Understand the Speed of Light of Game Input Latency

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Fredz
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Understand the Speed of Light of Game Input Latency

Post by Fredz »

Understand the Speed of Light of Game Input Latency
by Timothy Lottes

Link : http://timothylottes.blogspot.fr/2013/0 ... input.html

Very interesting article about latency that was published last month, discovered it thanks to a tweet from John Carmack.

Excerpts :

Dissecting PC DX WDDM Graphics Stack Latency
Two to three frames of latency is typical of the PC driver stack. For this reason, many serious gamers disable v-sync, accept tearing, and attempt to maximize frame rate --- all in the name of reducing input latency.

OpenGL and Latency
For OpenGL in Linux at least (and maybe even in Windows for some vendors drivers), route from CPU to GPU is much more direct (no WDDM layer in Linux). Also in OpenGL one can use glFlush() to force the CPU to kick off commands to the GPU instead of waiting for the push buffer segment to fill up.

Quick Estimates of PC Latency
I've removed the driver buffering problem by my CPU input thread writing directly into a pinned memory buffer, with the GPU reading input from pinned memory directly then computing the view matrix and storing to the constant buffer right before the GPU renders the view-dependent part of the frame.

Continuing with a high-end GPU with a gamer mouse and good PC display: 1ms mouse + 6ms drawing + 2ms v-sync window + 8.3ms scanout + 5ms display = 20.3ms. The PC remains king in terms of latency.


Understanding Tiler GPUs and Latency with Low Frame Rates
Mixing low frame rates and a tiler GPU is a latency nightmare. Add this latency to the latency of the tablet touch interface and now you understand why touch based twitch games won't work on iOS mobile...

And yes, he ordered a Rift : http://timothylottes.blogspot.fr/2012/0 ... v-kit.html :)
MSat
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Re: Understand the Speed of Light of Game Input Latency

Post by MSat »

It's good to see developers tackling the issue of input->pixel latency. Even if they're not working on VR content, at least the majority of the rules still apply. The more developers pushing for lower latencies in the OGL/D3D and driver pipelines, the better.
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Fredz
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Re: Understand the Speed of Light of Game Input Latency

Post by Fredz »

He's actually working on VR content, so his research will be very valuable for programmers who target the Rift.

http://timothylottes.blogspot.fr/2012/1 ... l-now.html :
Timothy Lottes wrote:I'm using GL instead of DX for my game (personal independent project), and here is why.
http://timothylottes.blogspot.fr/2012/1 ... 2790916854 :
Timothy Lottes wrote:As for stereo, I'll be supporting the Rift and HDTV 3D via the standard non-driver enabled methods (like split screen)
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cybereality
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Re: Understand the Speed of Light of Game Input Latency

Post by cybereality »

Interesting arguments for OpenGL. The guy certainly sounds like he knows what he's talking about.
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Re: Understand the Speed of Light of Game Input Latency

Post by MSat »

Fredz wrote:He's actually working on VR content, so his research will be very valuable for programmers who target the Rift.

http://timothylottes.blogspot.fr/2012/1 ... l-now.html :
Timothy Lottes wrote:I'm using GL instead of DX for my game (personal independent project), and here is why.
http://timothylottes.blogspot.fr/2012/1 ... 2790916854 :
Timothy Lottes wrote:As for stereo, I'll be supporting the Rift and HDTV 3D via the standard non-driver enabled methods (like split screen)

I wasn't talking about anyone specific. Then again, maybe all this talk about reducing latency is mainly coming from those interested in VR. :)
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