Arrrghhh!!! slow motion problems

SimonW wrote on 8/16/2006, 6:58 AM
Hi all,

Now, we all know the old 'shoot interlaced and then double the fields for slow motion' trick. Well, I love Vegas because it does do this quite well. WHEN IT FEELS LIKE IT!!

I've just finished some shots that have been filmed in just this very way. I put one of the shots on the timeline (project properties set to Deinterlace interpolate fields as it should be). No problem with this clip.

So I put another clip on the timeline, shot in EXACTLY the same way with EXACTLY the same shutter speed. I slowed it down in EXACTLY the same way as the last clip. I even tried copying the event properties from the last clip to this one. But can I get the damn clip to slow down properly without Vegas adding resampling style motion blur? CAN I HELL! Nothing I do can get this, nor any subsequent clips, to properly slow down using the half speed, interpolate fields trick. Nothing!

What on earth is going on?! I shot all the clips in the same location one after the other. The motion blur is not on the original clips. I shot them interlaced at 1/125th shutter (1/100th wasn't available). But as I mentioned, the first clip I dropped on the timeline works absolutely fine. It slow moed with no trouble at all. No ghosty motion blur, just pure smooth slow motion. Why the hell won't the other clips do the same?

I've tried velocity envelopes, Adjusting the playback rate in the Event Properties Box, time stratching. everything I can think of. And still it won't cooperate. I've tried disabling resampling, and all that does is makes the playback jerky.

Its as if Vegas is just performing a simply deinterlace, and then slowing down the clip. Rather than making a frame out of each field which it SHOULD be doing (and USUALLY does)!

Comments

johnmeyer wrote on 8/16/2006, 7:07 AM
Things to try:

1. Right click and in Properties, Disable Resample.

2. Double-check that Quantize to Frames is turned on (it should be on). Then, grab each edge of the event that isn't behaving and trim (shorten the beginning and end by one frame -- or lengthen by one frame if you the event has more video and longer works better. This will force the edge to snap to frame boundaries.

3. Before doing slow motion, make sure (in event properties) that playback speed is 1.000.

Those are the only things I can think of. The Quantize to Frames seems like the most likely problem.
SimonW wrote on 8/16/2006, 8:56 AM
The quantizing to frames makes no difference. Disbling resample doesn't make any difference either (apart from making the slow motion juddery.

The other clip (now clips, for some reason some of the others have started slowing down properly for no reason), all have resampling switched on.

Usually when this method of slow motion is working properly I can see the very top field of the picture flickering where the alternate fields are being alternated to create a whole frame. On the clips that are working properly this is visible. But not on the clips that aren't working right.

There just doesn't seem to be any explanation for it.
TheHappyFriar wrote on 8/16/2006, 2:10 PM
is it safe to assume they are two different shots with different actions? if the second shot had a lot of action the motion blur could be on the tape, even though it shouldn't be.

how are you previewing the clips? Anything less then "good -full" may not look like what the final output will be.
winrockpost wrote on 8/16/2006, 4:05 PM
...........Now, we all know the old 'shoot interlaced and then double the fields for slow motion' trick......


No we all dont know that, try starting another project and drop the second clip in first and see what it does .
SimonW wrote on 8/17/2006, 12:55 AM
The blur isn't on the original. If I do a straight interploated deinterlace without slowing the clip down there is no motion blur.

Preview is set to Good-Full.
SimonW wrote on 8/17/2006, 1:00 AM
"No we all dont know that"

Okay. Its a neat trick. You shoot the footage that you want to slow down in the cameras interlaced mode (because I'm in PAL land thats 50 fields per second), and with a 1/125th shutter to eliminate or minimise motion blur. Any faster a shutter than this and the slow motion will be jerky.

Now, start a Vegas project set to progressive scan, and deinterlace method set to Interpolate fields. Now the trick is that when you slow down the clip by exactly 50% speed (or stretch it exactly 200%) Vegas (and indeed other NLE's) will create a full frame out of each individual field. This halves the resolution vertically, but allows for very smooth 50fps slow motion.

When performed by a really good denterlacer such as Algolith the results are even better. I might have to resort to this.
DJPadre wrote on 8/17/2006, 8:40 AM
"This halves the resolution vertically, but allows for very smooth 50fps slow motion."

No it doesnt... it doesnt halve anything.. in fact what it uses is thehalf res FIELD as an interpolation reference.. the rest of frame is resolved from that field. In the ned, you still end up with the same 720x576, however the reference is half that...

IMO, i find that shooting progressive scan then halving the speed offers the truest slowmotion possible without the jeriness or stutter, as the pulldown of 2:2 is diched and the second frame (which isnt within the visible Progressive 50i stream) is used as a reference.. so this way u have a full frame reference as opposed to a field which is half the res of the orignal..

slowmotion in any digital format in post is a tricky thing..

dont forget, vegas has a supersampling option... in ur situation, i would strecth to 75%, crank up the supersample, then render as uncompressed DV AVI...
from there, reimport then re-render and slow it down again, now render THAT to DV uncompressed...

By doing it this way, youre creating reference frames or fields (depends on how ur rendering)
The more reference frames u have, the smoother the slow mo...

its moments like these, that the unpredictable nature of slow motion in post gets my goat.. its for this reason i stil believe the best method is to overcrank....