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Turbulent Displace Expression problem

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Mark Jobe
Turbulent Displace Expression problem
on Mar 1, 2011 at 9:49:01 pm

I've got a cloud image being deformed by a turbulent displace effect with an expression added to the evolution which is time*100 The option to cycle the evolution is switched off.

At 05:27:17 in the timeline the effect stops displacing which I only noticed because it looked like the effect was popping. Is there any reason why the evolution would stop working after 5 minutes? There are no keyframes anywhere in my composition. The only thing driving the effect are the parameters and the expression.

If anyone could offer a solution that would be a great help.

Thanks


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Walter Soyka
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 2:06:08 am

[Mark Jobe] "At 05:27:17 in the timeline the effect stops displacing which I only noticed because it looked like the effect was popping. Is there any reason why the evolution would stop working after 5 minutes?"

Yup -- at 5:27;21, with an evolution of time*100, you're at 91x +9.4 degrees. Doing the math, that's 32,769.4. The frame before, you're at 91x +6.1 degrees (or 32,766.1).

When a 16-bit area of memory is used by a computer to represent a number, it can count up to 65,536 (2^16). Since rotation can be positive or negative, we have to halve that number. As soon as your expression's value crosses 2^15 (32,768), you get unpredictable results because the computer can't count high enough with the amount of memory it has allocated to that number.


[Mark Jobe] "If anyone could offer a solution that would be a great help."

Keep the total rotation value below 32,768. Turbulent noise doesn't cycle like fractal noise does. Also, this old editorial wisdom might come in handy -- if you can't solve it, dissolve it!

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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Mark Jobe
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 2:41:35 am

Holy carp, that's some sort of alien technology sent back from the future answer right there. Thanks for such a thorough explanation. I need the effect to work for a 20 minute long scene so i'll definitely have to tweak the figures.

Thanks again explaining what was going on.


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Walter Soyka
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 2:59:46 am

To try to clarify a little, this is kind of like the old Y2K problem, where computers stored years in only 2 digits instead of 4. The year 1999 was stored as 99. The year 2000 would be stored as 00 -- but so would the year 1900. As soon as you exceed the maximum value (with Y2K, it was 99), you get unpredictable results.

After Effects is apparently using at 16-bit number to store the rotation value. 1 bit goes to signaling whether the number is positive or negative, and the remaining 15 bits store the number. 15 bits of binary can count up to 32,768. Once you exceed that number, it's analogous to trying to count above 99 -- you need another bit (or another digit in the number) to continue counting.

Honestly, that limitation is arbitrary and undocumented -- you might consider filing a bug report [link].

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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Brian Charles
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 2:38:19 am

It appears you've hit the limit of the number of degrees and revolutions the effect will support.

As it approaches a total value of 100 complete revolutions it stops.

Try this: in a test comp set the evolution from 0 to 100 in 3 seconds. It will stop just shy of 100. (93.6 in my case)

In your case it seems to be stopping just shy of 50, not sure why.



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Mark Jobe
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 2:52:18 am

I changed the expression to time*10 and the effect seems to be working throughout the whole 20 minutes as I scrub through the timeline. I'll do a render test but fingers crossed that seems to be fixed! Thanks to you both for getting back to me.

Mark


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Walter Soyka
Re: Turbulent Displace Expression problem
on Mar 2, 2011 at 3:07:03 am

[Mark Jobe] "I changed the expression to time*10 and the effect seems to be working throughout the whole 20 minutes as I scrub through the timeline."

time*10 will work ten times longer than time*100. Since it fails after 5 minutes with time*100, and you need 20 minutes (4 times longer than you have), you should be able to get away with time*25 (one quarter of the speed) if you need the look of a faster evolution.

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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