Solid modeling

Solid modeling is a feature in Roblox Studio that combines multiple parts together to form a UnionOperation, simply known as unions. Through the additional usage of NegateOperations and IntersectOperations, they can be used to create more complex shapes that are not normally possible using only primitive shapes. Unions serve as an alternative to meshes, which require creation from 3rd-party software to be imported into Roblox. Solid modeling was introduced on August 1, 2014,^[1] arising from a Hack Week project. In 2017, Roblox introduced a new version of solid modeling titled CSGv2,^[2] designed to address the limitations of the original system, while possessing some of its own.

Behavior[]

Solid Modeling section in Studio — The Union, Intersect, Negate, and Separate buttons in the Model tab of Studio.

Solid modeling supports block, sphere, wedge, corner wedge, and cylinder part types only. The faces formed by a respective part inherits the color of the part, allowing for parts of a union to be colored differently. Setting the UsePartColor property to true will keep the union's colors uniform. Most of the union's other properties inherited from BasePart (e.g. material, customPhysicalProperties, collision) are taken from the first selected part (or the calling part in scripting).

Unions previously had uniform scaling, in a similar way to cylinder and sphere-shaped parts. This behavior was updated to be non-uniform as part of a rollout starting in August 2022.^[3]

In some cases, it may be better to use regular parts instead of a Union. Using fewer Unions can improve load times. If CollisionFidelity is set to Default, Hull, or Box, considerably incorrect physics interactions may occur, especially for shapes with holes. However, they take less resources to handle compared to PreciseConvexDecomposition.

Union[]

Unions are multiple parts, unions, or a combination of the two, fused into a new object that is capable of interacting with other objects.

Creating a union is simple. Select the parts you'd like to combine and then click the Union button.

Intersect[]

Unions can also be formed along the overlapping spaces of two or more non-negative parts.

Negate[]

NegativeParts are parts that can subtract from a Union or a Part to create a new Union. The same limitations apply to negation as does when creating a union. Double negation does not create positive space for unions.

Negation requires one or more NegativeParts and one or more Unions or Parts. These also need to be intersecting with each other. When your NegativePart(s) and your regular Unions or Parts are now connecting, select them and click the Union button to subtract.

Separate[]

Unions can be separated to correct errors or to make adjustments to geometry. Select a union you'd like to separate, then click the Separate button.

Differences between CSGv1 and CSGv2[]

Both systems have differences that can lead to some cases where one would be more desirable to use than the other.

CSGv1[]

CSGv1 makes edges on geometry appear smooth.
When working as expected, it negates and unions parts accurately.

Drawbacks

CSGv1 has a maximum of 2,500 triangles, which can be very limiting in some cases.
Union generation is more prone to error than CSGv2.

CSGv2[]

CSGv2 is less prone to error than CSGv1.
The maximum triangle count is 5,000.

Drawbacks

Unions generated with CSGv2 are prone to corruption, and not being able to load.
The lack of smooth shading support makes edges on geometry stand out more and appear sharp.