View Full Version : How would you make a plastic bag floating in the air?

12-23-2014, 08:32 AM
I need to make an animation of a floating bag floating across the street with the motion and distortion of the plastic bag as realistic as possible. Whats the best way to go about this? Thanks!

12-23-2014, 08:52 AM
I'd start off with something like the Leaves demo in the Bullet documentation to animate the movement, save that out as a MDD, then apply that to a high poly bag model that you can deform with a turbulence or similar texture, set to use world co-ordinates.

Sorry AJ, don't have time to do a demo scene for you this time, :D

12-23-2014, 09:02 AM
No problem thanks for the advice! :) one question - what is an MDD?

12-23-2014, 09:06 AM
that is saved point cache, deformations of the object are baked in to mdd format.

could work nicely as pinkmouse says, but using fractal deformation displacement might not deform well if you want it to interact with the street or hitting a building etc.

I think it should be enough to make the bag a bullet deforming body, and use bullet forces to push it around, balance gravity settings, maybe envolope them.

there is also an option to use cloth fx for it, wich reacts to the old legacy dynamic system with dynamic wind.

not sure if I got the time to test, have to see later tonight.


12-23-2014, 03:35 PM
a little fast testing of bullet dynamics, I think it will be the best option, it is fast, you should have enouth divisions on your plastic bag, subpatch it, and set the mode to last..that is needed for it to work.
add a null, and make it a forcefield, from there you can set it to different modes, right now I am testing velocity, from there you can access the velocity vectors, and use the textures "T" button...and add different fractals..this way you are telling the force to use fractal textures to drive the wind so to speak.

one has to use the right texture and scaling etc..that is the tricky part perhaps until you get a hang of it, you could also add another null and make that a vortex field..there are so many options, finding the fastest way with best result, that is the trick to learn.

the soft body behaves nicely anway..both in swirling and touching ground or other objects, but you have to set right density lower, linear stiffness a bit to get realistic results.

12-23-2014, 03:46 PM
I must also mentioning this when using kinematic or force dynamics, to see the radius affected by the force or kinematic objects, go to world properties and show collision shapes, by default I think most of those tools are set to the scale of 1, so adding a force field and driving it with textures, it will only affect the bag when it is inside of that radius, so that main texture effect will turn and swirl your bag only when the bag is in that area, but it will continue to move dynamicly and will be pushed outside of the force area, so it sorts of continues with the initial motion given, but it will not continuosly be affected with that force and the texture fields that drives it, so you should actually just select your null/force, and size it..and you will also see that the force field guide is scaled accordingly, so after that your field will cover whatever scale radius you have set.

might post a sample later on.


12-23-2014, 04:09 PM
Don't you think it would be better to move the bag yourself, but have bullet work out the deformations?

I had a play around and either the bag didn't move or it shot of into outer space. :D

12-23-2014, 04:52 PM
Don't you think it would be better to move the bag yourself, but have bullet work out the deformations?

I had a play around and either the bag didn't move or it shot of into outer space. :D

you can of course do it that way to, though I tend to go for the full physical aproach, so if I by any chance need more bags, or other items affected along with those forces, it will be based on the same dynamics, and not keyframed motion..depends of course what needs to be done, probably faster to do it as you say, and if that is all you need.
I want to understand the forces fully though..for further projects, so why not start with a plastic bag, "sometimes thereīs so much beauty in the world, I just canīt take it" (american beauty..plastic bag scene) :)

It indeed shoots out in outer space if not set properly, you could lower density, or strenght etc.
wind is also often gusty, and naturally if there is a strong constant wind, the bag will continue to flow away until hitting obstacles, just like warm ballon lights we will se in new years eve over here,
but for twirly plastic bag, it often just catches a gust wind, then the wind settles a second or more, then start again and on and on, so enveloping the wind might help.

I hope for the lw team to get particles implemented and work with the new bullet forces as well, hopefully if they can work on the existing particle system so it works properly with hypervoxels, and all the other set ups I am customed to use, particle birth and other particle effects etc...perhaps they need a completly new system, I donīt know...I just hope for it to work on the older system with enhancements, turbulenceFD should be able to read and use particles that are pushed with bullet forces, so we can add fluid smoke and what else needed in conjunction with bullet forces and debris, now I am spinning of topic, so Iīm signing off now for a good meal and night snack.