RTSS SCANLINE Sync Confusion [Different GPUs for rendering & for outputting]

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RustyClark
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Joined: 22 Jun 2022, 15:25

RTSS SCANLINE Sync Confusion [Different GPUs for rendering & for outputting]

Post by RustyClark » 22 Jun 2022, 15:34

I've been stuck trying to get scanline sync to work. I used the guide found here

phpBB [video]


Everything appears on screen as it seemingly should but when I alter the "sync line 0", now matter what value I put in there the "sync total" remains at 0 and nothing seems to happen at all to the bar graphic on the left side. Vsync is disabled in the game. I think I've tried everything, including running at a bog standard 1920x1080 60hz in the older game Left for Dead with a real fullscreen exclusive enabled, but still nothing happens. I have a Samsung QN90B if that makes any difference.

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Re: RTSS SCANLINE Sync Confusion

Post by Chief Blur Buster » 22 Jun 2022, 20:56

RustyClark wrote:
22 Jun 2022, 15:34
I've been stuck trying to get scanline sync to work. I used the guide found here
https://www.youtube.com/watch?v=sq87lxuVQeg

Everything appears on screen as it seemingly should but when I alter the "sync line 0", now matter what value I put in there the "sync total" remains at 0 and nothing seems to happen at all to the bar graphic on the left side. Vsync is disabled in the game. I think I've tried everything, including running at a bog standard 1920x1080 60hz in the older game Left for Dead with a real fullscreen exclusive enabled, but still nothing happens. I have a Samsung QN90B if that makes any difference.
For simplicity:
- Is VRR disabled on your system?
- Are you using full screen exclusive mode?
- Are you initially testing on a single-monitor system (temporarily disable the extra monitors of a multimonitor system)?
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RustyClark
Posts: 2
Joined: 22 Jun 2022, 15:25

Re: RTSS SCANLINE Sync Confusion

Post by RustyClark » 22 Jun 2022, 23:11

My apologies. I have two GPUs and didn't realize the game was being run on a different GPU than the one outputting to the monitor. Your post reminded me to check that and when I made sure the game was rendered on the same gpu with the display output, it started working. Thanks!

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Re: RTSS SCANLINE Sync Confusion [Different GPUs for rendering & for outputting]

Post by Chief Blur Buster » 23 Jun 2022, 16:32

RustyClark wrote:
22 Jun 2022, 23:11
My apologies. I have two GPUs and didn't realize the game was being run on a different GPU than the one outputting to the monitor. Your post reminded me to check that and when I made sure the game was rendered on the same gpu with the display output, it started working. Thanks!
Yes, that's a tricky situation.

While there are many ways this can happen -- this situation commonly occurs with external GPUs (e.g. rendering on external GPU and outputting to the display connected to the internal GPU). Basically, the framebuffers is (copied, bitblitted, whatever) from one GPU to the other.

This situation also occur with repurposed cryptocurrency mining cards (GPUs without video outputs), where you've obtained a cheap/free retired mining card and use it for gaming instead. There are many cheap outputtless GPUs on the used market as crypto mining gets rid of these cards for pennies on the dollar. It takes a bit of hackery to get them to game, done by giving video-outputting responsibility to a different GPU (the internal GPU).

From the perspective of RTSS Scanline Sync --
This is confusing because the source of the tearing will be generated by the outputting GPU, not the rendering GPU. This is a very tricky situation for RTSS Scanline Sync, since the rasters (aka pixel row number currently being spewed on the GPU output) of the two GPUs are generally not in sync with each other, even when outputting the exact same timings & resolution.

As video output ports are essentially serializations of a 2D imagery on a 1D output, it is outputting one pixel row at a time from the outputting's GPU's perspective. Because of that, RTSS needs to sync to that scanline number of the outputting GPU, not the rendering GPU.

TL;DR: When you have separate GPUs for rendering and for outputting -- you want to sync RTSS to the outputting GPU.
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Forum Rules wrote:  1. Rule #1: Be Nice. This is published forum rule #1. Even To Newbies & People You Disagree With!
  2. Please report rule violations If you see a post that violates forum rules, then report the post.
  3. ALWAYS respect indie testers here. See how indies are bootstrapping Blur Busters research!

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