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Thumbnail Images

PhotoPrism uses the disintegration/imaging package to generate thumbnail images that are displayed in search results and in the full-screen viewer.

The smallest configurable size limit is 720px, which corresponds to the fit_720 thumbnail type. All sizes up to 720x720 will be generated by default and should therefore always be available.

Besides their obvious use in the frontend, the indexer uses thumbnails for color detection, face detection, and image classification, see Advanced Settings in the User Guide.

Server API

Like most commercial image hosting services, we have chosen to implement a cookie-free thumbnail API to minimize request latency by avoiding unnecessary network traffic:

  • When a browser requests static files such as images from a server over HTTPS, it is generally unnecessary to send a cookie with each request if the URLs cannot be guessed, so for most practical use cases
  • One possible use of cookies may be to prevent the user from intentionally or accidentally sharing confidential thumbnail URLs with others
  • This is possible with most image hosting services/social media sites, and could also be considered a feature if you just want to share a few thumbnails without a lot of bells and whistles
  • Once an image has been downloaded by someone else, blocking the original URL provides little additional security, as digital copies are just as good as the original, see info box below
  • Keeping that in mind, previously shared URLs can be invalidated by changing the security token in your config
  • This will invalidate the browser cache on all connected devices, requiring previously cached thumbnails to be downloaded again
  • Be aware that frequent token changes result in performance degradation and a poor user experience.

In addition to better performance, a major advantage of cookie-free thumbnails is that they can be easily integrated into a content delivery network (CDN), since there is no need to check cookies or add other complex logic on edge servers.

Web Security

Digital copies are as good as originals: Once shared and downloaded, such images should be considered "leaked" because they are cached and can be re-shared by the recipient at any time, with no sure way to get all copies back, even if the download URL becomes invalid or the service is shut down completely.

Any form of protection we could provide would essentially be "snake oil", could be circumvented, and would have a negative impact on the user experience, such as disabling the browser cache or context menu.

For the highest level of protection, it is recommended to shield your private server from the public Internet. Always use HTTPS, a VPN and/or ideally TLS client certificates and make sure that only people you trust have access to your instance.


Since most users have only one domain/host name and modern web applications can store authentication tokens in localStorage, our REST API does not require or use cookies by default.

Image Endpoint URI

Authenticated with SHA1 hash and additional security token:


In public mode without security token:


The thumbnail size and aspect ratio are specified at the end, in this case tile_500, which means 500x500 pixels.

Video Endpoint URI

Authenticated with SHA1 hash and additional security token:


In public mode without security token:


Videos can be streamed if the browser supports it. The format is specified at the end, in this case MPEG-4 AVC.

Thumbnail Types

The smallest configurable static and dynamic size limit is 720px, so all formats up to fit_720 should be generated by default while indexing.

Higher settings allow PhotoPrism to generate thumbnails with more detail in higher resolutions - either statically (while indexing) or dynamically (on-demand).

Name Width Height Common Use
colors 3 3 Color Detection
tile_50 50 50 List Preview
tile_100 100 100 Maps Preview
tile_224 224 224 Mosaic Preview
left_224 224 224 TensorFlow
right_224 224 224 TensorFlow
tile_500 500 500 Cards Preview
fit_720 720 720 Mobile, TV
fit_1280 1280 1024 Mobile, HD Ready TV
fit_1920 1920 1200 Mobile, Full HD TV
fit_2048 2048 2048 Tablets, Cinema 2K
fit_2560 2560 1600 Quad HD, Retina Display
fit_3840 3840 2400 Ultra HD
fit_4096 4096 4096 Ultra HD, Retina 4K
fit_7680 7680 4320 8K Ultra HD 2, Retina 6K

See /internal/thumb/sizes.go for details.

Storage Folders

Generated thumbnails are cached in sub-folders of /storage/cache/thumbnails based on the SHA1 hash of the original file, for example:


Downscaling Filters


Bilinear interpolation takes a weighted average of the four neighborhood pixels to calculate its final interpolated value. The result is a much smoother image than the original image. When all known pixel distances are equal, then the interpolated value is simply their sum divided by four. This technique performs interpolation in both directions, horizontal and vertical. This technique gives better result than nearest neighbor interpolation and take less computation time compared to bicubic interpolation.


Catmull-Rom is a local interpolating spline developed for computer graphics purposes. Its initial use was in design of curves and surfaces, and has recently been used in several applications. Catmull-Rom splines are a family of cubic interpolating splines formulated such that the tangent at each point is calculated using the previous and next point on the spline. The results are similar to ones produced by bicubic interpolation with regards to sharpness, but the Catmull-Rom reconstruction is clearly superior in smooth signal region.


The Lanczos interpolation function is a mathematical formula used to smoothly interpolate the value of a digital image between its samples. It maps each sample of the given image to a translated and scaled copy of the Lanczos kernel, which is a sinc function windowed by the central hump of a dilated sinc function. The sum of these translated and scaled kernels is then evaluated at the desired pixel. Lanczos interpolation has the best properties in terms of detail preservation and minimal generation of aliasing artifacts for geometric transformations not involving strong down sampling. However higher order Lanczos interpolation requires high computational time, which makes it unsuitable for most commercial software.


Blackman is a modification of Lanczos that has better control of ringing artifacts.

Further Reading