Question / Help Does OBS re-encode output from a capture card?

[t0lkien]

I've got the Avermedia Live Gamer Portable, and I know that just now OBS doesn't play well with it, but for my own clarification - when OBS *does* work with a capture card, does it re-process the video stream coming in from the capture card? Or is there conceivably a way to just pass the source stream on without touching it? I ask because if the former - as I have read online - that defeats the purpose of a capture card doesn't it?

Also, is it conceivably possible to let OBS work with the card so that it presents itself as an alternative video source to the card and routes it only that information that has been selected (eg. a dedicated game window)? This would be incredibly useful (unless I'm missing something and there's already ways to do this).

 

[paibox]

I'm not even quite sure what you're talking about. OBS gets video frames from the capture cards, but because not all capture cards use the sample color format, they have to be converted to RGB color, no color loss comes from this. The stream output itself is YUV 4:2:0 output, some color loss may occur when sending the data to the stream itself, but there's really no way around that.

I'm not quite sure what you think capture cards do or how they work, but in almost all cases, the capture cards themselves have to convert the source video to a YUV color format that they then send to the DirectShow drivers, you usually do NOT get the full color range from the console, or the colors you may see on your TV. The cards that do support full RGB 0-255 range are very expensive, and no one is all that likely to have one.

What you see in OBS is what you get. There's no re-encoding, only color conversion to RGB values. As far as the Live Gamer Portable goes, it (just like all other USB 2.0 capture devices capable of HD resolutions) sends compressed video data, which is why it currently doesn't work natively in OBS itself.

 

[t0lkien]

Thanks for the reply, and your patience. I may have misunderstood what capture cards can do. I'm (trying) to use mine to take some load off the CPU by using it in PC mode (i.e. it takes a HDMI video out from the PC which is in "duplicate monitors" mode in windows, so essentially the PC gets one version of what's on the screen and the LGP gets the other). From the reading I did I *thought* the LGP would send the compressed video capture back to the PC at the desired resolution (in my case 768x480 down from 1680x1050), thereby saving the PC CPU the heavy lifting of compression. I thought OBS (or similar software) would then take that compressed video data and send it to the stream (in this case Twitch). So I don't want to make use of OBS's own capture/compression functionality, if that makes sense.

The software that comes with the LGP connects directly to TwitchTV, but of course it sends the entire screen as that is what it has captured and compressed. I would like to make use of OBS's Window selection feature (so only the game window is captured and compressed), but of course that's not possible right now.

Or have I completely misunderstood? Is there no benefit to using a capture card on a PC with OBS (and other software like FFSlit, XSplit etc.)?

 

[paibox]

In general, there is no benefit from using a capture card in a one PC setup. One exception may be if there's a game that is hard to capture using one of the other software methods, but in general (for instance in the case of Game Capture versus a capture card) you are just trading some GPU load for CPU load.

I personally don't know what the encoder on the LGP is like, but if it's anything like the one on the Live Gamer HD, you would need very little CPU to encode at the same quality as that one.

Capture cards were always intended for capturing external sources, but because of XSplit's generally poor software capture performance (screen region, etc.), people ended up using capture cards to capture their desktop/screen for streaming. While a capture card can be faster than using monitor capture (unless you're on Windows 8), Window capture with Aero on and Game Capture will always result in less CPU load for the capture process itself.