Thumbnail Generation and Storage¶
Introduction¶
PhotoPrism currently 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.
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 |
Storage Folders¶
Generated thumbnails are cached in sub-folders of /storage/cache/thumbnails based on the SHA1 hash of the original file,
for example:
/storage/cache/thumbnails/1/a/3/1a30c1f...9_100x100_center.jpg
Downscaling Filters¶
Linear¶
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.
Cubic¶
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.
Lanczos¶
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¶
Blackman is a modification of Lanczos that has better control of ringing artifacts.
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.