How to Resize and Crop Videos using FFmpeg

Optimizing videos for different platforms and devices is essential for delivering a seamless user experience, improving load times, and reducing bandwidth costs. Whether you’re preparing videos for mobile, desktop, or social media, resizing and cropping are key steps in adapting content efficiently. In this guide, we’ll walk through how to use FFmpeg to resize and crop videos with precision, covering practical techniques, performance insights, and example commands to make video processing straightforward and effective.

Why resizing and cropping matter?

Key metrics:

• Video file size: Larger videos can exceed several gigabytes. Resizing can reduce video file sizes by up to 70%, making them easier to upload, download, and stream. For example, a 4K video that’s 10GB in size could be resized to 1080p, reducing the file size to around 1.5–3GB, depending on the compression.
• Performance optimization: Videos that are optimized for bandwidth can load up to 3x faster on mobile networks with lower resolutions (e.g., 720p vs 1080p).
• Social media standards: Each platform has its ideal video resolution:
  1. Instagram: 1080x1080 for square videos, which are 1:1 aspect ratio.
  2. YouTube: 1920x1080 for HD videos, optimizing the viewing experience across various devices.
  3. TikTok: 1080x1920 for full-screen vertical videos, a 9:16 aspect ratio.

Resizing and cropping are essential for addressing these requirements, ensuring the best performance and compatibility across devices.

Prerequisites

• FFmpeg installed: Ensure you have FFmpeg installed on your machine. Check ffmpeg version with:

1ffmpeg -version

• Basic command-line knowledge: Some familiarity with the command line is helpful.
• To check video resolution before resizing or cropping, use:

1ffprobe -v error -select_streams v:0 -show_entries stream=width,height -of csv=s=x:p=0 input.mp4

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Step-by-step guide to scaling and resizing with FFmpeg

1. Basic scaling to a specific resolution

The simplest way to scale a video is by specifying the desired width and height. FFmpeg provides a scale filter, which lets you define custom dimensions.

Command example:

1ffmpeg -i input.mp4 -vf "scale=1280:720" output_720p.mp4

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In this command:

• -i input.mp4 specifies the input file.
• -vf "scale=1280:720" applies a video filter (-vf) to resize the video to 1280x720 resolution.
• output_720p.mp4 is the name of the resized output file.

Data insight: Resizing a 4K video (3840x2160) to 720p (1280x720) reduces the file size by about 80%, from an average of 10GB to 1.5GB, with minimal visual quality loss.

2. Maintaining aspect ratio

When resizing, preserving the aspect ratio (width-to-height ratio) is crucial to avoid stretching or squishing the video. You can use the -1 parameter in one of the dimensions to maintain the original aspect ratio automatically.

Command example:

1ffmpeg -i input.mp4 -vf "scale=1280:-1" output_aspect.mp4

Here, setting -1 for the height tells FFmpeg to calculate the height based on the original aspect ratio.

3. Scaling for different platforms

Platforms like YouTube, Instagram, and Facebook have recommended resolutions and aspect ratios. Here’s how to scale to a specific platform:

• Instagram square video (1080x1080):

ffmpeg -i input.mp4 -vf "scale=1080:1080" output_instagram.mp4

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• YouTube HD (1280x720):

1ffmpeg -i input.mp4 -vf "scale=1280:720" output_youtube.mp4

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Scaling to these standards ensures that your video appears optimally across platforms.

4. Using scaling for lower bitrate (bandwidth savings)

When reducing resolution for mobile or web distribution, it’s often essential to lower the bitrate for bandwidth efficiency. FFmpeg allows you to specify the bitrate along with scaling.

Command example:

1ffmpeg -i input.mp4 -vf "scale=640:-1" -b:v 500k output_mobile.mp4

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Here, -b:v 500k specifies the video bitrate to 500 kbps.

Use case: This command reduces the resolution to 640x360 and sets the video bitrate to 500 kbps, making it ideal for low-bandwidth environments like mobile 3G/4G networks.

Performance metric: Reducing the bitrate by 50% can reduce file sizes by up to 60% with negligible perceptual quality loss in mobile-optimized videos.

5. Scaling with advanced filter

If you need more control over scaling quality, FFmpeg’s lanczos scaling algorithm is a great option. It improves the clarity of resized videos but may take longer to process.

Command example:

1ffmpeg -i input.mp4 -vf "scale=1280:720:flags=lanczos" output_highquality.mp4

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Lanczos scaling uses a more sophisticated resampling technique, improving image sharpness compared to basic linear or bilinear scaling.

6. Scaling based on percentage

You can also scale videos by a percentage instead of an absolute resolution. This can be useful for quickly creating smaller versions without calculating exact pixel dimensions.

Command example:

1ffmpeg -i input.mp4 -vf "scale=iw*0.5:ih*0.5" output_halfsize.mp4

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In this example:

• iw and ih stand for "input width" and "input height," respectively.
• iw*0.5 and ih*0.5 scale the video to 50% of its original dimensions.

Use Case: Ideal for creating video thumbnails or smaller versions of the original content.

