SolidModels are models constructed with CSG, which are made out of unions.
Solid Modeling is a tool to make complex shapes in ROBLOX. Starting with very simple shapes , it is possible to create almost any shaped object you can imagine.
Using Solid Modeling offers several advantages beyond creating new shapes. An object created by Solid Modeling is a single part. This can result in a significant part count reduction in your game which will increase performance. If your game was using models comprised of many parts, consider combining the parts with Solid Modeling to make your game run faster.
The key function when using Solid Modeling is called Union. To use Union, select the parts you want to combine and click the Union button. This will create a new part called Union (which you are free to rename).
Solid Modeling allows for more than just joining two shapes together. Shapes can also be removed from parts using the Negate button. To use Negate, select a part and click the Negate button. Now this part is a negative part (it will turn red and translucent to indicate this). Whenever this negative part is Unioned with a normal part, the section of the parts that overlap will be cut out. Note that any basic Part (Part, Sphere, Wedge and Cylinder) or even a Unionized part can be negated and used to cut out of other parts.
You can undo the negation of a part by selecting the negative part and clicking the Negate button.
If you do not like the result of a Union you can undo it by selecting the Unioned part and clicking Separate.
There is a limit to how many parts can be generated. If a Union would result in a part with more than 2500 triangles the operation will fail and Studio will alert you to the error in the window.
At the moment a part made with Solid Modeling can only use one material. If you try to Union two parts with different materials, the resulting part will only use the material of one of the parts. If you need to make a model with multiple materials, consider only using Solid Modeling on the parts that share the same material.
This restriction is expected to be changed in later versions of Solid Modeling.
Tips and Tricks
When making a complicated model with Solid Modeling, it is usually easier to make the parts you are working with very large initially and then scaling down the resulting union after the object has been completed and you are satisfied with the results.
A Unioned or Negated part can only be scaled uniformly (when scaling a part all the dimensions must be the same proportions). If you need to change the size of a part in a Solid Modeling composition in only one dimension it may be easier to un-negate or separate the part you need until it is a basic part which you can scale in any direction you like.