Creating Fluid Simulations on Sketchfab

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Sketchfab Master Jakob Baldwin uses Sketchfab stop motion to publish fluid simulations. This tutorial is part of our Special Effects series.

Fluid simulation can be tricky to create, really difficult to make realistic and it’s not really possible to upload and display natively on Sketchfab, but there is a workaround.

Here is an example of fluid simulation in Sketchfab (warning: the embeds in this article are quite heavy):

This tutorial is meant for Blender, but the general principle works for all 3D applications. In short, Sketchfab supports stop-motion animation: if you upload a .zip file with several objects and a .timeframe text file, Sketchfab will display the models in stop motion. If you export every frame of a fluid simulation as a separate .obj, it can then be displayed on Sketchfab.

Creating your fluid animation

I won’t go into details on this part, but Digital-Tutors has a great tutorial on fluid simulation with Blender. All the options for fluid simulations can be found here in your Properties window. For a basic setup like the one above, there are then 3 steps:

  • Create your shapes: a large box, with your fluid starting shape (i used a simple sphere) inside the box
  • Go to both shapes and press the enable Fluid physics button, then, under fluid settings change the box’s type to “Domain” and your fluid shape to type“Fluid
  • Select your domain object again, set the final resolution and click the “Bake” button

Wait a bit, and press alt+a to play your fluid simulation! If you want to add static objects, create them, and if you want them to interfere with your fluid, give these objects an “Obstacle” under fluid settings.

Converting simulation to several .obj’s

Select your domain object (which should now look like fluid, not a box) as well as any static objects, and export as Wavefront (.obj). Before you press export enable “Selection only” and “Animation”. Export to an empty folder, as this will create a lot of files.

Expect it to take a while. When you’re done, zip all the model and material files into an archive.

Writing .timeframe file and upload

image07Now to create your sketchfab.timeframe file, which is basically a text file that tells the engine which order to display your models, and for how long. The text file you need to create looks like this →

Each line consists of a number declaring how many seconds the specific object will be displayed for:

  • For 30fps the number would be: 1sec/30frames = 0.033333 sec
  • For 24fps the number would be: 1sec/24frames = 0.041666 sec
  • You get the idea..

I created my .timeframe files in notepad ++, since it allows to edit whole columns with alt+drag-click, but you could create the file in almost any text editor. The important thing to note is that the .obj’s have the correct names in the correct order, and that when you’re done you save the file as “sketchfab.timeframe” (Exactly this name, otherwise it won’t work)
Add this .timeframe file to the archive with the models and materials, and upload it to Sketchfab.

Post processing on Sketchfab

image02Now your fluid simulation is uploaded, head to 3D settings and remember to set the model to start with animation playing. Afterwards go under your material and play around with the settings until you’re satisfied with your fluid.
For realistic fluid like the one to the right I would recommend PBR with following settings:

  • A light blue albedo colour
  • A metalness value of 100,
  • A very low roughness value of about 3-7.
  • A high specular value (Mine here is 87).
  • About 50% transparency, either blend or additive.
  • Also decide on whether you’re going single sided or double sided, both options have their charm.

You can also experiment around with a more cartoony, simple blue fluid, BUT remember that you can’t really add textures to your fluid, since the geometry changes around every frame. (Well If you’re up for it you could, but it would take some time + many textures).

Technical discussion

Fluid simulations are really heavy for Sketchfab to display, since you’re essentially uploading 100 or more large geometries, and Sketchfab has to load all of it. After experimenting around, I’ve come to these conclusions on increasing performance:

  • Keep it short. For every second you want to show, you’re uploading another 30 or so meshes.
  • Keep it simple. The aforementioned resolution setting you set before baking decides how complex each mesh becomes. Set resolution to 10 and you could upload a one minute long fluid simulation, but it will look ugly. Set resolution to 200 and you’re looking at two seconds of beautiful fluid animation before your browser crashes. I used 45 and 60 for my three scenes.
  • It also seems like your setup matters, if you’re throwing fluid at more fluid (see tap scene below), the total polygon count for each mesh is still quite low, but if you’re throwing fluid at a floor, it splits up and every droplet creates more geometry, especially under higher resolutions. A good solution is to delete droplets with an “outlet” geometry.
  • You could “cheat” and use a lower framerate. Before you bake just change the “Speed” setting under domain object physics settings, speed it up by a 100% and you can use half the framerate in your .timeframe file. Doesn’t run as smoothly, but hey, it increases performance
  • A really good way to get more precise fluid simulation WITHOUT increasing your polycount, is to set the resolution very high when baking the simulation, but before exporting, you put a “decimate” modifier on your domain object. For example, if you’ve tested and found that a resolution of 150 works for your scene, bump it to 300 and then throw a decimate with a 0.5 value on the model, it will halve the polycount, but keep all the small drops
  • Also if you want any static objects in your scene, like a glass or cup, keep it low poly, as the static object has to be in every single .obj, even though it stays still…

Have fun playing around with fluid!

About the author

Jakob Baldwin

Danish youth with a passion for all things 3D

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