Houdini Retime FLIP Sim -- Short Version

hey everyone just a quick plug up front if you're finding these tutorials useful please help me out and subscribe to this channel if you'd like to help out even more pass this along to some other artists who you think would also find it helpful and ask them to subscribe as well so this is gonna be a hopefully a much shorter walkthrough of how I retimed this flip sim from my last tutorial this is this is what I got from my last tutorial which is just using we see if it'll play back just using standard doc node standard surface tension standard reseeding I was really happy with with the results but I I wanted to be able to read time this and as you probably know if you've ever tried to reach time I'm just gonna copy this over this is a game dev free time tool I'm just gonna pull this over to show you what what this would look like if I attempted to the read time at the points just go everywhere it's because of the way that the the flip solver handles reseeding the it doesn't maintain consistent point counts on some frames that'll insert points and other frames that will kill those points off so you since the retiming relies on consistent point counts or consistent point id's it just doesn't work so what here's what I ended up with after doing a little bit of research online and finding some help and some forums which if you want more detailed explanations of where I got that help check in the description here there'll be links to the files that I that I referenced and also there's a much longer video I just want to get this one done as quick as possible for for those who don't have time to watch a long-winded explanation this is working great and what I ended up doing was using a custom hole filling algorithm that I found on an odd force forum hosted by a guy named Carl Richter I'll show you that forum they'll be a link to this in the in the description it's really helpful this is the this is the test that that Carl put up and you can see that with his hole filling algorithm when she built in bops you can really maintain these he's cool thin sheets which is great but also because of the way that he's built it it maintains consistent point counts and I'm gonna break that down just a little bit but I didn't just pull in his pull in his file and and only use that as he made it I was able to there's one thing I that that I didn't love about what what he had going on and that's the fact that these kind of tendrils just by by way of the but sort of just how his hole filler functions these as points separate new points are birthed at a certain threshold but what that doesn't really allow at least you know my initial poking around with it it never really allowed like this droplet will never break off because as it gets further away these points are gonna be consistently birthed in between those just by nature of what he's doing and I didn't really like that but as I was as I was looking for for help one of the things I did find was an older file from a really fantastic Houdini artist Alejandro Ashbury and he's written his own custom surface tension tool which was helpful to poke around three he's got a link to this work-in-progress of it he you know this guy is kind of the king of these Sims he's does amazing work and he's written a lot of custom tools which he writes about the process he's been interviewed about the process both on like grit markets and a couple different websites he's really he's really kind to share some of his knowledge but he's not able to share his actual files those are he'll tell you that those are in-house tools but this is a work in progress file that kind of shows some of the methods that he's using or the methods that he was using four years ago when he posted this and I grabbed this file and started poking around to see what he was doing and it led me to some something really interesting this is his file and this is sort of the the heart of where I where I was able to get my setup working how I needed it to I'm just gonna send a few frames of this and again look for these file I'll post links to these those webpages I had pulled up so in his and his flip solve or on his flip solver the entry he's giving his surface tension from this sob solver that he's created and in there again like I'm not gonna try to explain everything that he's got going on you can grab the file and check it out for yourself but the interesting part of this so he's pulling in the points and in in essence what he's doing he's using this this surface which has just pulled in from from the dock net like this is the flip solver creates this surface by default but he's using it he's converting it to an isosurface are converting it to eventually to a VDB and then using some smoothing on that and then snapping the points in his sim to that surface is actually snapping it to the unsmooth surface and then snapping them also to the smooth surface computing the difference and setting some attributes based on that difference so that's how he's getting his surface tension but what was really interesting about this to me was I never thought about pulling in these these fields and what I ended up doing I'm gonna hop back over to my file what I ended up doing was taking the hole filler Bob and then this is the file from Carl Richter off of the odd force for him so he's got this again a Saab silver that he's plugging into a sourcing tab and on this he's just he's pulling in his top geometry was just his points and he's running this Bob setup to pull in the point cloud open it evaluated based on radius and Max points and then everything in this loop is what's evaluating those gaps and eventually adding points when certain criteria are met and it also it matches the velocity of the points around it as well so that's how he's doing is hole filling and because this is happening and then stop solver when I want to points burst one it's gonna get a higher like you have your original points your original points in the soon and when new points are getting birth because nothing's dying those new points are gonna have a higher point number than the than the ones present originally and because this operates every frame that's going to be true every frame so every frame when a new point is birthed it's gonna get a higher point value than all all of the points that are already present that's exactly the kind of if you want to think about it as receding vesting exactly the kind of reseeding that you need to retime a sim so i'm gonna hop back over to my sim and i'm gonna up in here I mean they will simulation just for a few frames to get something going and you see all these brightly white colored points those are all of the new points that are being birthed into into my sim so again what I did was kind of take a combination of Alejandro's curvature his surface tension based on mean curvature flow and the whole feeling either algorithm from Carl Richter I put that in a stop solver that that's this operating on the particle velocity you know I'm I really didn't this may work and plugged into some of the other the other inputs here but I plugged it into the particle velocity could honestly because that's that's where Alejandro had his his surface tension calculation going on so that that's where I started because he already had these you know pulling in this surface and converting it smoothing it he or