expression help

[jeromeOlivier]

Hello,

I'm currently going over Dan Alban's Inside Lightwave 7 chapter 14 & 15, but still haven't made it half way.

I'm currently working on an animation and need to have smoke rise out of the chimney of a train as it speeds along a set of tracks.

The smoke is a single sided polygon attached to a particle emitter using FX-Linker. The polygon has the image of a puff of smoke with an alpha channel to round out the edges.

I am able to get everything working except for one thing: I need to make the polys scale over distance, so that as each puff of smoke drifts away from the emitter, its scale increases UNTIL it reaches 1 (normal scale)

I know I can do this with expressions, but I'm not sure how.

Here's a picture that might help make what I'm trying ot say clearer:

[Mylenium]

This is not possible with the current versions of LW, since there is no way to acces particle data with expressions. From what I read you possibly would want to use HVs in sprite mode which would yield the same result.

Mylenium

[jeromeOlivier]

Thank you for replying.

But it's not the particles I'm trying to manipulate, only the polygons that have their motion affected by the emitter.

I know how to do it using sprites, but then I have to render using raytraying to maintain transparency - VERY long renders...

I guess another possibility would be to render the smoke separately and then comp it together in After Effects, but that seems like a waste of time also.

[Emitter.Position.Z]*.1

...or something like that attached to the X and Y of the scale channel of the smoke poly would work, but it is the relative distance between the poly and the emitter that needs to be calculated. Is there a way for me to fetch the information from the range finder plug-in and use that to create the expression?

AArgh!!! This is so frustrating! :(

[Mylenium]

Hummho, this would mean you had to bake particle motion using FX Linker... Not a very wise thing, 'cos it will be a major part of work and you will end up with lots of unnecessary keyframes for each particle. The expression would then have to be something like(just a suggestion)

[(Particle.Position.Z-Emitter.Position.Z)/(Time-Emission_Time)*scale_multiplier]

Unfortunately all those fancy tools like range finder are pretty useless for expressions 'cos you (once again) cannot access their data. You would have to write the trigonometry yourself.

Mylenium

[jeromeOlivier]

Mylenium,

Merci beaucoup! I understand now. I finally went the other way and simply used hypervoxel sprites with clip images.

I should definitely learn more about expressions. If you have any recommendations regarding tutorials or reference material, please let me know.

Again, thanks for your help.