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TOTAL DURATION: 3h 7m 37s
Solaris is Houdini’s environment for layout, lookdev and lighting and is built on USD, the open source Universal Scene Description format created by PIXAR. This makes Solaris a great environment for interacting and authoring USD scenes and assets. USD is an optimized system for organizing your assets that allows for efficient workflows for teams or individuals.
In this course, you will continue the conversation about what USD is, and how you author it using Solaris. Some of the topics that will be covered include: bringing USD files into Solaris, how to work with USD layers, and an analysis of what is happening inside of USD files. The goal for this course is to build a strong foundation of USD concepts in Solaris in order for you to be able to build scenes and workflows.
In this course, you will build a foundation of authoring USD in Solaris, learn how to analyze how USD is written to disk, and discover how to build simple scenes in Solaris.
The Sublayer LOP is the most simple way of bringing in a USD file, and it does not modify the Scene Graph Tree from the incoming file. The Reference LOP imports USD files as well, but it has the ability to set where the incoming Scene Graph Tree is rooted on the stage’s current SGT.
The Asset Reference LOP is similar to the Reference LOP, but allows for variants to be specified when loading from disk. This LOP also has transform controls on it that lets you position each asset in our scene.
References let you “Payload” a primitive. This opens up options for you to unload large numbers of geometry from the viewport, and make complex scenes easier to deal with. It is important to note that Payloads will still render if “unloaded”.
The Stage Manager LOP is an artist-friendly way to import USD files to the stage, control their hierarchy, and transform them within our scene. With this node you can also modify a stage that is connected to the input.
The “Primitive” LOP can create primitives in bulk using a text parameter to specify the paths for the primitives. This can be used to set up scene or shot hierarchies before any geometry is created.
The “Transform” LOP works in a similar way to what you would expect with a Transform SOP. However, you can get unexpected results if a primitive path isn’t specified. An “XForm” LOP can create a primitive of “xform” type, but in Edit mode it works almost the same as a “Transform” LOP.
There is no concept of “Groups” in USD. Instead you can make GeomSubsets on USD meshes in order to bind materials to sections of the imported geometry. These GeomSubsets take up lines in our Scene Graph Tree, however they themselves are not selectable or transformable.
Similar to subsetting based on Groups, you can also subset based on Attributes coming from SOPs. In this case you can use attributes with unique values (integer, string) in order to drive the GeomSubsets.
Schemas are structures defined by USD to bring in data and format it properly for use in USD. This can refer to attributes that are renamed, creating the correct primitive type, or mapping attribute classes, to name a few.
When importing geometry from SOPs that have non-standard attributes they are referenced in USD as a “Primvar”. These attributes can be found in the Scene Graph Details pane, and their contained values can be inspected there as well.
There are two LOPs specific nodes for SOPs: LOP Import Node & LOP Import Cam. The “LOP Import Node” SOP is a Geometry node that allows geometry to be imported from LOPs. The “LOP Import Cam” node is an Object node that brings camera data to the “/obj” context.
Layers are a way to organize your USD into segments, each with their own save paths. Solaris tries to automatically set these up for us, but it is good to keep track of new layers being created. We can visualize layers in two places: the Scene Graph Layers pane and the halo colors around LOP nodes.
The “Layer Break” LOP is the most absolute way to split layers. Everything that is created/referenced/read above a Layer Break node will not be saved by downstream ROPs.
The “Configure Layer” LOP works similarly to a Layer Break LOP, but it has a built-in parameter to specify the save path for the layer (instead of using a USD ROP). We can use a USD ROP at the end to force these files to be written, and create a master file that sublayers the individual layers.
You can overwrite primitives by sub-layering a primitive with the same USD Hierarchy. This will overwrite primitives created earlier in the network. Using this method you could overwrite geometry but keep the material assignment.
In USD “Opinions” are a way of resolving which values to author when two Scene Graphs are layered on each other. This is dictated by the LIVPRS Strength Ordering. Furthermore, leaf values on the tree have higher precedence. Solaris also gives us a way to prefer certain Strength in opinion when layering.
