# constantValues Values for the function-constant inputs that the shader graph declares. ``` var constantValues: MTLFunctionConstantValues ``` ## Discussion Function constants are baked into the compiled shader at program creation time and can’t be changed afterward. Use them for values that don’t vary at runtime — for example, a feature flag that selects between two code paths — to eliminate runtime branching and produce a leaner shader. To vary a value across frames or instances, declare it as a regular graph input and supply it through [`inputValues`](/documentation/RealityKit/PortalMaterial/Program-swift.struct/Descriptor-swift.struct/inputValues) or [`setParameter(name:value:)`](/documentation/RealityKit/PortalMaterial/setParameter(name:value:)) instead. Two descriptors that share the same shader graph but differ in their function-constant values produce distinct compiled programs that don’t compare equal. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)