7. Automating multiple resolutions for adaptive bitrate streaming

For Adaptive Bitrate Streaming (ABR), you may want to generate multiple versions of the same video in different resolutions. Here’s how to create three standard ABR versions (1080p, 720p, and 480p):

1ffmpeg -i input.mp4 -vf "scale=1920:1080" -b:v 5000k output_1080p.mp4
2ffmpeg -i input.mp4 -vf "scale=1280:720" -b:v 3000k output_720p.mp4
3ffmpeg -i input.mp4 -vf "scale=854:480" -b:v 1000k output_480p.mp4

By preparing these versions, you can offer users different resolutions based on their internet speed and device capabilities.

Data point: A video with resolutions at 1080p, 720p, and 480p will dynamically adjust depending on the viewer’s network speed, reducing buffering and improving user experience. Statistics show that videos with multiple bitrate versions have up to 30% fewer buffering events during playback.

Troubleshooting common scaling issues

1. Aspect ratio distortion: If your video looks stretched, double-check that you’re using -1 for one of the dimensions to maintain the aspect ratio.
2. Output file size too large: Try lowering the bitrate (-b:v), which can help reduce file size while retaining decent quality.
3. Video quality drop: If you notice significant quality loss, experiment with flags=lanczos for a better resampling algorithm.

Cropping a video with FFmpeg

FFmpeg offers the crop filter, which allows you to specify the exact dimensions and position of the crop area within a video.

1. Basic cropping

To crop a video, you need to specify:

• width and height of the cropped area.
• x and y coordinates to set where the crop should start.

Command example:

1ffmpeg -i input.mp4 -vf "crop=640:360:100:50" output_cropped.mp4

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In this command:

• 640:360 sets the width and height of the cropped area.
• 100:50 positions the top-left corner of the crop area 100 pixels from the left and 50 pixels from the top of the original video.

This command will create a 640x360 video cropped from the specified (100, 50) position of the original video.

Use case: Removing black bars or centering the action in a video, for example, when preparing content for mobile or social media.

2. Center cropping

If you want to crop from the center of the video, you can use FFmpeg’s iw (input width) and ih (input height) variables to calculate the center coordinates.

Command example:

1ffmpeg -i input.mp4 -vf "crop=640:360:(iw-640)/2:(ih-360)/2" output_center_cropped.mp4

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This command crops a 640x360 region fromthe center of the video by calculating (iw-640)/2 and (ih-360)/2 as thestarting coordinates.

Use case: This is especially useful for videos that were filmed in wide aspect ratios but need to be adapted for platforms that favor square or vertical formats.

3. Command example:

If you need to convert a video’s aspect ratio (e.g., from 16:9 to 4:3), you can use the crop filter to remove excess width or height.

For example, to crop a 16:9 video to 4:3 without distorting the image:

1ffmpeg -i input.mp4 -vf "crop=ih*4/3:ih" output_4by3.mp4

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Here:

• ih*4/3 calculates the width that would achieve a 4:3 aspect ratio based on the original height.

Data insight: By cropping the width and adjusting the aspect ratio, videos will fit the display requirements of platforms or devices that prefer 4:3 over 16:9.

4. Automating cropping with FFmpeg's in_w and in_h

If you are unsure of the exact dimensions or want to automate cropping based on video resolution, use in_w and in_h (input width and height) variables.

Command example:

1ffmpeg -i input.mp4 -vf "crop=in_w/2:in_h/2" output_half_crop.mp4

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This crops the video to half of its original width and height, starting from the top-left corner.

5. Removing black borders

If your video has black borders, you can use ffmpeg’s cropdetect filter to detect and remove them automatically.

Command example:

1ffmpeg -i input.mp4 -vf "cropdetect=24:16:0" -f null -

This will output the suggested cropping values in the console. Once you have these values, you can apply them manually with the crop filter:

1ffmpeg -i input.mp4 -vf "crop=WIDTH:HEIGHT:X:Y" output_no_borders.mp4

Replace WIDTH, HEIGHT, X, and Y with the values returned from cropdetect to achieve an automatic, precise crop.

Insight: Black bars typically account for 10–20% of video content on social media platforms. Removing them can save bandwidth and improve content visibility.

Integrating with FastPix API

Fastpix provides an easy-to-integrate API to stream both live and on-demand content seamlessly using adaptive bitrate streaming. Using FastPix you can automatically scale video and get multiple renditions for ABR support.

Here is an example of how you can scale videos using create media by URL:

1{
2  "inputs": [
3    {
4      "type": "video",
5      "url": "https://static.fastpix.io/sample.mp4"
6    }
7  ],
8  "metadata": {
9    "key1": "value1"
10  },
11  "accessPolicy": "public",
12  "maxResolution": "1080p"
13}
14

Here, you can specify the max resolution of your video files to cap the max resolution of your ABR ladder up to 4k resolution.

Use case: Ideal for streaming services that require multiple resolutions to cater to different devices and network speeds.

Conclusion: Why resize and crop for better performance?

By resizing and cropping videos strategically, you can achieve:

• Faster loading times: Smaller file sizes for quicker streaming and downloading.
• Improved video quality: Resizing with the right scaling algorithms like Lanczosresults in minimal quality degradation.
• Enhanced user experience: Tailored resolutions for each platform, device, and network condition ensure videos look great everywhere.

Through a combination of resizing techniques and cropping strategies, FFmpeg empowers video content creators to optimize their work for both storage and delivery efficiency.

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