FLIP Tank Basics for Beginners in Houdini

this video will discuss the basics of setting up a flip simulation and how to use the water line feature in Houdini to create the illusion of a larger volume of water from a small flip tank setup I'll first start off with a very basic setup of flipped tank simulation then I'll tweak the flip tank so it's possible to view the water collision in the middle of the tank next I'll set up multiple RBD objects so I can drop multiple objects into the flipped tank then I'll go over how to use the water line in the flip sim to create an illusion of a larger volume of water from the small flip tank last I'll have a discussion on how all this is useful in it can be used in combination with Houdini's ocean tools to create ocean collisions there are two other videos that continue directly from this one that you may find useful they're titled flip simulation render sound and flip simulation and infinite water flows like rivers if you already have a understanding of the flipped tank basics and know how to use the water line please check out those other two videos links are in the description the easiest way to learn flipped simulations is by dropping random objects into a body of water like a flipped tank I have this teddy bear character here that I created an indie brush and I'm going to drop it into a flip tank to demonstrate different parameters used to configure a flip simulation so let's create the tank now there's something used from the ocean tools there's something called an ocean source node so this actually will create a flipped tank for us automatically which is very handy now once you drop down the ocean source note it gives you an error right away that's because this wasn't really designed for flip tank simulations it's obviously for the ocean tools however if you go to the surface evaluation and then for type change it from ocean to flat and that error will go away now you have your tank of water well water particles right now now let's fill this so we have a volume of water now I'm gonna go and drop down a null it's always a good idea to put down a No so I'm going to turn off the render flag for the flip tank because I'm not going to be rendering the actual tank the flip simulation will contain the water fluid and we're going to be rendering off of that so flip stem dive in obviously we need a top net let's go inside lit objects up so this is the default cube that it comes with the flip object obviously we need a solver top that's not anything without the flip solver and let's drop them the basic gravity now let's hook up the flip object now let's before we do that I like to bring everything into the node contacts of the flip stem node so drop down the object Marsh - you know the flip object now we can bring in the tank so it's conveniently right everything is under the flip simulation will flip tank it's right here now it's not giving it's not bringing in all the particles obviously that's because this input type for the flip object the input type is set to surface saw if you've seen my other videos I've usually provided this soft path with the geometry so if for example if we have a cube here so I'm gonna delete this layer this is just for demonstration purposes this cube it's right here let's go back into the dotnet this lip object if we go back and feed in the test cube so we have oh that's okay let me let me use a spirit instead it's just because the default default geometry for the flip object is also a cube I want to distinguish the difference so this is what I've usually had oh this is no longer a cube this is what I've shown in the previous videos I've usually provided the flip object soft path with a geometry which in this case I'm showing is a sphere and that's what it uses it automatically converts that geometry that's pure geometry into a body of water for me into fluid particles which can be best illustrated so this is only for visualization purposes for the flip object visualization the sprite is kind of large for me so I kind of like the particles so this looks a little bit better but you can leave it this is only for visual purposes now back to this the input type is a surface saw so that's telling Houdini that for the flip object I'm providing you with a polygon can you please fill it in with water for me since the ocean source that we used over here that we have created here at this flip tank already has particles now we want to use these particles so let's go back to the simulation and we can actually tell it I want to use particles I want to give you a particle field so let's go back to the tank so there's our flip tank with water particles there's also one more thing we need to fix if you zoom out I'm gonna change the background so it's a little darker so you can see you'll see this wireframe this purplish wireframe cube that's the volume limit of the simulation now our body of water is much smaller let's link the size of this in the flip solver volume motion volume limits I'm going to copy it so go to the box eyes right click copy parameter flip tank that's our ocean source there's also a size now I'm gonna paste this in so mouse is over is hovering over the size of the ocean source right click we want paste relative references now this will link those two parameters dynamically together let's click it and I'll show you what I mean now you see this it it provides a path to where that flip solver was if we left click this size one time they'll actually show us what the value is of the flip solver now if we go to the flipper okay we don't eat this anymore motion volume limits 50 let's let's change this to 5 5 I want it a little smaller for demonstration purposes now this box size is now five five five let's go back and see what size that flip tank is five five five so these two parameters aren't dynamically linked now there's also one more parameter I want to link it's the Box Center