The format specs are named following a very simple naming convention described in the Formats page.
Generating thumbnails in JPG formats and a very cool GIF with multiple scenes:
Static thumbnails in JPG and PNG format
Simple Format pattern: jpg|png:$widthx$height
Generate thumbnails in either jpg or png format from the source video.
3 ways to generate thumbnails
By using the number option which is the default, you choose to generate a given number of thumbnails. The images will be spaced equally.
By using offsets (Array), you specifically select 1 or many offsets in second where you want to take the thumbnail.
With interval, you specify an interval in seconds.
Static Images Parameters
Simple Format pattern: gif:$widthx$height
Create an animated GIF from the source video. Width must be <= 500px.
All the generated GIF files are optimized but keep in mind that the result is usually pretty fat: 1-5MB for 5 seconds. Prefer using a small width value to have good quality/size for your preview.
The height is automatically calculated according to the given width and the original aspect ratio.