convert from 50p to 25p

[tiffytaffy]

I have a little camcorder that record progressive videos with 1920x1080 and 50 full frames per second.
That is too much for my old hardware. What is a good way to change it to 25 full frames?
I want to use ConvertFPS. But then I have to use the zone parameter, because the new frame rate is less than 2/3.
Which value is useful in this case?

Thanks

[sneaker_ger]

I want to use ConvertFPS. But then I have to use the zone parameter, because the new frame rate is less than 2/3.

Wait, what?

Just drop every second frame, in AviSynth:
SelectEven()

Or x264cli:
--vf select_every:2,0

You might also consider downsizing to 720p50 to keep the full temporal resolution instead, if your hardware is fast enough for that.

[TheSkiller]

Forget about ConvertFPS(), it'll do no good here. There are basically two options:
1) dropping every other frame
2) dropping every other frame and adding motion blur to prevent the image from looking stobe-like which it probably will if it's regular handheld camcorder footage.
You can do this for example with QTGMC, this will give you standalone motion blur (with halved framerate)


QTGMC( "Faster", ShutterBlur=3, ShutterAngleSrc=90, \
ShutterAngleOut=180, SBlurLimit=8, FPSDivisor=2, \
InputType=1, TR0=0, TR1=0, TR2=0, Rep0=0, \
Rep2=0, SMode=0, SLMode=0, Sbb=0, NoiseProcess=0 )

The goal for motion blur that is considered optimal is a motion blur angle of 180 on the final output. In other words half the distance that an object moves between two frames should be the "length" of the motion blur.
You need to tweak ShutterAngleSrc for the correct shutter angle that was used during filming, to do that you need to know the shutter speed used by the camcorder during filming.
For example if the shutter was 1/100 while recording 50p then ShutterAngleSrc=180. If the shutter speed was 1/200 then ShutterAngleSrc=90. If the shutter speed was 1/50 then you don't need to generate any additional motion blur as ShutterAngleSrc would be 360 and the output shutter angle is per definition halved when every other frame is dropped (becomes 180).

[2Bdecided]

Whether you need motion blur depends on the shutter speed of your camcorder. If your camcorder defaults to a very fast/short shutter speed (especially likely in bright sunlight), then relatively fast movement can look really horrible if you simply drop every other frame - almost like a rapid series of still photos with no connection between them. However, if your camcorder defaults to a 1/50th shutter, then you already have what some film makers judge to be the normal/ideal amount of motion blur for 25p.

If you do add motion blur to video shot with a faster/shorter shutter, it'll look less stuttery / more blurry with the blur, which subjectively looks better - but it'll turn a trivial operation (dropping every other frame) into a processor intensive one. If you have an older PC, this could be quite slow.

The cheats way is just to merge adjacent frames; it gives a kind of blur, but introduces visible double images when things move quickly. It's quite quick to do.

Cheers,
David.

[smok3]

The goal for motion blur that is considered optimal is a motion blur angle of 180 on the final output. In other words half the distance that an object moves between two frames should be the "lengh" of the motion blur.
so if i shoot at 50 fps i should not use 1/50, if i shoot at 500 fps, i should not use 1/500? where did you get that from? and how is shutter angle relevant today?

motion blur angle of 180 on the final output.

blur angle, huh?

[Ghitulescu]

The shorter the exposure, the clearer the image. Since this is the reason for using higher framerates, one should not forget to shorten the exposure, at the expense of light performance and the DOF.

[sumawo13]

so if i shoot at 50 fps i should not use 1/50, if i shoot at 500 fps, i should not use 1/500? where did you get that from? and how is shutter angle relevant today?

Shutter angle determines how long the image is exposed for. A 180 degree shutter would mean each frame in a 50p sequence would be exposed for 1/100th of a second, not 1/50th of a second as one might expect.

The formula is explained here:

http://en.wikipedia.org/wiki/Shutter_speed#Cinematographic_shutter_formula

[smok3]

sumawo13: i did actually saw the internals of the "real" 35 film camera once or twice, but i can't see why would one use shutter angle with "digital" video cameras.

[Gavino]

sumawo13: i did actually saw the internals of the "real" 35 film camera once or twice, but i can't see why would one use shutter angle with "digital" video cameras.
Think of it as a metaphor.
It's just a way of expressing the ratio of exposure time to frame duration, as a number between 0 and 360.

