Use the -vf scale filter in FFmpeg to adjust video dimensions. For example, ffmpeg -i input.mp4 -vf scale=1920:1080 output.mp4 forces a specific resolution. To maintain aspect ratio, use negative values like -1 or 0 for width or height, ensuring the video fills the screen without distortion.
Understanding Video Resolution Problems
Video files often arrive with incorrect or non-standard dimensions due to encoding errors, camera settings, or previous conversion attempts. These issues manifest as black bars, stretched images, or playback failures on devices that expect standard resolutions like 1920x1080 or 1280x720. FFmpeg provides robust tools to correct these geometric properties without re-encoding the entire stream if only metadata needs updating, though visual changes require processing.
When a video has an odd dimension (not divisible by two), many codecs will refuse to encode it because macroblocks must be aligned. This results in errors like "width not divisible by 2" during the transcoding process. Fixing this requires scaling the video to even dimensions before finalizing the output file.
Basic Scaling Commands
The most common method to fix width and height is using the scale video filter. This filter allows you to specify exact pixel dimensions or use proportional scaling. Below are the essential commands for different scenarios.
Forcing Exact Dimensions
If you need a video to be exactly a certain size, regardless of its original aspect ratio, use both width and height parameters. This may distort the image if the original proportions do not match the target.
ffmpeg -i input.mp4 -vf scale=1920:1080 output.mp4In this command, -i input.mp4 specifies the source file. The -vf flag invokes the video filter chain. scale=1920:1080 sets the target width to 1920 pixels and height to 1080 pixels. The result is a full HD video, but circles will appear as ovals if the input was square.
Maintaining Aspect Ratio
To avoid distortion, let FFmpeg calculate one dimension based on the other. Use -1 for width or height to indicate that the value should be auto-calculated to preserve the aspect ratio.
ffmpeg -i input.mp4 -vf scale=1920:-1 output.mp4This command scales the width to 1920 pixels and automatically calculates the height. If the input is 4:3, the height becomes 1080. If the input is 16:9, the height remains 1080 (since 1920/16*9 = 1080). This ensures the video fills the width without stretching vertically.
ffmpeg -i input.mp4 -vf scale=-1:1080 output.mp4Conversely, setting height to 1080 and width to -1 ensures the video fits within the height constraint while adjusting width proportionally. This is useful for vertical videos or when preparing content for mobile platforms.
Handling Non-Standard or Odd Dimensions
Sometimes videos have dimensions like 1921x1081, which are invalid for many codecs. You can combine scaling with rounding functions to ensure even numbers.
ffmpeg -i input.mp4 -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" output.mp4This complex filter expression uses trunc(iw/2)*2 to round the input width (iw) down to the nearest even number. Similarly, it rounds the height (ih). This prevents codec errors while keeping the video as close to its original size as possible.
Another common issue is unwanted padding or black borders. If the video has letterboxing, you might want to crop it instead of scaling. However, scaling is generally safer for fixing resolution mismatches across different devices.
Optimizing for Different Platforms
Different platforms have strict requirements. YouTube, for instance, prefers H.264 encoded videos with even dimensions. Instagram Reels require a 9:16 aspect ratio. Using FFmpeg, you can enforce these constraints efficiently.
For social media, you might need to pad the video to fit a specific canvas. While scaling changes the visible content, padding adds black bars. To force a 9:16 aspect ratio from a landscape video:
ffmpeg -i input.mp4 -vf "scale=1080:1920,pad=1080:1920:(ow-iw)/2:(oh-ih)/2:black" output.mp4This command first scales the video to 1080x1920. Then, the pad filter adds black padding around the video if it doesn't fill the frame perfectly, centering it. The black argument defines the color of the padding.
If you are converting between formats, such as MP4 to AVI, ensure the resolution is compatible with the target container. Some older AVI players struggle with high-resolution H.264 streams. In such cases, downscaling to 720p or 480p improves compatibility.
Convert MP4 to AVI using FFmpeg on macOS demonstrates how format conversion interacts with resolution settings. When changing containers, always verify that the video codec supports the new dimensions. Compressing Large Talk Video shows how resolution affects file size. Reducing width and height significantly reduces bitrate requirements, leading to smaller files with acceptable quality.Verifying the Output
After running the FFmpeg command, always check the output file. Use the following command to inspect the new dimensions:
ffprobe output.mp4Look for the line containing Stream #0:0(eng): Video: ... , 1920x1080, .... This confirms that the resolution has been applied correctly. If the dimensions are still incorrect, review your filter syntax for typos or missing quotes.
Playback testing is also crucial. Open the file in a media player to visually confirm there is no distortion. If the video looks stretched, revisit the aspect ratio calculation. If it has black bars where none were expected, check if the padding filter was inadvertently included.
Frequently Asked Questions
Why does my video look stretched after scaling?
This happens when you force both width and height to fixed values that do not match the original aspect ratio. Always use -1 for one dimension to preserve proportions, or manually calculate the ratio to avoid distortion.
How do I remove black bars from a video?
Black bars are part of the video frame. To remove them, use the crop filter instead of scale. Specify the exact coordinates and dimensions of the visible area to trim the borders effectively.
Can I scale video without re-encoding?
No, changing resolution requires re-encoding the video stream because pixel data must be recalculated. You can copy audio with -c:a copy to save time, but the video portion will always be processed.
What causes 'width not divisible by 2' errors?
Most video codecs require even dimensions for macroblock alignment. If your source has odd numbers, use the trunc(iw/2)*2 trick in the scale filter to round dimensions down to the nearest even number automatically.
Does scaling reduce video quality?
Downscaling (reducing resolution) generally maintains good quality if done properly. Upscaling (increasing resolution) often introduces blurriness or artifacts because FFmpeg must invent new pixel data that did not exist in the original file.