Getting Started with Houdini Engine: Brief Anatomy of an Asset

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Learn about Asset generators and output generators.

A Houdini Digital asset has a number of fixed and user defined attributes. The Asset generator has it’s own set, and the Output generator a different set.

And if you’re wondering what Asset generators and Output generators are… well that’s why you need to watch this video. Carry on!

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- In order to get you started with Houdini Digital Assets, allow me to give you a brief overview of how they look inside Cinema 4D. For reasons of simplification, I will refer to them just plainly as assets from now on because Houdini Digital Assets is kind of a tongue twister. So here we have an asset called Simple_test_asset. First of all, it's green and it's green because everything is okay and it's working properly. In contrast, I can show you another asset which is red. Red means that there is something missing. I haven't been able to complete my calculations. So always remember when the asset is red, that means something is missing. When the asset is green, that means that everything is okay. Now let's take this one. It seems okay. It has a hierarchy. If I expand it, you will see it has something underneath. Now the green icon, the top one is called an Asset Generator, and actually it represents the totality of the asset itself. If I click on it, you will see that I have an Asset tab in my Attributes and I have the Asset, a Scale, some buttons here, a Priority. Then I have the Infos. We will deal with these later on, and then I have the parameters. The parameters indicate that the specific asset has been designed by someone in Houdini to only have these parameters exposed. In this case, it's just a simple menu. If I change the menu, you will see that something changes in the viewport and that is by design. What parameters we can have is up to the asset creator. Now let me show you something else. If I go back to the volume replicator, I'll select it and go to the Asset tab. Let me expand this. You will see that this specific asset has an extra group, which is called Inputs, and they are both empty. These are standard Cinema 4D object links, and the reason that this particular asset is red is that it requires both these inputs to be populated with some sort of geometry in order to give us something that is useful. Let me show you what I mean. I'm going to make this smaller again, and I'll make a cube, and I'll make a sphere. Scale tool, I'll scale my sphere, and what I'm going to do is select the volume replicator, and drag the cube in the volume object. It's still red and this Sphere in the Object to Clone link. And now our asset has turned green. That means that the minimum requirements have been satisfied, and now the asset can do what it's designed to do, and that's why we have the difference between red and green so that we know that an asset needs a bit more information. What that Information is is dependent on what the designer of the asset wants. Now one thing I need to explain is that if I don't want to see the actual imported objects, for example, the cube and the sphere, which is somewhat inside that cube, I need to go and turn them off. And now we can see that we have these spheres. I can take my cube and make it bigger, and you will see that the replication adjusts to the volume of the bigger cube. The task the volume replicator has been designed to do is to create copies of the sphere inside the volume of the cube. The number of spheres can be defined by a parameter called "Number of copies" which I can change to my liking. Both the inputs and this specific parameter has been exposed by the creator of the asset. It's up to the creator to define how many parameters and what these parameters are going to be, if any. Now let me point to the following. You can see that under the asset generator, we have this blue little polygon with the H for Houdini. This is the output generator. This is the object that's generated after the asset has been cooked, and you will ask me, "Why did you say cooked?" Well, a cook is a standard Houdini term that means that the asset has been calculated. Each series of calculations that produces an output is called a cook. If I change a parameter, every time the parameter changes, we are actually forcing a new cook and that means that the data we are changing is pushed to the Houdini Engine. There is a calculation and that calculation gives us the result we see in our viewport. That is a cook. If I press Play, every frame forces a cook if something is changing. So cooks can be triggered by refreshing some of the parameters of our scene. Anyway, more on that later. What I want you to understand is that we have the asset generator, which can be red or green depending on if it's working or not. And it won't be working if something is missing. In this case, it was the inputs. In other cases, it may be other things. We'll see these as we go. The result of the cook is represented by an output, and outputs can be meshes or particles. We are not limited to only one output. Let me show you some examples so you can get a better idea. I know I'm jumping from topic to topic, but I want to give you an overview. All these things will be addressed over the next few weeks in high detail. Let me change and go to a simple particle generator. It's green, which means it's working. If I expand it, you will see that we have a mesh generator, a mesh output generator, and we have a particle output generator. These are the two types of outputs we can get from an asset. And what I can do is select this. You will see that there are no parameters because when I designed this specific asset, I didn't want any parameters to be exposed or promoted, as they say in the Houdini lingo. If I press Play, you will see that the asset will cook on every frame and produce particles which are falling. Let me show you another similar asset. Here I have the simple particle generator, but it has an extra two cubes. If I expand this, now you will see that I have three output generators, which are meshes, and one which is particles so we're not limited to the number of outputs we can have on each asset and that is something you need to remember. Everything you see here has been defined by the creator of the asset. To wrap up this brief overview, let me show you a couple more things. If I expand the hierarchy, you will assume that this relationship is robust, but I can pull this outside the parent asset and still it will respect the parameters from the asset. This means that I can make a copy of this. I'll press command and drag. I'll move it here, and when I select the asset and change this, you will see that both objects change. Now there's one more thing, though. When I came back to the simple cube, it moved back to its original position. This is actually not a bug. If I select the output generator, you will see a different set of parameters so the asset has a different set of parameters, and the output has a different set of parameters. Outputs do not have user defined parameters, but we have these little radio buttons here, which define the behavior. I'm just going to show you one. Update PSR, if I turn this off and move this out of the way, then when I change this, it will not go back to its original position. If this is on, then the position is defined by the asset generator. We will see the rest in following videos over the next few weeks. That was just an overview, and I hope you enjoyed it.
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