This post is a contribution from Vitaly Lyamin, an engineer with the SharePoint Developer Support team
The Video REST API provides an ability to discover and interact with videos. We’ve seen a number of issues retrieving thumbnails where the “ThumbnailURL” endpoint returns an endpoint (e.g. /pVid/video.mp4.PNG?VideoPreview=1) that is not suitable for use with an AAD app access token and is only accessible via cookie-based authentication (passive or active). Fortunately, there are a few solutions for this.
Solution 1
The Video REST API “ThumbnailStream” endpoint can be used with and without the width (in pixels) parameter.
Examples
https://tenant.sharepoint.com/portals/hub/_api/VideoService/Channels('ac784366-a780-4e21-8bb6-5851c3288906')/Videos('4cafb0e8-8013-40d9-9d5f-370d90e5edc7')/ThumbnailStream
OR
https://tenant.sharepoint.com/portals/hub/_api/VideoService/Channels('ac784366-a780-4e21-8bb6-5851c3288906')/Videos('4cafb0e8-8013-40d9-9d5f-370d90e5edc7')/ThumbnailStream('100')
Resources
https://msdn.microsoft.com/en-us/office/office365/api/video-rest-operations
Solution 2
The “GetPreview.ashx” handler can also be used with several different inputs (simple example below).
Examples
https://tenant.sharepoint.com/portals/hub/_layouts/15/getpreview.ashx?path=https%3A%2F%2Ftenant.sharepoint.com%2Fportals%2FChannel1%2FpVid%2Fmov_bbb.mp4&resolution=1
Resources
Non-MS Resources:
http://www.n8d.at/blog/image-renditions-available-in-modern-team-sites/