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Put parameters after /t/, in any order, separated by commas.
The full parameter table lives in the API reference. This page is the shortest path to the things people actually build.

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Recipes

A thumbnail that always has the same dimensions
c_fill crops the overflow, so every thumbnail comes out exactly 400x300 whatever the source ratio. Use c_pad,b_rgb:f5f5f5 instead if you would rather letterbox than crop. An avatar cropped on the subject
g_face picks the most visually salient region rather than the geometric center, which on a portrait is normally the face. Add r_max for a circle:
Rounded corners are cut to transparency, so pair them with f_png, f_webp or f_avif. On a solid background, b_rgb:ffffff fills the corners and lets you stay on JPEG. A banner at a fixed ratio
ar crops to the ratio, w sets the size. You do not need h. The smallest file each browser can take
Request no format and Openinary serves AVIF, WebP or JPEG depending on what the browser accepts. See automatic format selection. A specific format

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Responsive images

Nothing special is required: build a srcset by varying w. Each width is rendered once and cached from then on.

All image parameters

The full table: crop modes, gravity, corners, quality, formats.

Automatic Format Selection

How the default format and quality are chosen.

Upload & Cache Warming

Pre-generate variants so the first visitor hits a warm cache.

Signed URLs

Restrict which transformations can be requested.