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24 fps smoothness

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Glenn Takakjian
24 fps smoothness
on Jan 17, 2012 at 6:28:02 am

I rendered a file from C4D at 30fps and ran it through AE for final output. It plays beautifully.

I then rendered out the same file from C4D but at 24 fps and then brought it into AE. Made sure it came in at 24... timeline at 24... output at 24.

When I play the two outputs side by side the 24fps is noticeably less smooth than the 30.

On a Mac Pro 12 core. Up to date Snow Leopard. Preview and Quicktime playback shows a noticeable difference.

It's not a terrible jitter... just slightly jumpy from frame to frame.

Is this normal. Is 30 fps the way to go for on computer playback? I've been looking for articles discussing this but can't find anything.

Thanks



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Tudor "Ted" Jelescu
Re: 24 fps smoothness
on Jan 17, 2012 at 7:55:41 am

24 fps is a convention that comes from the days of film. It's at the border where the eye can still perceive a sequence of images as movement, relatively continuous, and at a decent cost. 48 fps for example is a lot smoother, but it would have been costlier to shoot and develop and create work prints for that much film. So it's a compromise that became a standard in time. Some folks like that 24fps feel and use it in video as well because it gives the impression of a film production and thus all the glamour associated with it.
For computer playback, the higher the frame rate the smoother the image, the crisper the edges of the objects in your video and the gfx. It all depends on how big you want your file to be and how much your hardware can handle. Some of the frame rates that people use are 50p or 60p.

Tudor "Ted" Jelescu
Senior VFX Artist


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Chris Wright
Re: 24 fps smoothness
on Jan 17, 2012 at 10:20:07 am

keep in mind, 24fps works best hand-in-hand with 1/48 shutter, so the motion blur hides the fast motion judder. Now if you want smoothness with no blurring whatsoever, you'll need to choose a higher fps.


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Glenn Takakjian
Re: 24 fps smoothness
on Jan 17, 2012 at 3:37:41 pm

Interesting... when you say 1/48 I assume you are referring to the Shutter Angle in the Advance Tab of the Composition Settings? How does that translate to Shutter Angle in Degrees?

My C4D render was taking about 6 minutes per frame. For a 10 second animation , 24 vs 30 fps saved me about 6 hours of render time. Unfortunately I was under the impression it would be smoother.

Live and learn.

Thanks for the response.

TAK



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Dave LaRonde
Re: 24 fps smoothness
on Jan 17, 2012 at 7:49:18 pm

No, he's talking about motion picture projectors in the movie theaters that flash light on the same frame of film twice before advancing to the next frame.

I'm still trying to figure out how that's germane to After Effects myself.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Andrew Somers
Re: 24 fps smoothness
on Jan 17, 2012 at 8:42:25 pm

No, he's talking about motion picture projectors in the movie theaters that flash light on the same frame of film twice before advancing to the next frame.


No he's not - he's talking about the EXPOSURE LENGTH when shooting film at 24 FPS - it is germane to AE and other such software as when you are adjusting the motion blur settings, you are (in effet) adjusting exposure length.

In a camera that uses a 180 degree shutter, the exposure length (at 24 fps) is 1/48. A 1/48 exposure give a certain amount of motion blur - to emulate that, you set your motion blur setting in After Effects (or whatever program is rendering the motion) to that length to emulate the look of film at 24 fps.


This is not related to projecting in a theater. In a theater, the projector uses a two or three blade shutter, so that each frame of film is flashed on screen two or three times between pull downs. i.e. 48 or 72 flashes per second. This is done to reduce flicker and is not related to motion nor motion blur.


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Walter Soyka
Re: 24 fps smoothness
on Jan 17, 2012 at 10:04:12 pm

[Glenn Takakjian] "When I play the two outputs side by side the 24fps is noticeably less smooth than the 30. It's not a terrible jitter... just slightly jumpy from frame to frame."

As the others have said, it could be a mismatch between your frame rate and your refresh rate, or it could be too much motion between frames without enough motion blur to cover it.


[Glenn Takakjian] "My C4D render was taking about 6 minutes per frame. For a 10 second animation , 24 vs 30 fps saved me about 6 hours of render time. Unfortunately I was under the impression it would be smoother."

