ARTICLE AD BOX
I’m building a web-based video editing SaaS using Next.js (frontend) and Node.js + FFmpeg (backend export).
The app has two export workflows that share the same export service:
Rankings export – multiple video clips are ordered and exported as one stitched video
Split Screen export – two videos are combined into a top/bottom split layout
Problem Description
Preview behavior (works correctly)
Preview is rendered using React + CSS
Multiple clips play smoothly in the Rankings editor
Ordering is correct
Split Screen preview correctly shows both videos
Subtitles are visible in preview
Export behavior (broken)
FFmpeg export completes without errors
Output file is generated successfully
Rankings export only includes the first video clip
Remaining clips are ignored or cause glitches
Split Screen export only shows one video input
Subtitles do not appear in the exported video
So the issue is not playback, but incorrect FFmpeg rendering logic.
Relevant Technical Details
All source videos are valid MP4 files
Videos may have:
different resolutions
different frame rates
Export is triggered via a Node.js backend using child_process
No FFmpeg errors or warnings are logged
Export currently runs a single FFmpeg command
Suspected Causes
Rankings export
Only the first input video is passed to FFmpeg
No proper concat logic is applied
Clips are not normalized before concatenation
Split Screen export
FFmpeg is invoked with multiple inputs but without a proper filter_complex
Only one input stream is mapped to the output
Subtitles
Subtitles exist only in frontend state (DOM/CSS)
They are not burned into the video during FFmpeg export
What I’m Trying to Do
Rankings export
Take an ordered array of MP4 clips
Normalize them to:
same resolution
same frame rate
same codec
Concatenate them into one smooth MP4 output
Split Screen export
Take two MP4 inputs
Combine them into a top/bottom layout
Output a single 1080×1920 video
Subtitles
Burn subtitles into the exported video so they appear in the final fileWhat I’m Looking For
Correct FFmpeg approach for:
sequential concatenation of multiple clips
parallel compositing (split screen)
Common mistakes that cause FFmpeg to silently export only one input
Best practices for handling mixed FPS/resolution clips during export
What I’m Not Looking For
Frontend preview solutions
iframe/embed-based approaches
UI-level fixes
Key Question
What is the correct FFmpeg strategy to ensure that:
all input videos are included in the export (not just the first)
multi-input layouts (split screen) are rendered correctly
export output matches the preview behavior?