he had that set up so I I left it there but what I'm doing with it I'm not using Alejandro's mean curvature flow setup to do anything with surface tension I'm still using the stock surface tension gas micro solver what I'm using a bit of his setup for is to isolate points where I want this whole filler to work like I don't want the whole filler to work everywhere because that never allows this separation and if like if you've ever smooth VDB surfaces with like if you ever tried to measure your sims using BDB surfacing and smoothing you know that like thin areas break up and that was kind of the key for what I what I'm looking for it like this is my sim this is the dot geometry that is pulling in but what I want to do is just group the parts based on this smooth this smooth volume and you'll see that in these thin sections that groups not present and that's that's really what I'm looking for and because in setting this group by the bounding volume now I can say hey this whole filler that Carl Richter provided I don't want you to run over this whole sim I only you to run on on these grouped points which are the thicker parts of the volume and and that allows for the me hug them and get out of the sin that allows for for this to break up a bit what you see here you see all these drops you know if I disable that group those droplets will never form and again you know as I explained like these points that are being births have all just drive this point home my original oops is trying to cash something my original sin the geometry coming in as I think it's 160 8241 points when you come down to my my cash sim oh and the other interesting thing is if you follow my last tutorial I created this ramp attribute that I'm visualizing by color here as a way to add paint gradiation in at render time through my thermal material to this black value the white value is just a visual representation of this attribute that I created called ramp and I know that ramp never gets to to full zero and the only reason I'm mentioning that is because you'll see let me also just isolate this to ramp my highest point value is over here because it's got that higher that higher rent value but as yeah my just at frame three all these bright white again and this bright white is because I've got a I've got a color attribute and coming into the sim and these new these new points that are being birthed by the soft solver aren't inheriting any of these attributes so they're they're colored differently and that's actually it's actually helpful for me to have something to grab onto to to do some work here in just second but again I think that my highest point number going into the same 168,000 some odd points you can see that these new points have a ramp value of zero come on come on come on yeah so that was my highest point value but our my highest point number before this this hole filler kicked in and as the hole filler kicks in look it's just adding points to it it's not it's not reordering any of the points that are already present it's just looking at gaps and kicking new points in and that that's the heart of why the why my retime works and you can see I just as for the nuts and bolts of the retime I'm just using the game dev free time you can use the the standard three times or the old version the old way of doing it with a time time blend in a time shift I'm using the game dead one because I really like this this ramped control as I play this back and you can you can slow this down really really really aggressively and it seems to hold up very well and that's the bulk of it that's it you know using those couple of tools the coral Richter's hole filling Bop setup is great I understand some of it I'm not gonna pretend to understand everything he's got going on I'm still trying to break that down and understand it so that that's ninety percent of what I'm using is his hole filling and the other the thing that really made it all comes together was was piecing in Alejandro's method of smoothing that VDB inside the soft solver because that allowed me to isolate the hole failing to only the areas where it needs it and not the areas where I wanted the SEM to break up and so just just by way of finishing this off I'll show you what I had to do to get my my ramp attribute back to color this properly this is nothing this is nothing complicated all I did was after my retime I grabbed this the splits op and what this is gonna do is split it's gonna split your your incoming geometry into two different these two different outputs based on whatever criteria you put in the group note I mean normally you would just put your group in there I don't really have point groups so I'm setting this point group on the fly this says if the ramp attribute equals zero kick it out on this side and and what that is basically that's that's the way that I isolated my new points from the point filler you can prod you this a different way you may even be able to set a point group inside that stop solver in your in your dock net you know there I was just kind of building this I was kind of figuring this out as I went so that ramp attribute that was all these new points getting the zero ramp attribute help me out because I was able to split that off so based on that criteria that I put in the group this group filled the new points are coming out the left side that means the old points are coming out the the right side so I just transferred my attributes from the from the right side to the left side and then now this isn't both this is in both groups this is still just the just the new points but now they have the right attributes on them and then i merged them back in and that was that was kind of all there is to it actually looks like I may have there we go some of the some of the points weren't we weren't getting the attributes couldn't I dismiss the threshold was too low yeah then I put they put the attributes on the new points from the old points merge them back together and then my my color ramp worked just as it should and and it holds the obviously it holds the the retime so that's so check the links check the links for those those files that Alejandro and Paul Richter put up there amazing that odd force forum which there's all there's also a link to is great so many so many talented artists and they're talking about how to maintain thin sheets and in small flips small flip Sims it's really great also the the grid markets link is is an interesting read to find out a little bit more about how Alejandro is approaching these these flip sims again he's really incredible doing doing amazing work and that's it and if this if this short video is not really explaining the the thought process enough or if you want to learn more about what I was doing here feel free to check out the longer video I just felt like I felt like it was a bit long after re-watching and after recording it so I wanted to do a really fast overview as well I hope you find it helpful check check out the other tutorials on here on YouTube subscribe and check out the check out my website as well what I found on net thanks so much

Channel: What I Found

Views: 3,558

Rating: 4.9316239 out of 5

Keywords: houdini, tutorial, cg, 3d, sim, simulation, sidefx, vfx, FLIP, FLIP sim, motion graphics, mograph, design, motion design, cgi

Id: 2okztk2OTmY

Length: 16min 55sec (1015 seconds)

Published: Thu Jun 18 2020