In the future, you may consider rendering motion blur as a post effect. You can render a motion vector pass (in addition to your other render passes without any motion blur) and feed it to RE:Vision FX's ReelSmart Motion Blur for very good results in a fraction of the time.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events


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Dave LaRonde
Re: 24 fps smoothness
on Jan 17, 2012 at 3:51:18 pm

As the other gentlemen have pointed out, the faster the frame rate, the smoother the motion. Having said that, there is a threshold frame rate in human perception of about 60 fps. That's the point at which motion looks real almost all people.

Douglas Trumbull, the special effects supervisor on 2001: A Space Odyssey, did a lot of research on how humans perceive frame rates, eventually developing the Showscan process. It sounds fancy, but in reality it's simply shooting & projecting film at 60 fps.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Andrew Somers
Re: 24 fps smoothness
on Jan 17, 2012 at 8:07:01 pm

When I play the two outputs side by side the 24fps is noticeably less smooth than the 30.



This is probably related to your computer's monitor refresh rate. What is your refresh rate set at?

For most computers/LCD monitors the*refresh* rate is 60 - and this is a problem for 24 frame video since the computer has to interpolate frames at odd increments to make up for the fact that 60 is not evenly divisible by 24.

Your monitor needs a refresh rate that is evenly divisible by 24, for 24 frame video to play smoothly - such as a rate of 72. OR, the refresh rate needs to be high enough that the interpolated frames are of short enough duration that they are not perceived as jutter.

This is partly why better home theater displays are capably of a true 24P refresh rate, or can refresh at the much higher 120 hz rate. 120 Hz is divisible by 24 by exactly 5.

If you monitor is refreshing at 60 (as most are) then 30 fps will look smoother than 24, especially for certain levels of contrast and motion.


ADDRESSING OTHER COMMENTS:

The integration point of human sensitivity in sight, sound, and touch for "individual impulse stimuli" vs "continuous stimuli" begins at approximately 12 to 16 Hz) though it can be lower, as low as 4 hz for touch). For stimuli with lower frequencies, we perceive individual impulses, vs a continuous stream - but related to this is the differentiation between "on" and "off" of the stimuli - i.e. contrast.

http://en.wikipedia.org/wiki/Psychophysics

I.e. in sound, where we transition rom hearing a series of distinct "clicks" vs a "continuous tone", or in touch, where we feel distinct "taps" vs a vibration. In vision, we begin to relate to motion in 8 to 12 Hs range, with smooth motion appearing above 16 to 20 fps.

FLICKER

The region between 16hz and 75 Hz is very dependent on contrast, that is, the difference between the absolute value of ON vs OFF. But also critical is the length of the ON vs the OFF signal.

But flicker can be perceived as high as 100 Hz and higher.

http://en.wikipedia.org/wiki/Flicker_fusion_threshold


The phosphors in old CRT type PAL monitors were formulated with a longer persistence than those for NTSC monitors because PAL refreshed at a 50Hz rate, and NTSC refreshed at 60Hz.

In your LCD monitor, the backlight is typically going to be operating at a very high rate (NOT related to the refresh rate), so it won't flicker - but the REFRESH rate will have an impact on the perception of *motion*.


MOTION

Motion is processed by a different area of the brain than still images. Things that help trick the brain into perceiving smooth motion include motion blur, number of images per second, and importantly the *equal distancing* of the stream of images.

While the region between 16 fps and 24 fps may have a varied perception of smoothness of motion, at 24 fps, smooth motion is "normally accepted" - but interestingly studies have shown that human's have a different emotional response to higher frame rates - between 24 fps and 72 fps, higher frame rates result in the perception of being "more real". There is no improvement in the "reality" sensation above 72 fps. A hypothesis is that as frame rates increase, the area of the brain that processes that experience shifts with higher frame rates.

In the case of creating a sequence of images intended to trick the brain into perceiving motion, the length of time eace image is exposed will affect the amount of motion blur.

Note that in the film "Saving Private Ryan", in the battle sequences, they set the camera shutter to a very short 45 degrees, to get a "staccato" sharp effect. While you still perceived motion, it had a "snappiness" to it.

Some film cameras allow the shutter to be opened as much as to 220 degrees, allow for increased motion blur - and some digital HD cameras allow for an "equivalent" 360 degree shutter - but in that case, the motion blur is SO long that it "feels like video".


