In China, there is a Houdini swindler who said that Houdini must make water according to the true proportion, that is, one meter is one meter

If the proportion is correct, otherwise it is wrong,
This person said that the scale can be reduced, but not enlarged,
The special effects engineer cannot change the scale, change the camera,
I want to hear the official explanation of proportion, whether he is right or wrong

zf3d
I want to hear the official explanation of proportion, whether he is right or wrong

not official, but based on my experience

Houdini FLIP is not 100% physical and there are many things that can cause physically implausible result even if you keep the scale 1unit=1m

but generally it's about whatever you decide if you make sure you adjust values of your forces etc accordingly
incorrect scale is mostly visible by how slow/fast splashes fall under gravity so you just need to adjust it for your units
(similarly Surface Tension will probably also need to be adjusted accordingly )

for example:
if you say 1unit=1m then for simple sims you want to keep gravity -9.81 since it represents m/s2
if you say that 1unit=1cm then your gravity would need to be -981 (cm/s2)
etc.

(many other POP forces are a bit arbitrary and should be tweaked visually as the @density doesn't really translate to @mass and POP forces have Ignore Mass on by default anyway, so you would need to be aware of a lot if you wanted to rely on truly physical inputs)

it's also pretty common to scale your scene (sources/colliders/etc) to m, run the sim and scale the output back to original size to avoid adjusting the forces or other settings if your sim setup is already tweaked to m

so it usually doesn't matter what scale the scene was built for since FX artist can usually adjust it (even differently for each sim if necessary)
of course it's ideal to be consistent

To zf3d, first of all, it's "Scale", and it's not " proportion" if you never travel outside the China, and if you don't know how to speak English, you will misunderstand yourself and others very easily. Learn English!
zf3d 你如果从来没有出国，走出过中国，没有在英文环境中生活，你的英文表达能力 会让对方产生误解， 你的英文水平直接让对方产生了误解，

To tamte, he is taking about "Scale" for example, you got a shot, there is a submarine coming from under the water in ocean, the asset Dep will give you the submarine geo as 1:1 scale to the real world scale, and going through layout Dep, then to the FX, in FX we Can NOT change the scale or the size of this submarine, and we have to sim the water use the real scale geo, you can't scale up or you can't scale down, this is basic, i'm sure in your company, you are doing this, FX artist is not allow to scale or change the size of geo coming from layout, there are some cases, but in most of time, we don't change the size of submarine, we will sim the submarine at 1:1 real scale in Houdini,not to mention, we will have many shots in a seq, i'm taking about this SCALE, you can't not change the size of geo to sim in FX. IF you need to, means, you are not good at Flip

To tamte, 一个镜头，一个潜艇上浮出水面，模型是来自资产部门的，和layout部门的，你是无法改变模型的大小的，你需要使用1：1的模型模拟你的海面 这个你是无法改变的

what Tamta is talking about is, in you flip sim, you can change the gravity or other setting, to make your sim looks better, but he didn't say, you can change the size of the object which you are stimulating
Tamta在他的帖子里面说的是， 朱峰社区 你给我听好了，人家说的是，你在你的流体动力学里面可以 改变参数 来获取不同的模拟结果，比如重力，让粒子下落的速度变快 或者修改改别的参数， Tamta并没有说 人工的 在特效部门内故意修改动力学模拟的模型尺寸！！！你搞错了！

you two are taking about two different things
你们两个人 在说完全两个不同的东西

Head of FX from XYZ FX

tamte
tamte

To tamte


you got a submarine geo from layout, which is 120 m long, you need to make this submarine coming out from water, when you put into your flip sim, are you going to keep sim it as 120m long object? or you will Scale it up 8 times, into 960M long object and sim it.

你拿到一个潜艇的模型，从layout部门，120米长， 你是直接用这个120米的模型做动力学模拟呢？还是 会去吧模型 放大8倍去，变成960米去做你莫flip模拟，
请回答我这个问题 谢谢

please give me your answer

thanks you

Jerry Jerry
you got a submarine geo from layout, which is 120 m long, you need to make this submarine coming out from water, when you put into your flip sim, are you going to keep sim it as 120m long object? or you will Scale it up 8 times, into 960M long object and sim it.

this is really a 2 questions
- can you do it?
I believe technically yes (purely from my experience), you can have your submarine 960 scene units long and adjust the physical sim units accordingly based on units they represent (forces, gravity, surface tension, viscosity) to get the same result as if the submarine was 120m long
as I said before, FLIP is not 100% physical, so there always may be additional tweaks you need to do at any scale to enhance realism (or additional solvers airfields, adhesion, ...), but if it was physical it's all about physical units, if all microsolvers inside represent correct units as stated above you should get correct behavior as if the sub was 120m (so you'd assume 1unit = 1/8m)

- should you do it?
in this case most likely not, I really don't see the reason to have to scale that submarine 8 times
my personal rule of thumb is to keep working in meters as long as the numbers seem reasonable as then you don't have to understand physics as much and defaults for FLIP will seem more plausible
so if someone handed me a submarine that is 960 unit long and told me it should be 120m, I'd rather scale it to 120 units for the purpose of the sim rather than adjusting the units in the sim, so yes I agree, it's reasonable to try to keep scale in meters for objects of reasonable size

however many small scale or macro shots I encountered (for example water interacting with a product that's 2cm in diameter) are easier to handle if you assume 1 unit is 1cm or 1dm and adjust FLIP sim accordingly, there is no point to force yourself to work with collider of diameter 0.02 and having particle separations 0.0001
while you can certainly still do that as long as you stay away from too small or too large numbers to avoid rounding errors, I personally feel safer to stay within reasonable decimal ranges and adjust my sim units rather than working in meters all the time
if you know what you are doing you should get the same results
same would apply to enormous scale sims, I'd rather do a physical units conversion to work with reasonable numbers than dealing with particle separation of 10 and floating point issues for example


as to the other points:
- I agree that it's important to be consistent especially when working in team
- so in a company, the workflow may be given by supervisor or lead, in which case FX artist shouldn't change the scale of the objects for the purpose of sims on their own without discussing with the Lead or Supervisor in case there are scale concerns or suggestions
- pipeline may already handle scale conversions to scale the scene to meters (or different chosen unit for particular task) in case the project's scale is different (decimeters are common due to more user friendly numbers for many objects we encounter every day)
so usually fx artists don't have to deal with scaling of the objects but they still need to be aware what units the objects are coming in since as I said for some jobs, shots or tasks the decision may be that we want unit to be dm or cm in Houdini for the greater good and convenience

i 'm agree with 90% of your posting

a little suggestion to you ”tamte“, for how i work, i always treat 1 houdini unit as 1 metre no matter what, i agree, it's hard to do a flip sim something like cap of wine water in the cap, it's not easy. but it's still feasible, you just need to know how to do it, find the setting to work in this small scale environment. once you find out how to do this small scale flip sim, next time when you get the same shot or similar shot, you can re-use the settings over and over again, that will save you time for a long run, without testing the setting every time if you going for the approach that scaling up the collision geo at different "times". for example, you might scale up for 5 times to make this shot easy to sim, but next time, you got different object, then you might need to scale up 10 times for that object. so each time you need to change your setting.

The way how i do it, find the correct setting once, and use it for long long time, just copy the setting , and sim it. as for today's CPU, 3-4 hours flip sim is nothing.