For hardware encoder, why select multiple of them? does it make it faster?

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What would you select for best quality youtube, idc about the size unless its absolutely huge (2x), id like to have av1 as ik its better with lower bitrate and lower quality but should i select qsv as well?

NVENC, QSV, and AMF are the hardware implementations in GPUs produced by Nvidia, Intel, and AMD, respectively. Which ones will be available on your system depends on what GPU(s) you have (including any GPU that may be built into your CPU).

Somewhere near that list on your screen is a button that will make Shotcut detect what is available on your system. If you think the list you show is missing some, you can use that button to detect your hardware. Or if you’re sure that the list is missing some, you can manually enable them. But if (for instance) you have an Nvidia GPU, and either your CPU doesn’t have a built-in GPU or it’s disabled by a system setting, then the NVENC ones are the only ones available to you.

Select the one you feel is best for your use. For Youtube, there’s really no reason to use h.264 if you have HEVC and/or AV1 available. You could try exporting your video twice, once with HEVC and once with AV1, choosing reasonable quality settings for each, and then see what you think about the file sizes and appearance.

In your case, there are three codecs available to you.
If you don’t go into details, you should consider the codecs in this order:

  • h264: high encoding speed, but large file size, the oldest codec.
  • hevc: average encoding speed and good compression, relatively modern codec.
  • av1: low encoding speed and better compression, modern codec.

Despite the fact that YouTube recommends compressing in h264, it accepts all of the above codecs without problems.

If speed is your first priority, then h264. If you want the most efficient compression - then av1. hevc is the golden mean, and good speed and compresses quite well. I usually choose hevc.