , preview images of graphic files as well as certain documents and video file types such as JPEG , BMP , GIF , PNG , TIFF , AVI , PDF , PPTX , DOCX and HTML are saved so that they do not have to be recalculated each time the thumbnail view is called up. This on the one hand relieves the CPU and on the other hand reduces loading times when a directory with a large number of preview images is to be displayed. The effect of long loading times is clearly visible when the thumbnail view is used on a DVD with thousands of photos and without a thumbnail cache.
In the list and detail view of the file explorer , files and folders are only displayed with a small icon next to the file name. In contrast, in the thumbnail view it is possible to see what the file will look like without having to open it. Images can be better assigned and text documents or presentations can also be found more easily. These thumbnails can also be displayed on a folder icon in Windows Explorer if there are files in this folder that have thumbnails.
The software is responsible for creating the thumbnails, with which these file types can also be opened. This means that the supported file types can vary depending on the system. The thumbnails of Microsoft Office documents are generated by Microsoft Office, for example. If Microsoft Office is not installed on a Windows system, thumbnails are not generated, saved or displayed for these file types. Under Microsoft Windows 10 , PDF files are often displayed without thumbnails, as Adobe Acrobat Reader DC does not support this function correctly in Windows 10. Adobe Inc. has provided a workaround for this. In order to avoid long calculation times, Microsoft recommends that developers generate the preview images when they are saved by the respective programs.
Central thumbnail cache
In Windows 2000, the preview images were saved in the alternative data stream if the operating system is installed on a hard disk with the NTFS file system. During a Windows installation on a FAT32 system, the file Thumbs.db was created hidden for the user in every folder . Windows ME also created Thumbs.db files.
Starting with Microsoft Windows Vista , preview images are stored in a central location to enable access to the images regardless of the storage location and to avoid errors with locally saved Thumbs.db files. The cache is stored in the directory %userprofile%\AppData\Local\Microsoft\Windows\Explorerin the files with the name thumbcache_ xxx .db (where xxx stands for the size of the preview images) and an index so that each preview image and the appropriate size can be found. Preview images are saved with a color resolution of 32-bit and in different sizes, mostly with 32 × 32, 96 × 96 and / or 256 × 256 pixels (but also in other sizes such as 1024 × 1024 pixels), whereby non-square images are proportionally reduced (the longer side is scaled to the respective resolution). Images are shown in the miniature view in their original size, i.e. not interpolated . Thumbnails of Office documents and PDFs usually consist of a preview of the original document and have to be recreated with each saving process. If documents that already have thumbnails in the cache change, the cache is also updated with the thumbnails. If there are no preview images in the cache yet, they will be created.
In addition to the central thumbnail cache, local Thumbs.db files can also be created under newer versions of Windows, for example when accessing hard disks that are not used on the local computer but via the network. This can cause problems when trying to delete these folders as the creation of the Thumbs.db file will lock the directory for a specific time frame. The creation of preview images on remote devices can be deactivated using the Group Policy Object settings.
forensic science
The directory within the thumbnail cache is not automatically updated when changes take place in the file system. Therefore, thumbnails of documents are retained even if they have already been deleted or moved to another location. Over time, the amount of space required on the hard drive increases . As part of Windows' disk cleanup, there is therefore the option of deleting thumbnails and clearing the cache in order to free up space on the hard drive. Disk cleanup can be called up via Windows Explorer.
The preview image database can be opened with special software and the saved thumbnails can be viewed, even if the associated files have already been moved or deleted. Depending on the resolution saved in the thumbnail, it can even restore deleted images. If necessary, these images can also be used in court.
Individual evidence
- ↑ Günter Born: First Aid for the PC: Solve every problem yourself! . Cambridge University Press, 2008, ISBN 978-3-8272-4306-5 , p. 500.
- ↑ Activate PDF thumbnail preview in Windows Explorer . May 9, 2017.
- ↑ Thumbnail Handler Guidelines . May 9, 2017. (English)
- ↑ Thumbnail Handlers . May 9, 2017. (English)
- ↑ Disk cleanup in Windows 10 . May 9, 2017.
- ↑ Thumbcache Viewer - software for opening thumbcache _ *. Db files (English)
- ↑ Windows thumbnail cache ( Memento from October 29, 2013 in the Internet Archive ) (English)