re-rendering flags waving in the wind

[SgWRX]

so i rendered out 100 frames and have 3 flags waving in the wind. i made a mistake and the flags are moving way too fast. i corrected the problem and get a nice slowish waving motion.

 

however the problem is, i've already rendered the scene with the waving flags. so i can't just re-render subset of scene (flags only) and overlay it. the old fast moving flags show behind the new flags.

 

the poles extend up past the building into the sky. they sky has a haze map for clouds in the mr physical sky.

 

thankfully, the moving flags are all in the sky and there are no foreground objects in the way.

 

what i'm doing now is, i applied a matte/shadow/reflection material to all objects in the scene except flags and flag poles. i set the matte to white. when i render, i get just the flags and background (mrphysical sky with haze map and the horizon/ground).

 

i can overlay this on top of the original render and just use a mask to cut off everything below the flags.

 

how else could i do this? is there a better way?

 

the BLUE flags in the attached image represent the new rendered flags. you can see the flags in the background are showing through. and like i said, fortunately, i can just add a mask in after effects to cut off everything below the flags (thus not interfering with the buildings).

[Fooch]

Is the shot moving lots?

 

Looks like its an easy comp scene ..

[SgWRX]

yeah, the camera swings to the left of frame. i just don't see a way of re-render the flags only with the background area.

 

what i should have done is render without the flags, then render the flags separately as an overlay. but oh well. thank goodness i only had the flags in front of the sky and not buildings. it ended up coming out OK.

 

i wonder how they'd do this in movies? maybe it's best practice to render a "background plate" and then other animated things on top of it for layering in post.

[Francisco Penaloza]

I think the way you describe is the best option, and actually you can turn off everything else, but the flag poles and Flags, then you'll get you flag animated with the same background sky. Just put it in top of your original shot and mask out what is not need it, giving some soft edges to your mask should not be noticeable.

in big productions, everything is a layer, in your case will be building, sky, foreground elements, cars peoples, flags, fog and everything else is in a separate layer, it sound over complicated but nowadays directors like to have full controls of every little thing.

When I do animated flags, cars or people, I always render it separated, there will be always a comment about, turn off that car, or change that person in front, or like in your case, your flag is moving too fast.

But as you mentioned a simple patch over your original shot will be fine and fast fix.

good luck.

[Fooch]

We would shoot the plates or cgi background plates etc.

 

Moving flags can be easily done via post if the camera angles moves are not extreme

 

Or bite the bullet and just re-render. Cant be a big scene to render. That said, our farm has a bit of muscle