Faucet Running Water

[tielax11]

I am trying to recreate the effect where running water becomes less opaque at higher speeds, in my instance when coming out of a faucet. For a while before it hits the floor, it is not as transparent as the speed creates bubbles that run with it until it hits the bottom. I have attacthed a picture showing what I am trying to explain. 

 

Simply brightening the water based on the particle's velocity is not enough, as you can see streaks the streaks of bubbles in the water, as well as the effect that the lighting has on the water. How would I create this type of effect in Houdini?

 

[danw]

Take the particles from the original FLIP sim, cull off any below a threshold velocity (-magnitude, so I guess I should say speed), and then ramp speed to pscale to smoothly grow them above that velocity... then render them as instanced spheres along with your surface mesh.  In order to keep them inside the water surface, you can set primary visibility off, so they only show up when hit by a refraction ray.

Ideally, reverse the inside/outside IOR on the bubble shader, but otherwise it could probably be a straight duplicate of your water shader.

 

(Edit) That should obscure the fast moving water, but it still might not be enough to actually give it that subtle white-ish look... You could try rendering the same spheres as a very faint uniform-volume object, or try dialing a slight hint of subsurface scatter on the bubble water shader.

Edited July 28, 2014 by danw

[tielax11]

Thanks for the reply! Have you had success with this before and is there a video or image showing the results of this method?

[danw]

I've used a similar effect to make a fast running flow of glacier water look like it had some energy, and to take away the perfectly-transparent-glass look, without making it look white and foamy.  I'm only suggesting it as a starting point, I don't know for certain if it'll work, it's just the place I would start from if I was trying to do it myself.  It'd be great to hear what you end up finding works best :-)

[teratera]

As a small tip I would suggest for you to check Add ID on the solver node, if you go the way Dan is describing. That will enable you to randomize bubble size without flickering (based on this id attribute rather the ptnum).

Edited July 29, 2014 by teratera

[tielax11]

As a small tip I would suggest for you to check Add ID on the solver node, if you go the way Dan is describing. That will enable you to randomize bubble size without flickering (based on this id attribute rather the ptnum).

 

By solver node do you mean the flip solver? 

[danw]

By solver node do you mean the flip solver? 

 

It's under the Particle tab of the FLIP solver, in the Behaviour section.

 

And yep, you could multiply a random number based on the ID as a seed, with the pscale you calculate from the velocity, to give it some variation.

I would suggest keeping that subtle... I don't think you'd get a huge range of bubble-diameters in clean tap water, they'd probably be fairly even.