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QTAKE 1.3 Slow Motion

Any news regarding the use of QTAKE

QTAKE 1.3 Slow Motion

Postby admin » Sat Mar 04, 2017 1:02 am

PROBLEM
One of the biggest problems of the video assist is the inability to playback smooth off-speed. Reason is obvious: cameras usually output 24fps, even though internally they are recording 48fps or more.

So, instead of 48 frames per second, video recording device gets only 24. Now in order to "simulate" how the shot would look like, you set the playback speed to 50% and play it back. Most devices will playback 12 frames per second, which is correctly stretching recorded time, but it's definitely not smooth. The best way to solve this is to record playback from the camera, but often there is no time to do it.

For past few years, we have been looking into this problem. First thing we did was frame blending. This means one frame dissolves into another to generate "missing" frames. We have achieved this in real-time, without rendering. But the result was nice only if there was no big difference between two frames. Not good.

Last year we have worked hard on the true frame interpolation that could produce missing frames closely matching real ones. This is much more complicated than the frame-blending. It requires a lot of computing power to calculate motion vectors and then warp one frame into another. Result was way better, but this approach needed rendering even on the fastest Mac. And it produced some artifacts when there was a parallax motion in the image. Not good.

SOLUTION
QTAKE 1.3 introduces the best way to handle off-speed with video assist - REVERSE PULLDOWN. Most of you know what PULLDOWN is: it's basically a method of converting 24fps film into 60i video by repeating frames in a certain cadence (usually 2-3-2-3). REVERSE PULLDOWN (or pulldown removal) is the process of getting back to original frame rate.

There is a button in QTAKE Project Window with this weird name, but powerful functionality. It detects and discards repeated frames and records only "unique" ones. Now imagine your camera is recording 48fps and outputting 60fps over 3G-SDI. QTAKE can detect those 48 original frames and record it into 24fps timebase clip. Playing back such a clip will result in a SMOOTH 50% PLAYBACK, just as if it was played back from the camera.

So, what do you need to do to utilize this awesome QTAKE feature? Just make sure you have a video card that supports 3G-SDI signal and make sure the output of the camera is set to 1080p60. In addition to smooth off-speed playback you will also get lower signal processing latency, because each frame lasts 16.6ms instead of 41.6ms.

The only downside of this method is the 60fps limit. Recording higher camera speed will result in a "simulated" off-speed playback, but still way better than if you were recording only 24fps.

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Re: QTAKE 1.3 Slow Motion

Postby nerdstation » Tue Apr 11, 2017 4:24 pm

So this wouldn't work on an Alexa mini which outputs 1080i60?
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Re: QTAKE 1.3 Slow Motion

Postby admin » Tue Apr 11, 2017 11:20 pm

Alexa Mini has 3G-SDI output. Are you sure you can't set the output to 1080p60?

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Re: QTAKE 1.3 Slow Motion

Postby merv320 » Thu Apr 13, 2017 12:19 am

Would Qtake be able to record 2 cameras in this way? If so, what kind of speed would you need for the hard drive ?
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Re: QTAKE 1.3 Slow Motion

Postby admin » Sat Apr 15, 2017 10:19 am

Sure, you can record two cameras this way, and it's not that much about the drive speed as it is about CPU, which needs to encode more frames per second. Easy way to test this it to try recording 2x 1080p60 and you will see if your CPU and storage can sustain it.

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Re: QTAKE 1.3 Slow Motion

Postby sheany » Sun Apr 23, 2017 7:16 pm

What would you suggest for a speed of 32 or 36 fps?
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Re: QTAKE 1.3 Slow Motion

Postby admin » Sun Apr 23, 2017 8:28 pm

This works for any speed up to 60fps.

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Re: QTAKE 1.3 Slow Motion

Postby mortenvb » Sun Apr 23, 2017 10:00 pm

It seems that when you enable this feature every render takes a lot longer even when not going for slowmotion.
E.g. for an quick render for an live-sync playback I do not need the extra frames.

Apart from that this is a great feature of Qtake
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