Cinema 4D: If at 24 fps, the equivalent shutter angle for a 1/48th exposure is 180 degrees. At 220 degrees, the equivalent exposure is close to 1/40th


24 FPS films in theaters: Most modern projectors use two or three bladed shutter to project the image on screen two or three time between pull downs - this eliminates flicker that would occur from a single bladed shutter cutting light off at 24 fps.


ShowScan: Showscan is not about making motion seem "more smooth" it's about making the total experience "more real". Running the film at 60fps allows for a small single bladed shutter, resulting in much more light hitting the screen. Also, Showscan is shot on 65mm (70mm projected) so the image area is nearly 4 times that of 35mm. The large image area and high frame rate together reduce the appearance of film grain (and dust), and greatly increase the perception of "being real". Showscan has a nearly 3D feel to it (without the need for glasses, LOL).


However, there are reasons that this is not necessarily desireable in a narrative film - Stu has this to say on the subject:

http://prolost.com/60p


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Dave LaRonde
Re: 24 fps smoothness
on Jan 17, 2012 at 8:42:08 pm

I find it amusing that the ONLY reason 24 fps is with us is because it was the minimum frame rate at which an optical sound track would work. Prior to talkies, it was 18 fps!

People go used to 24 fps: the frame rate of the movies. The frame rate of Hollywood! Of all those MGM musicals! The frame rate of escape. People got USED TO IT. They associate it with movies. And can the movies be wrong?

Never mind that it was the bare-minimum, bargain-basement, lowest-possible frame rate Hollywood tycoons could have. Why go with a higher frame rate? That's just more expensive film going through the camera, and less money in your pocket.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Andrew Somers
Re: 24 fps smoothness
on Jan 17, 2012 at 8:58:22 pm

No, prior to the introduction of sound, films were shot and projected at a widely varying number of rates - from 16 fps to over 26 fps.

In the silent era, each studio had their own standard rate system, and in theaters not controlled by the studios, some theater owners increased frame rates of projection from what was shot, in order to fit in more films per day.

In the sound era, 24 fps was chosen because a single standard was needed for sound, as variations in filming and projection would result in objectionable sound issues - among other things EARLY sound was NOT optical, it was on a separate disk (i.e. a record), that had to run in sync with the film.

Hysteresis synchronous motors allow for accurate motor speed based on AC line power frequency. 24 fps is easy to "sync" using a system built with hysteresis synchronous motors, as is 30 fps. But other "off" frequencies are less straight forward. (a HS motor running at 240 Hz needs only a direct 10:1 gearing ratio for one revolution of a shutter in the camera, etc etc).

24 fps is more economical to shoot than 30 fps, and gives acceptable motion smoothness.

Nevertheless, many Todd-AO "roadshow" pictures were shot at 30 fps, such as the musical "Oklahoma".


Optical sound tracks can work at various different frame rates, including rates lower than 24 fps - the 24fps was chosen more due to the need for a single standard, and a stable speed, which was easiest to achieve using line powered synchronous motors. In 35mm, consider that the film travels at 90 feet per minute. But in 16mm it's 36 feet per minute. Sound tracks don't care about fps, but feet per minute, and 16mm typically used optical tracks as well.


24 fps was considered the best trade off for economy (less film shot), stability, smooth motion, ease of engineering projection systems that ran at the rate, etc etc.


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Dave LaRonde
Re: 24 fps smoothness
on Jan 17, 2012 at 9:39:19 pm

Thanks for the detailed explanation.

It doesn't alter the fact that 24fps is nothing magical, it was made for budgetary concerns, but people got used to seeing it. Now anything else looks wrong, an attitude which I think will change over the next few years.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Glenn Takakjian
Re: 24 fps smoothness
on Jan 18, 2012 at 1:29:43 am

Alot of great info in your posts gentlemen. Thanks.

Just as a point of reference... my animation was a pretty slow camera move trucking, booming down and panning. Not really a need for motion blur. I rarely use motion blur in C4D. I generally always use Reel Blur.

I fear it was ambient occlusion, massive Displacement maps (high sub sampling) and significant soft shadow maps that slowed my renders.

As for the play back, I sense the refresh rate might be the issue. Somehow I assumed since a lot of work I do is for high def TV in 24fps that it would simply play correctly on my Apple Screen.

Now that I have some breathing room I am going to re-render everything at thirty (about 24 hours) and re-output.

Thanks again for all the input.



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