which is the position of the box so let's again hover over most hover over the box center right click copy parameter Center most hovering over the sensor right click paste relative references and then the path to the flip solver is is in there and then left click the center and you'll see the values in here let's play this let's play this simulation and see what happens the water just falls all the way down that's not what we want we want the water to stay inside the tank otherwise it's not very useful so for the flip objects I want closed boundaries now let's me play it again the water stays within the tank but it's not doing anything right now drop Oliver down into this pool of water and play the simulation again now I need a RBD object which stands for rigidbody I had brought in Oliver from into the flip simulation note so it's conveniently under this contacts now if we have an RPD object we need a RBT solver and we have two solvers we need to merge it together so both the solvers first together and then the merge plug the merge into the left input of the gravity now Oliver is a little too close to the body of water I'm gonna raise them up a bit so let's go back up to the top contacts go into Oliver of them up so I had created this beef early on so all I all it is does is move Oliver raises him above the ground by 10 units now let's go back to the simulation so now I'm gonna drop Oliver down into our flipped tank so let's play the simulation he falls does because gravity he's falling Oliver is falling is because the gravity is pulling him down as you can see Oliver goes right through this tank that's not what I want I know I want him to like stay on the ground so I'm gonna add in a ground plane I'm gonna merge the ground plane in two with the rest of the simulation the ground plane needs to be merged left to right so let's up here no crisscross I'm gonna change the viewport background back to the lighter color so we can see better but let's replay this now the ground is too high I'm gonna lower the ground a little bit so it's on the bottom that's good front view just make sure that body of the ground is right about the bottom edge of the flip tank I'm going to replay the simulation the splashes and what I wanted I wanted a more a dramatic splash this is because the body of water is very narrow and it's not very deep I'm gonna make the tank a little longer change the slips solver volume limit and it'll automatically update the tank size now I'm going to lower the ground a little bit since the tank is a longer restart the simulation Oliver is falling out of the tank which is fine for this demonstration I wanted to keep this volume limit as small as possible because I'm rendering while recording but ideally you would have a little larger tank at least the tank would be a little bit bigger so now that we have something basic working I can't see what Oliver is doing inside the tank I can't write because all the water particles are in the way there's actually something we can do about that in the flip tank we can create a narrowband particles so that will just create let's reset the sim as well that will create a narrow band at the top so no longer fills in the whole tank let's go back to the flip simulation the flip solver we also have to do the same thing flips over narrowband enable particle neuro band in the flip object for the input type we also have to tell it narrowband band as well so there's three places in the flip tank ocean source node particle tap enable narrowband particles and in the flip sim there's a flip solver node narrowband tab enable particle narrowband and again in the flip sim there's a flip object node initial data tab input type choose narrowband now after you do that it will ask for a surface volume and velocity volume now let's explain let me explain what the neural band does if you look in Houdini's documentation for the flip fluid object go under initial data now this was the surface soft up that tells who do you need to convert the geometry into the particles for me and then this is what we were using just right now at the particle field if we already have the particles which was the ocean source it already gave us the particles just use that now we're using the narrowband in the ocean source we specified narrow man so it tells us Houdini that we already have narrowband at the top it also requires us tell Houdini what is the surface volume and the soft path it wants a particle field now for the particle field we know how to feed it in in we had particles coming from the ocean source that gave us the flip tank but if you look carefully into the flip tank this ocean source actually creates two things for us particles and a surface volume so let's separate that I'm going to do a blast so I want the particles on the left side drop down to null so this is the particles now for the blast delete non-selected I only want the particles on the left side sorry render it first and then take a look now we only have the particles we no longer have the surface volume here now the other side I'm gonna do the same thing I'm gonna blast I want to delete everything but what I select so the surface is what I want to keep so delete non-selected it means delete everything other than what you specify drop down to null so surface volume here render it first now this has the surface volume here we no longer have those particles the points is one so now we have the particles and we have the surface volume so let's go back to the simulation clip object so here we want to feed in the particles which is what they're saying similar to the particle field similar to what we did for the particle field when we had here we we were feeding it a bunch of particles I like to bring everything in let's go back into the simulation slip object now we conveniently have everything in the same contacts so for the soft path I'm gonna feed in the particles the