Variable dt causes game to run twice as fast (DEF-3146) (SOLVED)

[Johan_Beck-Noren]

@dapetcu21

Wait, does that mean that checking variable_dt should turn off vsync? That doesn’t happen on OS X and that probably shouldn’t happen.

The way I understood variable_dt is that when it’s turned off:

The game displays frames on vsync

update() is called with a dt always equal to 1/update_frequency

update() might not be called every frame if the screen is drawn faster than update_frequency.

update() might be called twice for a frame if the screen is drawn slower than update_frequency.

And when variable_dt is turned on:

The game displays frames on vsync

update() is called with a dt based on diffing system time.

update() is only called exactly once per frame.

Did I understand correctly?

With variable_dt turned off the engine will set the GL swap interval to 60 / update_frequency (OpenGL set swap interval) and will only call script update() at each display refresh, with dt set to 1/update_frequency

With variable_dt turned on the engine does not care about vsync at all (vsync can be enabled/disabled in GPU drivers settings which we have no way of controlling) and will just update as fast as possible (calling script update() each time) with dt being calculated using platform timing functions . This might result in screen tearing (screen tearing) and big variations in dt if for example a fast frame is followed by a frame doing lots of heavy computations.

Defold does not have any decoupling between an engine update and calling script update() and we don’t skip frames to compensate for mismatching refresh rates, we rely entirely on hardware vsync (e.g. the combination of the set swap interval and the driver vsync-settings). A caveat; you can use set_vsync message to achieve software vsync in defold.

Perhaps the documentation around this should be improved. Questions about, variable_dt and vsync etc. pop up every now and then here on the forums. I’ll make a note of it!

Correct me if I’m wrong or missed anything @sven @Mathias_Westerdahl @Andreas_Tadic.