101 - Basic Frame Rate Interpolation

[velcrome]

hi vvvv’s

recently a customer came up with a very special angst, he was worried that his 25fps movies will have problems showing on a 60 Hz TFT. He asked specifically for a TFT with either 50 Hz or 75 Hz.

So I started to research the topic to make him less angsty, but it seems that i kinda poked into a wasps nest. The whole topic seems polarized between “who cares, don’t worry about it” and “this is a really horrible thing, it needs a lot of expensive hardware to get rid of it”.

the camera holds the skyline for a moment, then pans slowly down to street level. You recoil in horror as the picture comes completely unhinged. It stutters and shakes like a delirious madman. The buildings are seemingly in the throes of a bizarre earthquake. It hurts to watch it.

So not only is this a highly subjective topic, but also there is not a lot of systematic information about it. I will try to explain what i found out. Maybe you have your own informations you might want to share.

If a movie is played back with a frame rate smaller than the physical output, there needs to be some kind of frame interpolation. For example if the movie has 24fps and the display device runs at 60Hz, naively every even movie frame wouldbe hold for 3 display frames, every odd movie frame for 2 display frames.

this is called 2:3 Pulldown. It is not really a pretty sight (especially at scenes with slow panning), so smarter techniques are used, like interlacing or more advanced motion interpolation.

now there is some things i don’t get. say you don’t have a clear 24p movie, but a different fps, that cannot be interpolated as cleanly to the display devices framerate- is this not bound to have micro stutters at some point?

however, it seems to work well. at least i hardly ever saw something so aweful that i wanted to stop watching.

most current projectors and displays have inherent capabilities to do some more or less quality framerate interpolation. especially tv sets seem really good at it nowadays.

however, i don’t know what’s doing that in a computer context. say you use vlc to play back a video, is it the graphics card handling the frame interpolation? or some internal vlc-magic? is there any difference between vlc playback and a VideoTexture in vvvv?

My comprehension really struggles here conceptually.

The last thing that struck me was a related hardware thing. I read somewhere on the webz that practically all TFTs (but the 3d-ready ones) are running on 60Hz internally, even if they accept different framerates. Unfortunately I was not able to confirm this.

Can someone say if that is bullshit? I don’t even know how to test a given TFT for a thing like that.

[bjoern]

@velcrome

The last thing that struck me was a related hardware thing. I read somewhere on the webz that practically all TFTs (but the 3d-ready ones) are running on 60Hz internally, even if they accept different framerates. Unfortunately I was not able to confirm this.

I recently tested some displays in this regard and from my experience most displays only claim to support framerates other than 60 Hz but actually they don’t - in this case they pulldown as you said, which results in “judder”.

@wikipedia

Normally, when a fixed framerate display such as an LCD screen is used to display a video source whose framerate is less than that of the screen, frames are often simply duplicated as necessary until the timing of the video is matched to that of the screen, which may introduce a visual artifact known as judder, perceived as “jumpiness” in the picture. Motion interpolation intends to remedy this by generating intermediate frames that make animation more fluid.

Motion interpolation

I tested one display (Samsung 400 UX3) that showed this issue only when driven through display-port, DVI and HDMI were fine. So checking the different ports seems reasonable.

For testing:
A simple patch with quads running from left to right / up down should suffice. Set your display to e.g. 50Hz and the renderer accordingly. If the display really supports 50 Hz the motion of the quads should be smooth, whereas if a pulldown occurs you will perceive jumps/judder.

With the Samsung 400 UX3 I also did a side by side comparison, driving one display through DP and the other through DVI, filming it with a Highspeed Camera @ 300 FPS.

edit:
“decent” monitor reviews take this issue into account see for example:
Test Monitor Dell U3011 - Prad.de (sorry german only)

JudderTest.v4p (12.0 kB)

[sebl]

intereting topic

i think a quick and – in terms of image quality - the best solution would be to convert the video’s framerate with an editing software (or even rerender if possible). in AE for example you can then slightly play with the compositions motion-blur and shutter settings.