surface volume I'm gonna feed in the surface volume let's give this a try now we can see what Oliver is doing inside the flip tank and see how Oliver is interacting with the body of water I'm gonna make a couple more oliver's let's go to the Oliver I'm gonna make a copy of them I also need a copy of this this over however this is gonna be adding five so maybe 20 so this will be 30 got three Oliver's Solan so let's go into the simulation now since we want all three oliver's to interact individually with the flipped tank we need individual RBD objects as well you'll need a separate or BD solver for each well that was unexpected I third all over to land on the second one so I'll move the third all over a little higher I removed the last one a little higher so he doesn't bounce off the second there you so I ran the simulation with all three teddy bears dropping into the flip tank one after another and the thing I wanted to illustrate is that as the teddy bears drop in there's quite a bit of wash back as you can see the water runs up and down the edges of the flip tank it's very obvious when it comes here at this frame so you can see that this is a very obvious closed boundary flip tank and when you render it it's very obvious that it's it's a tank of course because we specified closed boundaries for the flip project we specify closed matter it's the water is stuck in this container but what if we wanted the illusion that water would go infinitely in all directions but still water be contained inside this tank now this is actually possible and the reason why you would do this I mean a simple fix would just be make the tank larger than you have and then zoom the camera in close and then you have the illusion of a large body of water however if you increase the size of the tank obviously the it would take longer to cook and it'll take longer to do anything on the computer and gets really heavy so this is a more optimization way now in the flip solver there's something called use water line so anything above so let's click at first anything above this water line will be will act as an open border on these on the edges of this tank and what that means is that this tank this closed boundaries becomes semi open so it's like an open boundary anything that is above this line above this line along the closed boundaries that means this these particles will just get really 'td and anything below it the water will just keep now how is this useful you won't have this wash back the water won't bounce off the boundaries and wash back into the center this will give you the illusion that it's an infinite body of water because the water is above this water line plane and passes the boundaries so it needs to be above and passing the boundary then it'll get deleted so no bouncing water on walls and no wash back so if nothing is bouncing off the walls it'll give you can't tell it that that's container if you blend this water with a larger piece of ocean ground it'll look like it's part of it so I'm going to lower the water line a little bit so there goes the first teddy bear second teddy bear and the third you'll see that as the water gets close to the boundaries it kind of disappears after that it doesn't bounce back there's less wash back there's less water moving back into the center so it doesn't climb all the way to the top as what we saw before let me try to find out here we have some a lot of particles flying off right here let's see what happens to it as it hits the edge now I disappear it just disappear right when it hits the edge I'll just be deleted so how is this useful I mean we still have just this body of water right here I mean it's obviously it's a container of water so this is useful for creating oceans that need infinite water surface or at least an illusion of an infinite water surface if you watch my previous videos on oceans I described how Houdini's ocean node work where you have a grid representing the ocean floor and Houdini's ocean you evaluate node will displace that grid into the shape of the ocean wave that's how you get the ocean floor and it generates very fast because it's normal cooking a simulation anything it's Emily just displacing geometry this is very different from flip simulations flip sims can infect water particles up into the air away from the pool of water so you get those pretty water splashes water droplets and collisions are then possible within flip zones however oceans you can't do it but if you wanted to cool if you wanted to do collisions in oceans you need the help of flip sims to combine the best of both worlds you would use this this flip tank sim and blend it with the ocean geometry to create the illusion that this water collision is happening in the middle of the larger pool like the ocean floor in this video I never talked about anything or even mentioned that top import known I'm mainly focused on the setup for the simulation but he want to render up your simulation the top import note is a must-have for your Houdini projects every time you use the top net node you need a corresponding top import node to extract all the data out and convert it into a format that you can then use for rendering you can't just render out the raw cooked top netsim it just won't work the flip tanks sim I showed in this video uses two different top import nodes to extract it which is covered in a different video titled flip simulation render setup again link in the description below if you already know how the top import and top import IO works then I don't think you need that video but if you're new to Nini please check it out thanks for watching and sticking to the end

Channel: bubblepins

Views: 35,675

Keywords: Houdini, sidefx, tutorial, FLIP Tank simulation, FLIP Tank, beginners, physical simulation, vfx, 3d vfx, 3d animation, water simulation, waterline

Id: k0P111aWW80

Length: 26min 15sec (1575 seconds)

Published: Sat Jul 27 2019