The VRayOSLMtl material The VRayOSLMtl material automatically compiles OSL shaders to object code (OSO format) upon shader selection. No following compilations occur (even at render events). If the loaded OSL shader is modified use the context menu options Reset or Reload of the Shader file button for your changes to take effect.
An OSL compiler that translates *.osl files to *.oso files can be found in the tools subdirectory of V-Ray's root installation folder.
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Open Shading Language version 1.10 is supported on macOS.
References
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OSL home page at SPI - Open Shading Language home page at Sony Pictures Imageworks
