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The mvhd atom or box of the original Quicktime MOV format supports a poster time variable for a timecode to use as a poster frame that can be used in preview scenarios as a thumbnail image or cover picture. As far as I can tell, the ISOBMFF-based MP4 format (.m4v) has inherited this feature, but I cannot find a way to set it using FFmpeg or MP4box or similar cross-platform CLI software. Edit: Actually, neither ISOBMFF nor MP4 imports this feature from MOV. Is there any other way to achieve this, e.g. using something like HEIFʼs derived images with a thmb (see Amendment 2) role?

<code>mvhd</code> box layout

The original Apple Quicktime (Pro) editor did have a menu option for doing just that. (Apple Compressor and Photos could do it, too).

Quicktime menu

To be clear, I do not want to attach a separate image file, which could possibly be a screenshot grabbed from a movie still, as a separate track to the multimedia container. I know how to do that:

I also know that some people used to copy the designated poster frame from its original position to the very first frame, but many automatically generated previews use a later time index, e.g. from 10 seconds, 30 seconds, 10% or 50% into the video stream.

Crissov
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    M4Vs are Apple's MP4 variant, not standard ISO. ISOBMFF mvhd does not have provision for poster frame. ffmpeg won't write it for MOV either. – Gyan May 24 '20 at 15:48
  • I see that both ISO/IEC 14496-14 (MP4) and ISO/IEC14496-12 (ISOBMFF) are [freely available standards](https://standards.iso.org/ittf/PubliclyAvailableStandards/) and have now checked the PDFs myself. The `mvhd` box is described in ISOBMFF 8.2.2 and lacks some of QuickTime's original MOV properties indeed, including `poster time`. MP4 does not extend (nor even mention) this box. I guess @Gyan therefore answers my original question, because I phrased it in a too narrow way. – Crissov May 25 '20 at 06:55

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