Maintaining surface area: Making a flag

[Ernst.Schneider]

Hello!

Noob question. I want to make a realistic flag that looks like it’s flapping in the wind (not animated). As flag fabric doesn’t stretch, how does one ripple, bend, etc. a surface without changing it’s surface area?

Thanks!

[pascal (Pascal Golay)]

Hi Ernst - you can get close by keeping track of the length of the edge and scaling (or ExtendSrf) accordingly -

FlagMaybe.3dm (46.3 KB)

Adjust the points on the wavy surface and adjust to keep the edge length (DimCurveLength, + History) close to 200.

-Pascal

[Ernst.Schneider]

@pascal

points on the wavy surface and adjust to keep the edge le

Thanks Pascal but I’m trying to create something lot more dimensional and flowing. Something like this:

[pascal (Pascal Golay)]

Yep, but you should be able to keep it close with the curve lengths and possibly a DimArea as well - then move points around as needed. There is no guarnatee any of it will behave like fabric - it will not, but you can probably get pretty close with the larger, simpler distortions and then add the smaller details with a bump map at render time.

-Pascal

[wim (Wim Dekeyser)]

Hi Ernst -
You could also look into simulating it with Kangaroo in Grasshopper:

Kangaroo Wind Control

Hi @zic_23 The flapping of a flag in the wind is not primarily driven by variations in the wind velocity - even in a perfectly steady wind, it will still flap, and that’s what’s happening in the simulation. What happens is a feedback loop - depending on their angle, some parts of the mesh catch more of the wind, so parts of the mesh move differently. The tension of the fabric makes all the parts interact, and it gets pulled into a new shape, which results in a new wind force distribution, and …

-wim

[DanielPiker]

Here’s a slightly updated version of the file in the link above to output a surface to make it easier to apply a texture, and an example of the baked result

flappingflagdemo_surf.gh (17.8 KB)
flappingflag.3dm (277.2 KB)