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Use ChatGPT, prompt: ffmpeg pipeline capture webcam to mp4 ChatGPT: >>> ffmpeg -f v4l2 -framerate 30 -video_size 640x480 -i /dev/video0 -c:v libx264 -preset v

Use ChatGPT, prompt: ffmpeg pipeline capture webcam to mp4 ChatGPT: >>> ffmpeg -f v4l2 -framerate 30 -video_size 640x480 -i /dev/video0 -c:v libx264 -preset veryfast -crf 28 -pix_fmt yuv420p output.mp4 Here's a brief explanation of what each part of the command does: - `f v4l2`: specifies that the input device is a V4L2 (Video for Linux 2) device, which is typically what webcams use. - `framerate 30`: sets the framerate to 30 frames per second. - `video_size 640x480`: sets the video resolution to 640x480 pixels. You can adjust this to your desired resolution. - `i /dev/video0``: specifies the input device to use, which is typically /dev/video0 for the first connected webcam. If you have multiple webcams, you may need to adjust this to the appropriate device. - `c:v libx264``: sets the video codec to use, which is H.264 in this case. - `preset veryfast``: sets the encoding speed preset to "veryfast", which is a good balance between speed and quality. - `crf 28`: sets the video quality using the Constant Rate Factor (CRF) method, with a value of 28. Lower values mean higher quality and larger file sizes, while higher values mean lower quality and smaller file sizes. You can adjust this to your desired quality/file size. - `pix_fmt yuv420p``: sets the pixel format to YUV 4:2:0, which is a commonly used format for H.264 video. - `output.mp4`: specifies the output file name and format. <<< Never used the `crf` or `pix_fmt` flags personally, but seems correct (the issue with ChatGPT).

ttflee 4y ago
You need `-pix_fmt yuv420p` for an iPhone to recognize a ffmpeg-encoded video file.
- djkoolaide 4y ago
  and if you're encoding an x265 file, it needs -tag:v hvc1 for iphone/quicktime to recognize it.
- jeroenhd 4y ago
  Do you happen to know why? Is this a hardware decode limitation perhaps, or a bug in the standard decoder?
  - 867-5309 4y ago
    licensing for a non-free closed-source platform
  - brigade 4y ago
    The capture pixel format depends on device - I think 4:2:2 is pretty common for webcams, which in H.264 requires profiles that aren’t widely implemented outside of x264 and ffmpeg.
  - izacus 4y ago
    YUV420p is the only pixel format that all H.264 compatible decoders MUST support. Other formats are optional. In practice that means that most hardware decoders (not just iPhones, also other phones, TVs, older PC GPUs) won't be able to decode YUV 4:2:2 or YUV 4:4:4 videos. It's kinda annoying, since YUV420 really messes up red text in screen captures for example. You need this parameter to force ffmpeg to convert the color space if your input isn't YUV420P (it defaults to keeping the input pixel format to avoid quality loss).
    - thrdbndndn 4y ago
      >since YUV420 really messes up red text in screen captures for example Is there an detailed explanation why red is especially bad after chroma subsampling? Have been wondering about this for awhile. I assume it's because red has low brightness/luminance (no idea if correct or not)?
      - klodolph 4y ago
        Yes, it’s because red is dim. Dark blue (like, RGB 0, 0, 255) also suffers, perhaps worse, but that shade of blue is hard to reaf even when it is reproduced perfectly.
- onion2k 4y ago
  You also need that same parameter for transparent videos.
thrdbndndn 4y ago
crf: "Constant Rate Factor" for x264 among other encoders. Think it like a output quality (higher = lower quality). I'd say 28 is way too high especially when you're using preset=veryfast (too bad quality). Since it's only 640-480, most of computers can handle fast or even medium in real-time. If not sure, I'd say just use default (by not giving anything). pix_fmt: pixel format. 99% videos use this format, and it has best compatibility. FFMPEG will try to keep the original format when do transcoding, so in most of time you don't need to specify. However, in this very case, since the input is from a webcam, the chance it uses some weird format is high. And you don't want to keep that in your final result. So it's good to specify it out here. My 2c: ChatGPT is great, but I recommend to read the comments it gives about each parameter, try to understand their purposes, and adjust accordingly if needed. Also having a rough idea about how FFMPEG pipeline works (mainly the order of input, output and their associated switches in arguments) helps a lot. Video process is a very complex thing and lots of time it relies on experience. Just be prepared sometimes your "typically works" command would break.
chx 4y ago
> Never used the `crf` or `pix_fmt` flags personally, but seems correct ( Let me translate: the AI might or might not be bullshitting, I have no way to know but I decide to believe in it. Why ?
- ly3xqhl8g9 4y ago
  Why do you trust the descriptions of `ffmpeg -help`? What if some evil daemon [1] went into the binary and changed completely the behaviour of the flags? Do you read the source code and verify the checksums for every program you run? In the real world, good or bad, very few people care beyond a first level of trust: does it work for my current issue? great, no?, try something else. Also, you disingenuously left out the parentheses from the quote: using a tool requires undertaking its downsides, if the downsides can be mitigated accordingly then the tool is useful. Millions of users put to good use imperfect tools daily. [1] https://en.wikipedia.org/wiki/Evil_demon https://en.wikipedia.org/wiki/Evil_demon
  - chx 4y ago
    ChatGPT is a stochastic parrot it doesn't even pass the sniff test, or first level of trust. Does it work? That's meaningless pretty much as everything else that ChatGPT spits out. The information contained in them is zero. If you read something ChatGPT wrote, you know exactly as much as you did before you read it. It sounds plausible and it's basically a zero day on human cognition. Be vigilant. You linked Wikipedia: I don't trust that either. If I want to actually know something I will follow the sources and then evaluate the trustworthiness of the sources. Indeed, most of the Scottish Wikipedia being written by an American teenager who didn't speak Scottish at all very strongly parallels ChatGPT.
izacus 4y ago
Yeah, crf sets the quality and 28 will make your video look like a blurry over-compressed mess. Congrats :)

