[FFmpeg-user] 4K 60Hz Directshow Video Capture

[Alex P]

I think I've figured it out. When I use nv12 or yuv420p as the input and output pixel format, I get x1 performance. If I use bgr24/rgb24 as the input and yuv444p as the output, I get around x0.3.

But even when I use bgr0 for the input and output, I get less than x1. Does anyone know what exactly bgr0 is? I can't find any information about it in my googling. 

In your testing James, what was the pixel format? 

-----Original Message-----
From: ffmpeg-user [mailto:ffmpeg-user-bounces at ffmpeg.org] On Behalf Of James Girotti
Sent: Monday, February 12, 2018 7:03 PM
To: FFmpeg user questions
Subject: Re: [FFmpeg-user] 4K 60Hz Directshow Video Capture

>
> ffmpeg -f dshow -video_size 3840x2160 -framerate 60000/1001 -rtbufsize
> 2100000000 -pixel_format bgr24 -i video="MZ0380 PCI, Analog 01 Capture"
> -c:v h264_nvenc -preset lossless -f null - Gives me the same error
>

That's surprising, I can get about 200fps using file-based/ramdisk "-c:v h264_nvenc -preset -lossless". Have you also tried "-c:v hevc_nvenc -preset lossless"? What's the encoding FPS that you're getting? You technically shouldn't be able get much more than 60fps as that's what your capture card is supplying. Can you monitor the "Video Engine Utilization" during encoding? In linux it's listed in the nvidia-settings GUI or "nvidia-smi dmon" on the CLI will show enc/dec%.


> ffmpeg -f dshow -video_size 3840x2160 -framerate 60000/1001 -rtbufsize
> 2100000000 -pixel_format bgr24 -i video="MZ0380 PCI, Analog 01 Capture"
> -c:v rawvideo -f null -
> Gets me nearly x1 performance when executing from a ram disk but
>
> ffmpeg -f dshow -video_size 3840x2160 -framerate 60000/1001 -rtbufsize
> 2100000000 -pixel_format bgr24 -i video="MZ0380 PCI, Analog 01 Capture"
> -c:v rawvideo raw.nut
> Only gets me x0.5 and the buffer overflows.


> Is there a way of accelerating rawvideo decoding? Would using my 
> colleagues 1080 make a difference? Thanks.


I think raw-video is already decoded. So no way/need to accelerate that.
You might try a different pix_fmt from your capture card while using hw-encoding, but you'd have to test. I don't know the internals, i.e. when the pixel format is converted during hw-encoding. So it might make a difference.

Changing pixel formats might be a concern if you are trying to achieve "100% lossless" capture. I've read that yuv444p should be sufficient colorspace for bgr24.

There isn't a lot of info out there on encoding speed differences based on GPU models. It's a complex subject, but from what I have observed the ASIC is tied to the GPU clock (I have observed that GPU clock speed increases as ASIC load increases). If that's true, then a GTX 1080, with it's higher max clock, could have faster encoding, but I have no data to back that up only.
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