Diagram: VSYNC OFF input lag of different parts of screen.

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Diagram: VSYNC OFF input lag of different parts of screen.

Post by Chief Blur Buster » 03 Jan 2014, 20:06

I created a diagram for this this G-SYNC thread, however, it's handy to have here because this is interesting general display/graphics discussion.

The input lag of different parts of a refresh are different (that's why the Leo Bodnar input lag tester has three flashing squares - top, center, and bottom edge of screen). Now, we know that GPU frames are rendered in sudden bursts, while a display is scanned out in a continuous fashion. See high speed videos of LCD refreshing to understand how the top edge of the display is refreshed first, then "scanned" to the bottom edge of the screen. This is still the status quo for LCD's, and is the best way since that's how data comes in from the display cable, even DVI and DisplayPort (pixels come in a raster fashion).

This creates input lag disparities at the top edge versus bottom edge of the screen. This is already complex to explain. And even more complicated to explain during VSYNC OFF. I understand this very well, so I've created a simplified diagram of VSYNC OFF input lag of different parts of screen, during 240fps @ 120Hz:


This illustrates how complicated it is to truly measure the input lag of rendered game frames, of different parts of a refresh cycle, during situations where game framerates run at higher than refresh rate. Hopefully this diagram helps explain things better. It keeps scaling as the framerates go higher, even far beyond refresh rate. The higher the framerate, the more frame slices, and the lower latency each frame slice becomes.

This is a simplification obviously, strictly geared to explaining input lag (of different parts of screen) during VSYNC OFF during framerates massively exceeding refresh rate.
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