Blur Busters Forums

Who you gonna call? The Blur Busters! For Everything Better Than 60Hz™ Skip to content

VSYNC ON lag behavior with big VBI (Large Vertical Total)

Advanced display talk, display hackers, advanced game programmers, scientists, display researchers, display manufacturers, vision researchers. The masters on Blur Busters.

VSYNC ON lag behavior with big VBI (Large Vertical Total)

Postby KKNDT » 18 Jul 2018, 08:36

Here's an example of the timing diagram of an LCD controller/panel:

Image


If we set the parameters in the CRU according to this

Image


Won't this make the actual scanout work like this diagram? It seems to add lag.

SCANOUT.png
SCANOUT.png (14.22 KiB) Viewed 851 times
KKNDT
 
Posts: 47
Joined: 01 Jan 2018, 08:56

VSYNC ON lag behavior with big VBI (Large Vertical Total)

Postby Chief Blur Buster » 18 Jul 2018, 18:19

Depends -- and only if you use VSYNC ON.

With VSYNC OFF, lag differences averages out to nil difference since a random unsynchronized presentation time will average out to being the same regardless of scanout velocity.

Now for VSYNC ON:

That depends on whether presenting the frame is timed to beginning of VBI or end of VBI. The frame and VBI are just endless infinite loop of scanout (like a filmreel) and the frame-presentation intervals can be simply arbitrary in theory -- so fundamentally it doesn't have to add lag. But yes, it is a double edged sword depending on how things are done.

Your question is still a good one for the future of input lag knowledge: Your question should be actually worded as "What is the prevailing frame presentation timing -- beginning of VBI or end of VBI?" -- and this becomes important with large blanking intervals -- and reducing or increasing input lag with VSYNC ON

During variable refresh rate mode, next-frame presentation coincides with beginning of refresh cycles, and historically in fixed refresh rate mode, next-frame presentation coincides with end of refresh cycles. With large VBI, it's favourable to sync next-frame presentation to beginning of refresh cycles.

There is a third-party solution though now. There is a new RTSS scan-line-number-based frame capping feature that will allow frames to sync to the end part of the VBI, rather than the beginning.

Current Windows default driver settings can add lag during VSYNC ON + large vertical totals -- for this purpose, I recommend using the brand new RTSS scan-line-based frame rate capping trick to achieve the "Quick Frame Transport" (QFT) lag-reduction effects. I'd recommend setting the scanline offset to approximately -25 or -50 (negative number) --- to present the frame just slightly before a new refresh cycle begins.

In programmerspeak, basically the Present() or glutSwapBuffers() API timing during VSYNC ON, should ideally now block until the end of vblank before flipping the frame & displaying. Instead of blocking until entry into vblank. This will allow games with lag-optimization logic to synchronize closer to the beginning of active scanout (via inputdelay techniques).

In the long term, graphics drivers will need to fix this to achieve the full "Quick Frame Transport" benefits of large blanking intervals. At least configurable -- e.g. Present() sync to beginning of VBI versus sync to end of VBI.
Head of Blur Busters - BlurBusters.com | TestUFO.com | Follow @BlurBusters on Twitter

       To support Blur Busters:
       • Official List of Best Gaming Monitors
       • List of G-SYNC Monitors
       • List of FreeSync Monitors
       • List of Ultrawide Monitors
User avatar
Chief Blur Buster
Site Admin
 
Posts: 5211
Joined: 05 Dec 2013, 15:44

Re: VSYNC ON lag behavior with big VBI (Large Vertical Total

Postby KKNDT » 20 Jul 2018, 10:05

Chief Blur Buster wrote:What is the prevailing frame presentation timing -- beginning of VBI or end of VBI?


OH Yes! Very accurate capture. This is exactly I alway want to know. Thank you for the explanation. :D
KKNDT
 
Posts: 47
Joined: 01 Jan 2018, 08:56


Return to Area 51: Display Science, Research & Engineering

Who is online

Users browsing this forum: No registered users and 3 guests