CMake: add hdArnold plugin detection and deployment

Adds HDARNOLD_ROOT and ARNOLD_LOCATION cache variables to detect and deploy
the arnold-usd Hydra render delegate (hdArnold.dll, ndrArnold.dll) built from
arnold-usd Arnold-7.4.0.0 against MtoA 5.5.0. DLLs land in usd/ alongside
their plugInfo metadata dirs so LibraryPath ../hdArnold.dll resolves correctly.
ai.dll is copied to the exe root for Windows DLL search resolution.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CMakeLists.txt

@@ -46,6 +46,28 @@ else()
     message(STATUS "hdEmbree: NOT found — set HDEMBREE_USD_ROOT to a USD build with PXR_ENABLE_EMBREE_PLUGIN=ON")
 endif()
 
+# ── hdArnold detection ───────────────────────────────────────────────────────
+# HDARNOLD_ROOT: CMAKE_INSTALL_PREFIX used when building arnold-usd
+set(HDARNOLD_ROOT "" CACHE PATH "arnold-usd install directory (the one containing plugin/hdArnold.dll)")
+# ARNOLD_LOCATION: Arnold SDK root (MtoA dir) — needed to deploy ai.dll
+set(ARNOLD_LOCATION "" CACHE PATH "Arnold SDK root (e.g. C:/Autodesk/mtoa/5.5.0/2026) for ai.dll deployment")
+
+if(HDARNOLD_ROOT AND EXISTS "${HDARNOLD_ROOT}/plugin/hdArnold.dll")
+    set(OPENUSD_HAS_ARNOLD TRUE)
+    set(HDARNOLD_DLL          "${HDARNOLD_ROOT}/plugin/hdArnold.dll")
+    set(NDRARNOLD_DLL         "${HDARNOLD_ROOT}/plugin/ndrArnold.dll")
+    set(HDARNOLD_PLUGIN_DIR   "${HDARNOLD_ROOT}/plugin/hdArnold")
+    set(NDRARNOLD_PLUGIN_DIR  "${HDARNOLD_ROOT}/plugin/ndrArnold")
+    if(ARNOLD_LOCATION AND EXISTS "${ARNOLD_LOCATION}/bin/ai.dll")
+        set(ARNOLD_AI_DLL "${ARNOLD_LOCATION}/bin/ai.dll")
+    else()
+        set(ARNOLD_AI_DLL "")
+    endif()
+else()
+    set(OPENUSD_HAS_ARNOLD FALSE)
+    message(STATUS "hdArnold: NOT found — set HDARNOLD_ROOT to the arnold-usd install dir and ARNOLD_LOCATION to the MtoA root")
+endif()
+
 # Collect source files
 file(GLOB_RECURSE CORE_SOURCES "src/core/*.cpp")
 file(GLOB_RECURSE UI_SOURCES "src/ui/*.cpp")
@@ -131,6 +153,33 @@ if(WIN32)
             )
         endif()
     endif()
+
+    if(OPENUSD_HAS_ARNOLD)
+        add_custom_command(TARGET UsdLayerManager POST_BUILD
+            # DLLs go in usd/ alongside their metadata dirs (LibraryPath "../hdArnold.dll")
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                "${HDARNOLD_DLL}"
+                "$<TARGET_FILE_DIR:UsdLayerManager>/usd/hdArnold.dll"
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                "${NDRARNOLD_DLL}"
+                "$<TARGET_FILE_DIR:UsdLayerManager>/usd/ndrArnold.dll"
+            COMMAND ${CMAKE_COMMAND} -E copy_directory
+                "${HDARNOLD_PLUGIN_DIR}"
+                "$<TARGET_FILE_DIR:UsdLayerManager>/usd/hdArnold"
+            COMMAND ${CMAKE_COMMAND} -E copy_directory
+                "${NDRARNOLD_PLUGIN_DIR}"
+                "$<TARGET_FILE_DIR:UsdLayerManager>/usd/ndrArnold"
+            COMMENT "Copying hdArnold plugin..."
+        )
+        if(ARNOLD_AI_DLL)
+            add_custom_command(TARGET UsdLayerManager POST_BUILD
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                    "${ARNOLD_AI_DLL}"
+                    "$<TARGET_FILE_DIR:UsdLayerManager>"
+                COMMENT "Copying Arnold runtime ai.dll..."
+            )
+        endif()
+    endif()
 endif()
 
 # Compiler warnings
@@ -189,6 +238,27 @@ if(OPENUSD_HAS_EMBREE)
     endif()
 endif()
 
+if(OPENUSD_HAS_ARNOLD)
+    install(FILES "${HDARNOLD_DLL}" "${NDRARNOLD_DLL}"
+        DESTINATION bin/usd
+        OPTIONAL
+    )
+    install(DIRECTORY "${HDARNOLD_PLUGIN_DIR}"
+        DESTINATION bin/usd
+        OPTIONAL
+    )
+    install(DIRECTORY "${NDRARNOLD_PLUGIN_DIR}"
+        DESTINATION bin/usd
+        OPTIONAL
+    )
+    if(ARNOLD_AI_DLL)
+        install(FILES "${ARNOLD_AI_DLL}"
+            DESTINATION bin
+            OPTIONAL
+        )
+    endif()
+endif()
+
 # Install Python runtime DLLs required by OpenUSD.
 install(FILES
     "$ENV{LOCALAPPDATA}/Programs/Python/Python312/python312.dll"
@@ -245,6 +315,14 @@ if(OPENUSD_HAS_EMBREE AND EMBREE_DLLS)
 elseif(OPENUSD_HAS_EMBREE)
     message(WARNING "hdEmbree found but no Embree DLLs detected — set EMBREE_LOCATION to the Embree SDK bin/ dir or check HDEMBREE_USD_ROOT/bin")
 endif()
+message(STATUS "hdArnold support: ${OPENUSD_HAS_ARNOLD}")
+if(OPENUSD_HAS_ARNOLD)
+    message(STATUS "hdArnold DLL: ${HDARNOLD_DLL}")
+    message(STATUS "Arnold ai.dll: ${ARNOLD_AI_DLL}")
+    if(NOT ARNOLD_AI_DLL)
+        message(WARNING "hdArnold found but ai.dll not located — set ARNOLD_LOCATION to the MtoA root (e.g. C:/Autodesk/mtoa/5.5.0/2026)")
+    endif()
+endif()
 message(STATUS "=======================================")
 message(STATUS "")