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Rendered AVI file playing in Slow Motion???

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Rudy Shalamar
Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 3:28:13 pm

I created an 8 second comp and rendered it out as lossless. Now when I go to play the AVI file outside of AE it plays back in slow motion. I can't figure out why it's doing this if the composition was set to 8 seconds?

I did do some time stretching in the comp but would that affect the render?

Thank you!


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Larry S. Evans II
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 4:22:16 pm

Time stretching does exactly that. If you make a clip in the comp 200% longer than the original, then it plays back half as fast (slow motion). Likewise, if you make it 50% of the original size it plays twice as fast. The length of the comp is just a "window" of time that the clip plays against. Altering the time on the clip will always produce some kind of temporal distortion.

Larry S. Evans II

Executive Producer

Digital I Productions


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Rudy Shalamar
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 5:12:50 pm

Oh OK. I imported some image sequences (30 images) but they played SOOO fast.

When I did the "time stretch" it looked perfect but like I said on played back it reallllyyy slowed down and what was set as a 8 second file played back at around 12 second.

How do I stretch them out to play slower? Meaning make a 1 second (30 image) sequence into a 4 second (30 image) sequence without doing the time stretch?

Should I create 4 seperate images for each image in the time sequence? Meaning. Image01a, Image01b, Image01c, Image01d. All of which are the same image but will now run 4 frames total.

Is there another way to stretch the frames?


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Larry S. Evans II
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 6:04:56 pm

There's not really a good answer for that. After Effects has some high-quality frame blending (assuming version CS3/CS4) for dealing with changes in the timescale, where it will actually try to create new frames based on the "gap" between the frames you have and where you'd want them on the time line.

The idea behind frame blending is that the intervening frames make a smoother visual transition between your existing frame 1 and frame 2, so that as they playback you don't get "jerky" motion. The concept, put in simplest terms, determines the average value of a pixel by looking at it's values in frame 1 and frame 2. If you re-time things and get a gap of two frames, then it calculates that mid-point, and moves backward toward each of the original frames, and caluclates an average value between the midpoint and the original.

The more frames you skip between originals, the more that has to be "made up" by the computer, so that long stretch operations can take significant render time and also look rather "phony" when done.

Even if the results are very accurate, what you will get is still a "slow motion" version of the original clip, it will just be smoother.

Now, you are talking about taking a 1 second clip and spreading it to 4 seconds, so all the motion in it will slow down to 1/4 of it's original speed. If that's how fast you want it to appear, then that's fine, and you can turn on the frame blending so After Efects will try to render accurate interframes to fill the gap (in this case, three new frames will need to be created for every existing frame).

Without frame blending, After Effects will simply copy each existing frame three times before moving on to the next one, resulting in a jumpy or jerky playback.

You turn Frame Blending on in the timeline, by checking the box under the little icon that looks like frames of film for each clip you want to use it on. You will also need to click the larger film frame icon in the top of the timeline bar to enable it for all checked layers. Checking the box enables a layer for frame blending. The larger icon turns on frame blending for preview, RAM Playback, etc. so that you can turn it off if you want to work on other parts of a project without having to wait for it to render. Just remember to turn it back on before sending your final work to the render queue.

Now, all that being said, I personally would say that if you have the opportunity to output the original image files at a speed more in line with what you want, that is the best solution. For example, I use Lightwave to create 3D animations that get comped into some video projects. The animations are rendered out of Lightwave to individual Photoshop files with their alpha channel, and then imported as a sequence into After Effects, which is fairly close to what you are doing.

If the timing of the animation is off by a little bit- and I would say never more than 25% or so- it is probably safe and more practical to use time stretching to get the animation to hit it's mark.

Think of it in these terms. If I need to make the animation 25% longer, then it will run 25% slower. This may be just fine, presuming that all other parts of my animation still match up with any other cues in the video sequence.

If they don't I may need to do time-remapping, which essentially moves specific points in a clip to a new point on the timeline, and adjust speed of playback to accomodate that. We actually see this used as a visual effect these days, particularly on CSI, where the motion is superfast, then goes to normal, then back to superfast. It's completely unrealistic, and if you have very large variances in your imported animation when you try to match your background video, then the final shot will look like an episode of CSI (which may not be your intention).

Thus any time-stretching or time-remapping is going to be an "unnatural" effect, and something you want to keep as subtle as possible. To keep it subtle, you have two choices, you can slow the whole thing down a lot and live with the clip being slow motion, or you can have a clip that only needs a bit of slowing down, so that the "unnatural" flow of time isn't noticed.

For an example of the former, consider that you are adding an animated spaceship flyover to your video clip, and it's zooming by 4 times too fast. If you slow it down by making it 400% longer than the original, it will move in "slow motion". However, if it is the only movement in it's respective clip, and the speed is artistically a match for your video clip, then no one will actually see it as slow motion. That is, because all the audience sees is the ship flying by at it's new speed, they assume that this is the speed it was originally produced at (we use overcranked cameras for this kind of thing all the time. A plastic model at 120 fps can look like a full sized object moving at "normal" speed when slowed down to 30fps).