[-Vit-]

When I added shutter blur to QTGMC I considered using shutter speeds as input. Unfortunately there is inconsistency and confusion with reported camera shutter speeds on digital cameras, especially with interlaced footage (frame rate vs field rate). I was concerned that people would enter the values they used on the camera, but not get the correct result. In any case, as Gavino notes, these are ratios not absolute measures. MVTools, which provides the core blur functionality uses values 0-100, which is numerically sensible. I chose shutter angle as it relates to a real world measure, albeit one that is less well known.

Shutter angle=0: Pin sharp, no blur at all even on the fastest motion
Shutter angle=360: Motion is blurred all the way from previous frame to the next. If you overlaid sequential frames, the motion blur would "connect" into a contiguous stream.

QTGMC allows ShutterAngleOut > 360 in some circumstances, in which case the motion blur in one frame overlaps that of previous and next frames for an extra blurry look.

Note that using ShutterBlur > 1 or higher SBlurLimit values helps to counter potential errors in the blurring, but they also reduce the blur level a little. So you would slightly increase ShutterAngleOut to compensate.

[TheSkiller]

so if i shoot at 50 fps i should not use 1/50Yes, in fact when shooting 50p (or 25i) then a shutter speed of 1/50 will give you a fully open shutter, the maximum amount of motion blur a camera can deliver. The "length" of the blur is as long as the distance an object moves between two frames/fields, Vit just explained that really well. Of course this may be a matter of taste also but if there's enough light to use 1/100 then I see no reason to use the blurry 1/50 shutter in such cases. I do know it is common for "professionals" to use 1/50 (or 1/60) all the time but from an objective point of view it is giving too much motion blur for the desired framerate of 50p or 25i although it is not as objectionable as it would be if you used 1/25 when recording 25p.

where did you get that from?
It is common film maker knowledge. Guess what's the commonly used shutter speed for hollywood movies which are filmed using a framerate of 24? Yes, it's not 1/24, it's 1/48. ;) And this applies to any camera, digital ones and 35mm film ones.

blur angle, huh? What's the problem, I'm just using the terms QTGMC uses (which happen to be based on a formula) so I don't confuse the OP. I think this is all quite understandable.

[smok3]

of course, i understand all that, but the film camera needs to move film and so it can't really have 360 degrees (it can't be 24 fps at 1/24 due to mechanical reasons) right?

will do some tests with 50fps and 1/50 and maybe 25fps with 1/25. any other interesting ratios?

[2Bdecided]

@TheSkiller - it's entirely subjective. Just like the people who love the "look" of 24fps over 50fps or 60fps.

The nearest thing to Nyquist sampling of time is a 360 degree shutter.

What we're doing with shorter shutters is trying to mitigate the effect of using too low a frame rate, at the expense of introducing temporal aliasing.

Strobing or blur. Pick your poison. Or pick "the look" you want - not that you can guarantee that any particular "look" wrt motion will be faithfully displayed on a modern TV.

Cheers,
David.

[Didée]

not that you can guarantee that any particular "look" wrt motion will be faithfully displayed on a modern TV.

That's an important point nowadays. The "original look" of 24p often is just "not available" anymore. With LCD TVs, it's a plain impossibility (Though some restraint(!) motion interpolation does improve the experience, compared to the full load of retina blurring/ghosting that you'll get otherwise). Plasma displays are somewhat more close, but still not the true thing ...

Technology improves, but somehow things don't get better. :D

[tiffytaffy]

Thank you for your interesting comments.

I realized that my camcorder record not a real 50p. I guess It doubles every frame, because when I watch frame by frame, I can see there is a motion after every second frame only.
That is why SelectEven() is a good choice in my case.
I try to find out some technical details about the shutter, but all I found is, that the camera has a rolling shutter. It has a CMOS sensor. I don't mean the rolling shutter effect, but it don't capture the whole frames at once.

[TheSkiller]

Yes, CMOS sensors always have a rolling shutter (which does not imply that the "jello" effect has to be visible all the time), so the top of the frame is captured not at the same time as the bottom.
However the time the whole frame, top to bottom, is exposed to the sensor is still there, the shutter speed. Some consumer camcorders allow the user to tweak or at least view the shutter speed, some don't. Usually such a camera uses a high shutter speed (shorter than at least 1/50 of a second) whenever there is lots of light (outside, especially when it's sunny), simply because that's the only thing left for it to do to prevent overexposure.