I have two videos that each have a different quality setting. How do I create a side-by-side comparison video of them with ffmpeg?
You can create a side-by-side comparison video using FFmpeg by using the hstack filter. Here's an example of how you can use FFmpeg to create a side-by-side comparison video of two videos:

```bash
ffmpeg -i video1.mp4 -i video2.mp4 -filter_complex [0:v]setpts=PTS-STARTPTS, scale=iw/2:ih[left]; [1:v]setpts=PTS-STARTPTS, scale=iw/2:ih[right]; [left][right]hstack -c:v libx264 -crf 18 -pix_fmt yuv420p output.mp4
```

Here's what each part of the command does:

    -i video1.mp4: This specifies the first input video.
    -i video2.mp4: This specifies the second input video.
    -filter_complex: This specifies the filter graph to apply to the input videos.
    [0:v]setpts=PTS-STARTPTS, scale=iw/2:ih[left]: This takes the first input video and sets the presentation timestamps, scales the video to half its width, and labels the result as left.
    [1:v]setpts=PTS-STARTPTS, scale=iw/2:ih[right]: This takes the second input video and sets the presentation timestamps, scales the video to half its width, and labels the result as right.
    [left][right]hstack: This creates a side-by-side comparison video by stacking the left and right videos horizontally.
    -c:v libx264 -crf 18: This sets the video codec to H.264 and the CRF value to 18, which provides a balance between quality and file size.
    -pix_fmt yuv420p: This sets the pixel format to YUV420P, which is a widely supported format for H.264 encoding.
    output.mp4: This specifies the output file name.

Note that this is just one example of how you can use FFmpeg to create a side-by-side comparison video. You can adjust the parameters as needed to achieve the desired output.
Can I do this without reencoding the video