Spotty emission from bitmap?

[ben_hamburg]

Hey there,

i have this freezing setup, where I´m emitting with a composite map as a mask.
I mentioned this in another thread, where it didn´t seem to work, until I increased the grid resolution.
Now I have simmed with around 125 mill cells, but now I get an undesired spotty look (see attachment).
I don´t know if this is in the nature of the setuo with masked emission, if its still not enough resolution, or if its something else.
It looks a bit like emission comes from small particles covering the masked area.
I´m running another test with higher velocity surface brush emission, maybe I was just not emitting enough (I also had to crank up the opacity curve quite a bit).

Or is it another setting like different advection or something?

[Svetlin.Nikolov]

Hmm, can you show me where to look? If you are using the fire/smoke solver, there are no particles involved unless you are using drag particles, but still, the emission comes from voxels.

Are you using surface force or volume brush?

[ben_hamburg]

I marked a couple of areas.
I guess then the spotty look is probably based on not enough voxels.
125 mill seems pretty high to me, but since there is wind and adaptive grid, it might still not be high enough ( I think initial gridsize is only 30 mill voxels).
I just did another testrender with higher emission, but the problem actually seems worse now. I think its the resolution, looks less spotty and more like voxels to me now, what do you think?

I also did a quick testrender of the drag particles and there seem to be some grid like structures forming on the surface, maybe thats another indicator for too low grid resolution?
I´ll do another test, cranking resolution up to 250 mill to see if that changes anything.

Also I don´t seem to get motion blur if I use "force on" for the dragparticles...wasn´t there an option somewhere to check to export velocity for drag particles, apart from the general export checkbox (where velocity and speed are checked)?

[ben_hamburg]

I am using surface brush by the way.

[ben_hamburg]

Outch...I canceled the higher resolution sim, I was up to 5gb per frame, thats a bit too much for an effect that might be barely noticed in the final product... I do think it looked better, from what I could tell:

[Svetlin.Nikolov]

Hey, sorry for the late reply! So drag particles get their velocity from the grid. They don't have their own velocity channel. You need to export the grid velocity in order to get moblur.

I was asking about the emit mode because there is no surface brush... let me try to explain it once again:
- Volume Brush and Volume Inject need the inside of the geometry. This is why the geometry must definitely be non-solid.
- Surface Force mode creates velocity coming out of the surface of the geometry. It can work with both solid and non-solid bodies. The outgoing velocity is the speed of the fluid coming out of the surface. In your case, I think you are using surface force. You could map both the outgoing velocity by a texture, meaning that fluid will be ejected faster in white texture areas, slower in gray areas and will not be emitted at all in black areas. You can also map the Smoke channel, regulating the density of the smoke in different places. To me it seems you should definitely be mapping the Smoke channel and optionally mapping the outgoing velocity, is this the case?

[ben_hamburg]

Hey Svetlin,

Yeah, thats all clear by now and is what I´m doing already, sorry for mixing up the names again. One of those things my brain refuses to save to disk...
I´m still more concerned about the spotty emission look.

Do you have any idea if its a problem with the grid resolution, or something more benine?
For now I´ve just played around some more with the opacity to get rid of the spotty look and settled for a bit lower grid resolution than in the last test.
I also got the particle shader motion blur working ( no idea what the problem was last time) and it looks pretty cool, I´ll post an updated picture once its done rendering.

Just one of those things where I´d still like to know why its not looking like I was expecting...

[Svetlin.Nikolov]

Chances are that this is the grid resolution - though the texture has infinite detail, the grid simulator emits from voxels, so each voxel just reads the texture from its middle point (just one sample) and uses this color...

[ben_hamburg]

Yeah, I´m thinking the same thing. Like I said, resolution seems pretty high to me, but I´m also bad at math...Especially close to the emitter surface it looks like there are little smoke voxels (in the middle picture, around the eyes for example). Checking Inscribed voxels is just important for liquid sims though, right? There is a part in the manual about rendeirng liquid in glass, where the voxels can be seen.

[Svetlin.Nikolov]

Ah, just the opposite - inscribed/circumscribed work only for the Fire/Smoke solver - the FLIP liquids talk directly with the geometry surfaces. Gotta check the docs and see if the liquid in a glass problems explanation needs revision...