Efficient Way of Stacking Two Videos Vertically Using FFmpeg

To stack two videos vertically using FFmpeg, you must use the vstack video filter rather than hstack, which arranges clips side-by-side. This process requires that both input videos share identical height and pixel format to ensure seamless merging without distortion or playback errors. The most common pitfall for beginners is attempting to use hstack for vertical arrangements, which results in a horizontal layout instead. Another frequent error occurs when source files have mismatched resolutions; FFmpeg will reject the command if the heights do not match exactly. By preparing your inputs correctly and applying the right filter chain, you can create professional-looking split-screen layouts suitable for presentations, reaction videos, or comparison content.

Understanding Vertical Stacking vs Horizontal Stacking

FFmpeg provides two primary filters for combining multiple video streams into a single output frame: hstack and vstack. It is crucial to distinguish between them based on your desired output orientation. The hstack filter places videos next to each other horizontally, effectively doubling the width while keeping the height constant. In contrast, the vstack filter places one video above the other, doubling the height while maintaining the original width. When constructing your filter graph, selecting the wrong filter is the most immediate cause of unexpected results. If you intend to show a speaker alongside their slides in a top-bottom arrangement, vstack is the correct choice. Using hstack would place the slides to the right or left of the speaker, which may not be the intended visual composition for many tutorial or presentation formats.

Ensuring Input Compatibility

For vstack to function correctly, all input videos must have the same height. They do not need to have the same width, although matching widths often produce the most aesthetically pleasing result. If your source videos have different dimensions, you must resize them before stacking. FFmpeg cannot automatically adjust differing heights during the stacking process, so pre-processing is required. You can use the scale video filter to normalize the height of your inputs. For example, if one video is 1080p and another is 720p, you should scale the 720p video to 1080p height. It is also important to ensure that both videos share the same pixel format, typically yuv420p, which is standard for most media players and web platforms. Mismatched pixel formats can lead to color distortion or complete failure of the encoding process. Concatenate Videos Using FFmpeg: A Comprehensive Guide explains how to handle various input inconsistencies, including resolution mismatches, which is a prerequisite skill for successful filtering operations like stacking.

The Command Structure for vstack

The basic syntax for stacking two videos vertically involves specifying both inputs with -i flags and linking them through the -filter_complex option. The filter complex allows you to manipulate multiple streams simultaneously before encoding them into the final output file. You reference the inputs using labels such as [0:v] for the first video and [1:v] for the second video. Here is the standard command structure for stacking two videos where both are already compatible in height:

ffmpeg -i video1.mp4 -i video2.mp4 -filter_complex "[0:v][1:v]vstack" output.mp4

In this example, [0:v] represents the video stream from the first input file, and [1:v] represents the video stream from the second input file. The vstack filter takes these two labeled streams and combines them vertically. The resulting output will have the first video on top and the second video on the bottom. If your videos have different widths but the same height, they will still stack correctly, but there may be empty space on the sides if the widths differ significantly. To center wider videos within the stack, you might need to add padding or scaling steps prior to the stack operation.

Handling Different Resolutions

When dealing with videos of different resolutions, you must normalize them before applying the vstack filter. Suppose you have a 1920x1080 video and a 1280x720 video. Since the heights differ (1080 vs 720), direct stacking will fail. You need to scale the smaller video to match the height of the larger one. You can achieve this by chaining filters within the complex filter string. First, scale the second video to match the height of the first, then apply the stack. Note that scaling changes the aspect ratio if you force a specific dimension without preserving the aspect ratio flag. Usually, you want to preserve the aspect ratio, so you calculate the new width based on the target height.

ffmpeg -i video_high.mp4 -i video_low.mp4 -filter_complex "[1:v]scale=iw*1080/ih:1080[low_scaled];[0:v][low_scaled]vstack" output.mp4

This command scales video_low.mp4 to a height of 1080 pixels, calculating the width proportionally. The scaled stream is labeled [low_scaled]. Then, [0:v] (the high-res video) and [low_scaled] are passed to vstack. This ensures both streams have the exact same height, allowing FFmpeg to merge them successfully. Increase Blur Power in Video Plays with FFmpeg: A Tech Support Guide details advanced filter chaining techniques that can be useful if you need to apply visual effects to individual clips before combining them.

Frequently Asked Questions

Why does my vstack command fail with an "Invalid argument" error?

This error almost always indicates that the input videos have different heights. FFmpeg's vstack filter requires all stacked streams to have identical vertical dimensions. You must resize the videos to match heights before stacking.

Can I stack more than two videos vertically?

Yes, you can stack any number of videos by adding more inputs and extending the filter chain. For three videos, you would use [0:v][1:v][2:v]vstack, ensuring all three share the same height.

How do I add black bars if the widths don't match?

If widths differ, you can use the pad filter to add black borders to the narrower video to match the widest one. Apply pad before vstack to ensure uniform frame sizes across the entire composite.

Does vstack affect audio tracks?

No, vstack only processes video streams. Audio tracks from the input files are ignored unless explicitly mapped to the output using the -map option. You must manually combine audio if needed.

Is vstack slower than hstack?

There is no significant performance difference between vstack and hstack. Both operations involve similar computational overhead for combining frames. Processing speed depends more on resolution and codec complexity.