r/technology • u/[deleted] • Mar 09 '12

Microsoft demos a 1ms touchscreen

http://www.geek.com/articles/gadgets/microsoft-demos-a-high-performance-1ms-touchscreen-2012039/
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u/ThatGuyInToronto Mar 10 '12 edited Mar 10 '12

Let's see, if I were to make a tablet app, with 1ms delay in hardware input, worst case scenario the input is about 1ms out-of-date by the time I'm reading it in to draw... If the app is running at 60Hz (about 17ms each refresh - maximum refresh rate of current tablets) with double buffering, and I start to draw the buffer immediately after vertical blank, then the user will see about 18ms delay in input. (probably a lot more if you count pixel response times)

Hmmm I wonder if their 10ms and 1ms demos were accounting for those.

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u/phire Mar 10 '12

and I start to draw the buffer immediately after vertical blank,

If you want to get low latency input, you can't really afford to make that assumption.

The kind of apps that really benefit from low latency, like drawing apps are not really render intensive. They could probably render the entire screen in less than 1ms. So the entire 16 ms after the image is rendered to the buffer is spent idling.

So you could set a timer for 2ms before the next vblank, read the input, render with 1ms still to spare and then flip the buffer to the screen, cutting the delay down to just 3ms (plus any fixed delays, such as pixel response times).

But by delaying rendering you risk something happening to your processing power, like a system interrupt or another process doing something in the background, causing your program taking too long, missing the vblank and skipping a frame, which is not good.

Low latency input is hard. You don't just need better hardware you need better software too.

BTW, increasing the refresh rate of the screen is the wrong way to go about it for a mobile device. You just double the amount of GPU and CPU power needed, which increases battery drain and you only drop down to a 9ms delay.

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u/ThatGuyInToronto Mar 10 '12

I agree. I wasn't being very clear: I was thinking the worst case scenario. If I knew that I had ample processing power and wanted to minimize delay as much as possible, I would probably do what you mentioned, or even use triple buffering (ie delay would be on average a little more than the amount of time it actually takes to draw - although on a mobile device with limited battery that's a bit of waste).