In the second case, let's presume that you are adding the same spaceship, but it also has an animated vapor trail coming out behind it. If you drop the speed by a great deal now, the expansion of the vapor cloud can look like it is moving in slow motion, and not be perceived as "real" by the audience. So in this case, you only want to have to alter the timescale by a small amount, because at some point the shot will look "fake".

If you are trying to get animation to match preshot video (and that's frequently what we compositors are doing), I find it is easiest to take my video (even a rough cut) into the 3D program and use it as a backdrop or projection. I think most 3D animation systems support this kind of thing nowadays, even what one would term "entry level". This allows you to get very close, if not dead on, to the final composite, and then you are only going to have to make small adjustments.

Of course, if you are stuck with a badly timed piece from a client and there is no budget, time, or option to recreate or reshoot, you are stuck with simply making them aware that if you make the video run four times longer, it will run four times slower, and no magic can fix that.



Larry S. Evans II

Executive Producer

Digital I Productions


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Rudy Shalamar
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 6:24:36 pm

Larry THANK YOU!!! Seriously great info. I really appreciate it.

One good thing here is the "jerky/choppy" motion is OK for this project. It is footage (image sequences) of people walking but made to look like they are part of a baseball card. Meaning, the choppy look would actually work but I am still short time wise.

The slow motion part is bad because you can see the blur (arms and legs) of movement so you know its slow mo.

So knowing that the choppy look (missing frames) is OK, can you think of a solution?

1. Maybe make 3 more frames for each frame (4 of the same frame for each image in the sequence).

or

2. Instead of running 30 frames a second, can i change ot to 7.5 frames a second?

Thank you again!!!!


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Larry S. Evans II
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 6:50:09 pm

Okay, knowing what you are trying to do gives me some better idea how to address the question. Unfortunately the answer is that you probably aren't going to be able to get what you want.

As the images you are using already have motion blur in them (as you say -"you can see the blur (arms and legs) of movement") changing the speed of playback is always going to look unnatural, because you will invariably get a "still" that shows that blur. There's not really a way to remove it (there are tons of ways to add it). I honestly don't know if frame-blending would address that or not.

Here is something that you might try, and I can't guarantee that it would work, but it's what I would try to do if I had a similar problem.

I would take the sequence you have a 1 second, and time stretch it to something like 1.5 seconds. then I would render that out to a new AVI file making sure I had turned Frame Blending on.

Once done, I would import that clip, and time stretch it by another 50% (make it 3 seconds long), again turn on Frame Blending and render it out.

You are still going to end up with a "slow motion" clip, but this might give you a more natural look to the motion blur. I really don't know.

Changing the frame rate will have exactly the same effect, and there are a lot of complications to having a clip with one frame rate in a comp with another. But essentially, if it moves at 7.5 fps against a background of 30fps, it will be the same slow motion shot.

After a certain point you have to decide that the clip is just not going to work.

BTW, I posted to your thread regarding how to get the "printed" look of the baseball card. That one, at least, I think I might have been able to help with. In this case though, I'd really see if you can get a better piece of video to start with, or you try to stretch it less.


Larry S. Evans II

Executive Producer

Digital I Productions


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Rudy Shalamar
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 6:55:01 pm

Larry, Thank you so much for your help on this. I really appreciate your efforts. I think I will ask for a more detailed sequence. I think they sent me every 8th frame or something crazy like that.

Thank you again!


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Larry S. Evans II
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 7:07:58 pm

It's entirely possible that if this was something sent via e-mail that they altered the frame rate to make the file smaller. When you import that into a 30fps comp, then you get a sped up playback, and when you try to slow that down, you get a crappy slow-motion playback.

Always best to work with highest quality you can get, even if it means taking the time and bandwidth to FTP or paying for a courier service to get it on a disk.

Good luck.

Larry S. Evans II

Executive Producer

Digital I Productions


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Rudy Shalamar
Re: Rendered AVI file playing in Slow Motion???
on Aug 26, 2009 at 11:56:26 pm

Larry,
I just re-imported the AVI clip (the slow motion one) into AE and then set the frame rate to 7.5 and re-rendered it.

Now it seems to be playing exactly how it played when I RAM previewed in AE. Choppy but it fits the look I am going for.

Can you think of a reason why this is bad idea? Does the quality degrade when you render an AVI clip at a lower frame rate?

Thank you again!!!


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Larry S. Evans II
Re: Rendered AVI file playing in Slow Motion???
on Aug 27, 2009 at 1:49:45 pm

Is it a "bad idea"? Anything that fits your creative vision is inherently a good idea, at least as far as the vision is concerned.

There are technical considerations involved. I think to get it to "stick" at 7.5 fps while the rest of the comp is running in 30fps requires placing it in a comp of it's own and nesting that in the 30fps comp with a specific setting.

Truthfully I haven't done that often enough to remember what the exact requirement is, but if you look around the forums here or in the tutorials at videocopilot.net you'll run across the specifics. It's probably in the manual/help files too, but good luck on finding it.

But bearing that it mind, if you can keep the "fixed" clip running as you want while the others run, you would be well on your way to getting it resolved.


Larry S. Evans II

Executive Producer

Digital I Productions


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