While you are used to working with the “Merge” SOP, its LOP counterpart can lead to hierarchy issues or duplicate prims. Here you will learn how the Merge LOP differs from its SOP counterpart, and understand how the different modes work via an example from the documentation.
USD allows you to create a scene with one file or multiple files that are sub-layered on each other. With multiple layered files, you have the ability to make changes that will not break the scene or require master scene/shot files to be re-created.
Since .usda creates human-readable files, let’s open one up and look at what’s happening under the hood. You will take a look at the code in the file and try to make sense of what the USD software is doing to compose the final scene/asset.
The Duplicate LOP is the simplest way to make instances of a primitive. It allows for those duplicates to reference the original, unless it is set to “Copy” mode. The duplicated primitives are able to be transformed incrementally from the original primitive.
The “Instancer” LOP is used similarly to the “Copy to Points” SOP or the “Instance” Object node. It uses points as locations to copy “Prototype” primitives to. These points can come from an Internal SOP network, External SOP network, LOP input primitives, or LOP input points.
Let’s take a look at the different types of instances that are available to you and investigate the features that each option does and doesn’t have. In this chapter, you will create a material override on the prototype primitive and on one single instance to see how the copy and reference scenes react. You will then open the .usda files to look at the code contained inside.
Instanceable references are one of the most efficient ways to create a copy of geometry in USD. However, they come with some drawbacks that you will investigate in this chapter. You will again open the .usda file to look at the code being authored.
Writing animation to a USD file can be tricky, so let's look at some of the best practices when dealing with animated USD files. Often the best way to save animations are in single USD files, which can feel counterintuitive when compared to the rest of Houdini.
There are times when you have to write large data sets to USD where factors like memory concerns might make writing a single file impossible. In these cases, you can turn to stitching in order to make files that can be more easily read by the USD software. In order to accomplish this, you turn to the USD Stitch Clips ROP.
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COMMENTS
traileverse 2 years, 1 month ago |
Truly awesome to see more USD/Solaris content. Looking forward to seeing advanced USD stuff as well.
lorsange 2 years ago |
Hi again Peter, get same problem here with #2 "Sublayer & Reference" section
Get errors on the Reference node:
Error Unable to find layer file: -
Warning Fixed invalid USD path: /_
Works only with previous "reference" node asset, and don't understand /`@soucename` variable, only with hardcoded /simple_scene
TamilAless 1 year, 11 months ago |
I have exactly the same problem. Any advice? Thx.
derek_lindell 1 year, 11 months ago |
could be an error with name spacing, I had a space in the name of the folder my hip file was in and it worked after I changed it?
derek_lindell 1 year, 10 months ago |
or could be the fact you're using houdini non-commercial which means it generates .usdnc files instead of .usda files!
andy.nicholas 1 year, 7 months ago |
The issue is that some of the output paths on the ROPs should be using $HIPNAME, but they're not.
All paths should be starting with $HIP/usd/$HIPNAME/
Chassaigne Victor 1 year, 11 months ago |
as said before , this is Truly awesome.
conomara 1 year, 9 months ago |
this is a great intro, thanks for taking the time to create
andy.nicholas 1 year, 7 months ago |
I re-downloaded this recently and it seems to work fine out of the box now, so it must have been fixed.
gustls2017 1 year, 7 months ago |
Hello, I'm a beginner at Hoodini. Sublayer & Reference Import -> C:/Users/hskwak/Desktop/Houdini/Solaris_Tuto_preset/usd/USD_with_Solaris_Authoring/scenes/simple_scene.usd path, but no files are called in because the folder has been created up to screens. Do I need to export usd individually? - Automatic translation is being used
hanyilun2 1 year, 7 months ago |
the hip file link seems expired, can anyone share it THX
norv2001 1 year, 1 month ago |
Great tutorial, thanks!
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