I’ll share the most common mistake I see from students every time I teach Houdini that inadvertently makes their FX look terrible as well as how to fix it.
Ever created an effect that you kind of thought looked good as you’ve been working on it. Bu then the more you looked at it the more you felt something is kind of off. Something’s not quite right.
Or maybe you were you know working on an effect and you’re just dialing up the numbers and it’s it’s an incredibly large value that you have to start putting in before you start seeing any real results and you’re wondering why in the world that’s happening well you definitely want to stay tuned for today’s episode because we will be talking about the number one most common reason why your effects look bad so stay tuned.
For those of you who don’t know me my name is Nelson Lim and I’m a digital artist and technologist some of the film and game projects that I’ve been involved with in the past include transformers pirates of the Caribbean, Fortnite and pacific rim.
In this channel I help cg vfx artists level up their mindset and their skill sets so that they can create more earn more and live more so stay tuned.
What is the common mistake?
So you found a model somewhere uh of a cool looking x-wing and you decided to bring it into your Houdini scene and you want to do some effects on it you start working on your simulation at first you notice that your particles seem to be moving really slowly so your first instinct is you just double those numbers and even after doubling those numbers it doesn’t seem to have much of an effect. have you ever experienced that before?
This is a problem that many beginners go through right at the initial stages. Now the secret that you need to know is that Houdini operates in meters. Now what do I mean by that? That means that on the Houdini grid by default one unit on the Houdini grid is equivalent to one meter.
Why does it matter?
Now, you ask me, “Nelson what does that matter?”
So here’s what’s happening. Chances are the model of the x-wing was modeled in some other package like maya. And if it was modeled in a package like maya the default units in maya is actually centimeters. So one unit on the maya grid is actually one centimeter.
Now think about it, if one unit is one centimeter, to build something that is maybe about one meter it would be a hundred units and then that gets exported out as an obj or some other format.
You bring it into your scene and what then you experience is an object that is maybe 100 units in your Houdini. It suddenly becomes 100 meters in Houdini and you don’t even realize it. Because you’re not really paying attention to that.
Why is this making my effects look bad? Well, that’s really simple. What you do is when you go back to that scene with your x-wing, go ahead and drop a digital human (like the houdini tommy digital asset) and you would immediately see that your whole x-wing is probably a hundred times bigger than it should be.
What that means is this whole time you had no idea how big the asset you brought in is. And you’ve been working in such a massive scale. And you you know it’s a complete beginner’s mistake.
How to avoid it?
So I’m gonna share with you a couple of tips on how to fix and avoid this problem. Stay tuned to the end because right at the end, I am going to share one more benefit of working in this way especially with regards to pyro.
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Working in real-world scale
Tip number one it’s best to always try to work with real-world scale when you first begin in Houdini effects especially and the reason is because many of Houdini’s solvers and the default parameters and everything is set to real-world scale so like gravity is 9.81 so if you don’t want to be changing a whole lot of different parameters and if you don’t have a lot of experience in terms of knowing which parameters change it’s always advisable to start off with real world scale and that is best practice even in production there are a couple of exceptions to that rule and that’s only if for example your project has really tiny scale maybe it’s a story about ants or the scale is really large it’s a story about giants for example see those are times where you may consider changing the scale okay
Tip number two. I’ve kind of already mentioned it which is it’s always helpful to drop in a scale reference in your scene and one of my favorites is to drop in like some sort of a digital human uh in Houdini I love to put just tommy and there’s a whole bunch of other human references that you can use if you look at the test geometry and I like putting in a human because our eyes are so used to how big a human should look in relation to so many other things that it really makes it a lot easier to look at it as a scale reference. You can do something as simple as putting in a one meter by one meter cube or a two meter by two meter cube which kind of roughly approximates the height of maybe a heroic character.
How to fix it?
So you find out that the assets that you brought in are just way too big so how do you fix them well a couple ways you can go about doing it.
Well the first way you can go about doing it is just go to the geometry container under scale you can just scale it down by however many times that you need it to scale down by.
The second way you can do it is to add a null node and parent the asset under the null node and then you scale down the null node.
Now the benefit of doing that through null node is that you can attach multiple assets under the null node and it scales all of them down.
What’s wrong with working out of scale?
I just want to quickly mention here that I know some of you are thinking, “Nelson what’s wrong with just increasing my values by 100 times?”
There’s really nothing wrong with that but here’s two reasons why i wouldn’t do that.
Well firstly I mentioned that there’s a ton of parameters in solvers and different Houdini effects setups all kinds of setups that are dependent on the scale so you need to know what those values are if they if your scale is different so there is a lot to get to and not all of them are equivalent to just multiplying everything and dividing everything by a hundred the second reason is what if your source scale was not centimeters but something like feet well that would just be absolutely confusing and as an effects artist you want to build a sense of familiarity with what values make sense especially in the empirical scale in terms of meters and not have to keep reference of different values depending on what scale you have there’s enough things to keep around in our brains.
So just don’t trip your brain up with having to remember so many different values for different scales.
The Pyro quick tip!
So here’s the quick tip about pyro now if you’ve been working on park in pyro for any amount of time you notice that actually the reference temperature is a very important attribute for you to understand and know because it determines sort of how hot of the fire is.
If you work an
imperial metric scale then the quick tip is you can actually find like temperatures of different gases and how they combust and you can use real world temperature scales uh references from that to sort of be a baseline from which you can start your effects from and then you can tweak from there. So that’s really beneficial but only if you work in real world scale.
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