ttflee 4y ago

You need `-pix_fmt yuv420p` for an iPhone to recognize a ffmpeg-encoded video file.

djkoolaide 4y ago
and if you're encoding an x265 file, it needs -tag:v hvc1 for iphone/quicktime to recognize it.
jeroenhd 4y ago
Do you happen to know why? Is this a hardware decode limitation perhaps, or a bug in the standard decoder?
- 867-5309 4y ago
  licensing for a non-free closed-source platform
- brigade 4y ago
  The capture pixel format depends on device - I think 4:2:2 is pretty common for webcams, which in H.264 requires profiles that aren’t widely implemented outside of x264 and ffmpeg.
- izacus 4y ago
  YUV420p is the only pixel format that all H.264 compatible decoders MUST support. Other formats are optional. In practice that means that most hardware decoders (not just iPhones, also other phones, TVs, older PC GPUs) won't be able to decode YUV 4:2:2 or YUV 4:4:4 videos. It's kinda annoying, since YUV420 really messes up red text in screen captures for example. You need this parameter to force ffmpeg to convert the color space if your input isn't YUV420P (it defaults to keeping the input pixel format to avoid quality loss).
  - thrdbndndn 4y ago
    >since YUV420 really messes up red text in screen captures for example Is there an detailed explanation why red is especially bad after chroma subsampling? Have been wondering about this for awhile. I assume it's because red has low brightness/luminance (no idea if correct or not)?
    - klodolph 4y ago
      Yes, it’s because red is dim. Dark blue (like, RGB 0, 0, 255) also suffers, perhaps worse, but that shade of blue is hard to reaf even when it is reproduced perfectly.
onion2k 4y ago
You also need that same parameter for transparent videos.

thrdbndndn 4y ago

crf: "Constant Rate Factor" for x264 among other encoders. Think it like a output quality (higher = lower quality). I'd say 28 is way too high especially when you're using preset=veryfast (too bad quality). Since it's only 640-480, most of computers can handle fast or even medium in real-time. If not sure, I'd say just use default (by not giving anything). pix_fmt: pixel format. 99% videos use this format, and it has best compatibility. FFMPEG will try to keep the original format when do transcoding, so in most of time you don't need to specify. However, in this very case, since the input is from a webcam, the chance it uses some weird format is high. And you don't want to keep that in your final result. So it's good to specify it out here. My 2c: ChatGPT is great, but I recommend to read the comments it gives about each parameter, try to understand their purposes, and adjust accordingly if needed. Also having a rough idea about how FFMPEG pipeline works (mainly the order of input, output and their associated switches in arguments) helps a lot. Video process is a very complex thing and lots of time it relies on experience. Just be prepared sometimes your "typically works" command would break.

chx 4y ago

> Never used the `crf` or `pix_fmt` flags personally, but seems correct ( Let me translate: the AI might or might not be bullshitting, I have no way to know but I decide to believe in it. Why ?

ly3xqhl8g9 4y ago
Why do you trust the descriptions of `ffmpeg -help`? What if some evil daemon [1] went into the binary and changed completely the behaviour of the flags? Do you read the source code and verify the checksums for every program you run? In the real world, good or bad, very few people care beyond a first level of trust: does it work for my current issue? great, no?, try something else. Also, you disingenuously left out the parentheses from the quote: using a tool requires undertaking its downsides, if the downsides can be mitigated accordingly then the tool is useful. Millions of users put to good use imperfect tools daily. [1] https://en.wikipedia.org/wiki/Evil_demon https://en.wikipedia.org/wiki/Evil_demon
- chx 4y ago
  ChatGPT is a stochastic parrot it doesn't even pass the sniff test, or first level of trust. Does it work? That's meaningless pretty much as everything else that ChatGPT spits out. The information contained in them is zero. If you read something ChatGPT wrote, you know exactly as much as you did before you read it. It sounds plausible and it's basically a zero day on human cognition. Be vigilant. You linked Wikipedia: I don't trust that either. If I want to actually know something I will follow the sources and then evaluate the trustworthiness of the sources. Indeed, most of the Scottish Wikipedia being written by an American teenager who didn't speak Scottish at all very strongly parallels ChatGPT.

izacus 4y ago

Yeah, crf sets the quality and 28 will make your video look like a blurry over-compressed mess. Congrats :)