Cause: Using "Cover" mode on a vertical video inside a horizontal frame. Fix: Implement a conditional ViewerFrame Mode that checks aspect ratio delta. If the source is taller than it is wide, switch to "Contain" automatically.
Many tools bind ViewerFrame Mode to a key like F8 or a dedicated camera icon. In Unreal Engine, it might be part of the “Game View” or “Immersive View” options. In Unity, similar functionality exists under “Game” tab or “Simulate” mode. viewerframe mode
Cause: Decoding a large source just to crop 80% of it.
Fix: Use canvas cropping pre-processing or request a fragmented MP4 where the viewer requests only the sub-region of the frame via HTTP range requests. Cause: Using "Cover" mode on a vertical video
Power users leverage the scale filter with force_original_aspect_ratio and setsar. pad=1920:1080:(ow-iw)/2:(oh-ih)/2" -c:a copy output.mp4
ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2" -c:a copy output.mp4
This is the classic "Contain" ViewerFrame Mode implementation.
When you have 25 participants in a grid, each cell is a different aspect ratio (some vertical, some horizontal). A unified ViewerFrame Mode policy is essential.