To keep mixed videos consistent in an FFmpeg YouTube stream, choose one output canvas, scale each source proportionally to fit inside it, pad any remaining space, and ensure the output uses square pixels. For a conventional landscape stream, a 1920×1080 canvas is one practical choice—not a requirement for every YouTube video. First check the source’s aspect-ratio metadata: blindly forcing square pixels can change how anamorphic footage appears.
Frame size, display shape, and pixel shape are different
Width and height describe a frame’s stored dimensions. Their ratio is not always the shape viewers see, because metadata can specify a sample aspect ratio (SAR): the shape of each stored pixel. The display aspect ratio (DAR) is the resulting displayed width-to-height ratio. A frame stored at one ratio can therefore display at another when its pixels are non-square.
FFmpeg’s FAQ explains: “Most processing filters in FFmpeg handle the aspect ratio to avoid stretching the image: cropping adjusts the DAR to keep the SAR constant, scaling adjusts the SAR to keep the DAR constant.” Inspect the source before overriding its aspect-ratio signaling; FFmpeg documents that setsar and setdar override metadata. FFmpeg FAQ.
Choose the output shape before filtering
Landscape: a fixed 16:9 canvas
If you want a conventional landscape stream, decide on a fixed canvas such as 1920×1080 and make every source fit that canvas. YouTube’s computer player uses 16:9 as its standard, but adapts to other aspect ratios. A 16:9 output is a production choice, not a rule that every YouTube video must follow. YouTube’s aspect-ratio guidance.
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Vertical: compose for 9:16
If the intended output is vertical, choose a 9:16 canvas deliberately—for example, 1080×1920—and decide whether to fit and pad or crop. A center crop of landscape material can cut off the subject or important context.
YouTube Live Control Room can stream horizontal 16:9 and vertical 9:16 at the same time. The vertical rendition is a center crop by default, so keep important content within the area that will survive that crop or configure the vertical layout when both views matter. YouTube’s guidance for dual horizontal and vertical live streams.
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Fit mixed sources into one landscape frame
To preserve the whole picture, scale proportionally until it fits inside the target, then pad the unused area. This avoids stretching and gives differently shaped inputs the same output dimensions. YouTube’s live encoder guidance specifies square pixels (SAR 1:1); FFmpeg’s documented scaling options include resetting SAR while accounting for input DAR. YouTube Live encoder settings · FFmpeg scale filter.
ffmpeg -i input.mp4
-vf "scale=1920:1080:force_original_aspect_ratio=decrease:force_divisible_by=2:reset_sar=1,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,setsar=1"
-c:v libx264 -pix_fmt yuv420p -c:a aac
-f flv rtmp://YOUR_YOUTUBE_INGEST/STREAM_KEYThis is a template, not a verified end-to-end command. Replace the ingest address and secret stream key with the values for your YouTube stream; do not publish the key. Use encoder options suited to your host and the current YouTube settings. Confirm that your installed FFmpeg build recognizes each filter option before relying on it. The filter approach establishes geometry, not your ingest credentials, available bandwidth, encoder build, or bitrate. FFmpeg scale filter · YouTube Live encoder settings.
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What the filter does
force_original_aspect_ratio=decreasescales the input down proportionally so it fits within 1920×1080 without enlarging either dimension beyond the target.force_divisible_by=2keeps the scaled dimensions divisible by two, which can help satisfy codec constraints when used withforce_original_aspect_ratio.reset_sar=1resets the sample aspect ratio while the scaling operation accounts for the input display geometry. The finalsetsar=1explicitly signals square pixels for the output.padcenters the fitted picture in the 1920×1080 canvas. A source that does not match 16:9 will leave unused space at the sides or top and bottom.
FFmpeg versions and builds differ in option support. If an option is rejected, check the installed version’s filter documentation rather than assuming the template will work unchanged.
Fit and pad, or crop to fill?
| Approach | What happens | Choose it when |
|---|---|---|
| Fit and pad | Preserves all source content; unused canvas areas become bars or other padding. | You need to keep the full image, including edges, inside a fixed output size. |
| Crop to fill | Fills the canvas, but removes image edges when source and destination shapes differ. | Filling the frame matters more than retaining every edge; confirm the subject remains in frame. |
FFmpeg documents fit_mode=cover as proportional scaling with cropping and contain as fitting with padding. Availability and interaction with other options depend on the FFmpeg version. Check the documentation for your build before using those options. FFmpeg scale filter.
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For uploaded video, YouTube advises against baking bars into the image because they interfere with dynamic player sizing. That guidance concerns uploads; for a live production, choose the framing you intend viewers to receive rather than adding bars by habit. YouTube’s aspect-ratio guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Normalize a playlist or mixed set of inputs
When clips have different dimensions, apply the same target-canvas logic to each one before combining or sending them onward. Consistent frame dimensions do not require identical source footage: fit-and-pad preserves each clip’s full image, while crop-to-fill trades edge content for a filled frame. Inspect representative clips, especially those with subjects near the edges.
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If you are preparing simultaneous horizontal and vertical live views, do not assume one crop will suit both. YouTube’s default vertical companion is center-cropped; plan safe composition or configure the vertical layout if the framing must differ.
Diagnose stretched, cropped, or inconsistent output
- The subject looks stretched: inspect both the image and its SAR/DAR metadata. Non-square pixels are common, and scaling normally adjusts SAR to preserve DAR. Do not blindly force
setsar=1if it would change the intended display of anamorphic material. FFmpeg FAQ. - Clips have different output dimensions: ensure each one is normalized to the same target width and height before combining or streaming.
- Bars appear around the image: they are the expected result when preserving the whole image on a differently shaped canvas. Use a crop/cover approach only if losing edge content is acceptable.
- The subject is cut off in vertical output: a center crop may have removed it. Recompose for 9:16 or configure the vertical layout when both horizontal and vertical views matter.
- The image remains distorted after filtering: a filter cannot repair footage whose content is already distorted or cropped. Check the source picture and metadata first. FFmpeg FAQ.
- FFmpeg reports an unrecognized option: option support varies by build. Check the installed filter documentation and choose an equivalent supported approach.
Check the stream before an event
Send a test with representative footage, including motion and audio/video, before going live. Monitor stream health and review encoder messages, as YouTube recommends. Confirm that the displayed geometry is right in the player and that important content survives any crop. YouTube Live encoder settings.
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