CVDisplayLink constant outputTime value

Question

I'm writing this post in case anyone else is having the same issue I've been having with the lack of documentation for the CVDisplayLink API.

Intro:

In my CVDisplayLink code I've been using the following code to obtain the deltaSeconds value between calls to its callback:

float deltaTime = 1.0 / (outputTime->rateScalar * (float)outputTime->videoTimeScale / (float)outputTime->videoRefreshPeriod);

It seems like this line of code is widely used across different apps & engines.

The issue:

While running my OpenGL app I've noticed that this value is now constant (0.016669 to be precise). I haven't made any big changes to account for this change of behaviour, other than using Mavericks and the new development tools.

Finding the cause has been a lost cause so far.

Answer 1

I've found what I believe is a good way to calculate the deltaSeconds between frames by using the following alternative code:

double deltaSeconds = (outputTime->videoTime - self.previousOutputVideoTime) / (double)outputTime->videoTimeScale;
self.previousOutputVideoTime = outputTime->videoTime;