Getting Started with Houdini Engine: Inputs Outputs Overview

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Learn about Houdini's inputs and outputs and how to use them.

Learn about Houdini's inputs and outputs and how to use them.

A Houdini Digital Asset may do nothing at all. In that case you shouldn’t bother making it. But if you want it to do something useful, make sure you know what inputs and outputs are, how to use them, what to expect from them and how to daisy-chain different assets together, to make cool stuff.



- It should be obvious by now, that any functional asset should at least generate one output, as an asset without outputs doesn't do anything useful. As far as inputs go, they are optional, and we can have fully functional assets that have no inputs whatsoever. So as a reminder, I want to repeat, that inputs and outputs, can be either meshers or Thinking Particles. As far as meshers are concerned, they can be plain polygon objects, primitives, generators, or even mesh outputs from other assets. And as far as Thinking Particles are concerned, they should be Thinking Particle Groups. Let me show you a few examples. I have a scene prepared for you, and it has two assets, both created by Side Effects. One is a mesher, and let me click on the mesher, and it has an input and some parameters; let me raise this up a bit, now this input should be a particle group. What I've done here is I've created a Thinking Particle setup, it's extremely simple...that's it; this is a Null, and this is a piston node. And I've added an expression that vibrates this null and creates the particles. So what I'm going to do now is press Stop, Rewind, select the mesher, and in order to drop here the group of Thinking Particles, I need to go to my Simulate, Thinking Particles, Thinking Particles Settings, and drag the group over here. Now you can't drag the null, you have to drag the group. Let me close this, rewind and press Play again. What this asset does, is it takes the particles and it meshes them. It's quite good when you do fluid simulations and the so forth. Good. Let me press Stop, Rewind, and if I wish to deactivate it, I'll select it, make this a bit bigger, go here and say Clear. You will see now that it's red, because it doesn't have an input, now you can just get rid of the output. Excellent. If I press Play again, you will see that only my particles are visible. Now let's check the other one. This is a Voronoi Fracture. And I will make a cube, and I will make it editable by pressing C. So this is a plain polygon object. I'm going to select my asset and drop the cube in here, and now I have a copy, I can make this invisible, and you will see that we have a fractured cube. Let me delete this...and when I delete it, it turns red because it lost its input, and I can get rid of the output. I can make a cube, and I am going to retain the cube as a primitive object, select the Voronoi Fracture and drop it in here, repeat the same exercise again, and you can see that I have a cube which is fractured. And, even if I add more edges, you will see that the fracture persists. Let me delete this again, let me delete this, and finally, let me add a more complex object, like something generated, like a MoText. Then I can drop the MoText in here, and again, I'm going to get a fractured MoText. So, as you can see, it works with all sorts of mesh inputs. Now let's do something else a bit more interesting; I'm going to use the output of the mesher, as an input for the Voronoi Fracture. So let me delete this, I'm going to go to the Mesher, repeat what I did before; Thinking Particles, Settings, select the group, drop it in here and press Play...lovely. Now I'm going to do something, and something odd is going to happen. I'm going to select this, and drop this in that. If I press play, you will see that we have a lag of one frame. Now, don't worry, this is normal. And the reason is that by default, Thinking Particles' priority is Expression Zero. So, what happens here is that the priorities are a bit all over the place. That's why there is added functionality on assets to define a manual priority. By default, when the automatic setting in the priority is set, then an asset executes at animation level. Now if I click on this you'll see that animation is before expression. So what is happening here is that after this mesh is created and we think that this will follow, actually it's not following, because this is trying to execute before the Thinking Particles. So it actually executes on the previous frame of the mesher. Anyway, this may sound complicated, it's quite simple, and it's a very simple solution. We need to select the first asset and say, "I don't want it to go automatic, I'm going to put it on generators, for example, zero, which executes after the expression, and I'm going to select this, and say, Generators One." So what we have here is, Thinking Particles on animation level, and then this one on Generators Zero, and then this one on Generators One. If I now rewind and press Play, you will see there is no lag. Even if I select it, you will see that everything lines up properly. So that's one thing you need to take into consideration when you're using inputs and outputs and all sorts of different things like generators and so forth, and that is the priority level. And that's why we have the ability to switch from automatic to manual to accommodate for these complex situations. To wrap up this video, I want to mention one more thing. Let me delete this asset, and let's play a few frames, and everything looks great. Going to stop the animation. Now, if for any reason you select this and you delete it, there is no big damage done. You can always select your asset and this button has been activated now. And you can press Update Generators. Whenever we've deleted an output or did something that needs the asset to update, then this button will activate and we can press it to force and update to the generate. So if I delete it, then I can select it and I can update it again. So this wraps up this video and let's continue with more fun stuff.
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