Advice with speeding up footage

[jabbason]

Hi, wonder if anyone can advise me with a quandary I’m in. I’m thinking of making a short film that requires the shot footage to be played back at very fast speeds, for example a 10 minute clip played sped up so that it only lasts 30 seconds. Now I understand that if I am using a DV format in PAL that the footage is subject to 25 fps, therefore I can expect to loose a lot of visual information through dropped frames.

Are the frames lost or are they merged?

Also would using a HD video camera increase the frame rate and preserve information at such speeds?

And finally is there another film format that could do this job better?

Would greatly appreciate any input on this subject, thank you.

John

 

[Akribie]

Replay rate for PAL will remain at 25fps, so you will always get a very speeded up effect if you compress time as you suggest. Apart from the obvious difference in quality between DV and HD, there isn't anything to choose between the two formats as far as speeding up is concerned.

The best way is to avoid any frame blending by choosing an integer factor.

If you are using DV, there are at least 4 ways to proceed...

1. Film normally and speed up on the timeline. This would result in a large source file from which most frames would be jettisoned.

2. Film normally and use a utility like Scenalyzer Live to do time-lapse capture from tape. This captures many fewer frames and so the file size on disk is much smaller. If you are not sure how long the final movie should be, you can be conservative about the capture frame interval and speed up (much less than in 1.) on the timeline.

3. Capture from a live feed using the same technique as in 2.

4. Use interval recording on the camera (if it offers that facility). In this case normally about 5 frames are captured each time, so extra processing is needed in post over interval capture via software into a computer.

I don't yet use HD, so am unaware if there are utilities that allow timelapse capture in that format. There are some HD cameras that allow faster frame capture for quite short periods (eg Sony), but this is the opposite of what you are trying to do.

I used the direct capture technique to record a near-total eclipse of the sun over several hours using Scenalyzer. Worked brilliantly except that foolishly I left the exposure on auto so didn't get the darkening (until I cheated) that was visible during the event.

 

[jabbason]

Thanks for the quick reply Akribie.

Why is it important to avoid frame blending? When does this occur? Does it have anything to do with interlacing? What is an integer factor?

I was planning to do it like you said by using the large source file and allowing premiere to do the frame jettisoning.

I suppose the main issue is the limitation placed on what I am proposing to do by the physical limitations of the medium itself. No matter what speed I choose to increase my footage by, the rate of lost frames will be proportional to the speed of playback. I suppose the ideal situation would be a video medium that would allow a playback resolution of say 60 fps, but I imagine this does not exist!

I thought therefore that frame blending (if I understand it correctly) would be a way of compromising by allowing information to be shared across the fewer frames by being blended together. Is this not effective?

 

[Akribie]

Frame blending is more relevant to slowing footage than speeding it up. An interpolated frame is always likely to be less sharp than an original, hence the advice to avoid it.

An integer factor is a whole number. So go 50 times to dump 49 frames between each retained frame. If you opt for, say, 34.6, then there isn't a clean way to do it so a blended frame is likely, with softer results. For this reason it is better to use speed control rather than use timeline resize to adjust speed.

But you can try this yourself on any old footage and see what you think.

 

[jabbason]

I see your point. In the past i have used slow motion whereby the frame blending helps to soften the transition between frames that would otherwise appear clunky.

I will experiment with footage speeded up with and without blended frames. Softer results are not necessarily a problem for what i require, i am more interested in preserving as much information as possible; however what i desire may not be practical, will have to just test it out i guess.

I have done timelapse photography in the past, but in that instance i was doing a shoot over a period of some 15 hours and so opted for a digital stills camera which i then dumped into the timeline of premiere. However for this project i was interested in having a set source file from which multiple rate changes could be applied.

The problem is that i am asking too much of the medium! However it is these same limitations which may create interesting results when i come to experiment with the footage. The subject matter is a lighthouse beam rotating. As i speed the footage faster and faster it gives the appearance that the beam is passing through the lens more rapidly. The problem i was having was that at the speeds i previously mentioned the light had stopped rotating altogether. The frames had removed to the point where the beam appeared to become fixed.

 

[Akribie]

You will need to do some math to ensure that your frame rate doesn't result in an interval that matches the rotation period of the light.

Not sure exactly what effect you are trying to achieve.

If you want the light at normal speed but the rest of the day rushing by, you could film the light at normal speed in the dark and then run the slow-mo capture in daylight with the camera locked off. By superimposing with a luma effect the light would rotate at normal speed whilst the rest of the image rushed by.

If you just want the light to go round faster, then there is a maximum amount you can speed it up before you destroy the illusion. You'll need to ensure that your speeding up is not too closely matched to the rotation period or it will appear to freeze the action.

If you have Premiere Pro CS4 or, better still, After Effects, time remapping is of a higher quality and you have more freedom to ramp the replay speed up and down as required rather than having a fixed speed per clip.

 

[jabbason]

When you say I will have to be careful that the frame rate doesn’t result in an interval that matches the rotation period of the light, what do you mean exactly?

No, I only want to experiment with the speeding up of the light's rotation from the stock footage of it operating at normal speed. I think it is inevitable that eventually the rotation speed will be so great that the light will merely appear to freeze in space. It is my intention to film the light at night, and if necessary use a shorter exposure setting to focus attention only on the rotating light source; it is not so important that the time-lapse be evident from the passage of clouds or changes in light etc.

When you talk about the time remapping functions on CS4 and After Effects are you talking about the ability to gradually change the frame rate across a piece of footage? Or is this something that would improve the quality of a set rate change across a single clip? (once I have the desired rate change it will be applied to the entire clip, I don’t need gradients of rate change)

 

[Akribie]

Well, it the rotation of the light takes 2 seconds and you choose to speed up such that you only take one frame every two seconds, then the rotation will appear to be stopped.

The benefit of time remapping is mostly related to gradual changes of speed, which won't apply in your case if you want a constant speed per clip.

 

[jabbason]

Yeah that’s what I thought. I’m hoping however that there might be a little variation for example in the definition of the lens flaring over the course of a longer source clip (as opposed to the effect if I were using a looped 2 second clip that was exposed to the same rate change, which I imagine would eventually resemble a still frame)

 

[Akribie]

It might be easier to use a combination of the strobe effect and lens flare effect on a static image to create an artificial light under your own control.

 

[jabbason]

thankyou for all your help akribie, time to start experimenting! May i ask, what do you do with film?

 

[Akribie]

I used to make and edit documentaries as a semi-professional and spent some years teaching the UK amateur movie organisation (

http://www.theiac.org.uk)

how to use NLE in the early days.

Nowadays I seem to do more editing for other people than my own productions, but video work is only a small proportion of my current activities so I frequent the forums to keep current between jobs.

 

[jabbason]

Oh i see, i thought you had to be in the industry in some capacity. I'm doing a fine art masters degree and tend to use alot of video for work and documentation. unfortunately for me the uni hasn't got a film department so i'm on my own! cheers for all the help, do you frequent this site often? would be good to pick your brains from time to time if you didn't mind!

 

[Akribie]

I may not have a fine arts degree - mine's in maths and computing - but I do know Premiere etc quite well.

I visit now and again.