From b1f8fc7d2e64a35c4effef6fdac2462a992fdd47 Mon Sep 17 00:00:00 2001
From: "Hans J. Johnson" <hans-johnson@uiowa.edu>
Date: Sun, 6 Sep 2026 11:29:00 -0500
Subject: [PATCH] COMP: Detect the VTK rendering backend from imported targets

VTK 9 does not define VTK_RENDERING_BACKEND, so the fallback to OpenGL2
always fired and every "STREQUAL None" guard was unconditionally true.
ITKVtkGlue therefore required a rendering-enabled VTK even though its
sources and tests already guard the viewer classes for the None case.

The same block appears in the two export-code strings, which run at the
consumer's find_package(ITK), so a downstream project re-ran the broken
detection as well. All three are changed.
---
 Modules/Bridge/VtkGlue/CMakeLists.txt           | 12 ++++++++++--
 Modules/Bridge/VtkGlue/itk-module-init.cmake    |  7 ++++++-
 .../Bridge/VtkGlue/wrapping/itkViewImage.wrap   | 17 ++++++++++-------
 3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/Modules/Bridge/VtkGlue/CMakeLists.txt b/Modules/Bridge/VtkGlue/CMakeLists.txt
index 74231aaff7e..3a3f9147773 100644
--- a/Modules/Bridge/VtkGlue/CMakeLists.txt
+++ b/Modules/Bridge/VtkGlue/CMakeLists.txt
@@ -85,7 +85,11 @@ set(
 set(VTK_DIR \"${VTK_DIR}\")
 find_package(VTK 9.1 NO_MODULE REQUIRED)
 if(NOT VTK_RENDERING_BACKEND)
-  set(VTK_RENDERING_BACKEND OpenGL2)
+  if(TARGET VTK::RenderingOpenGL2)
+    set(VTK_RENDERING_BACKEND OpenGL2)
+  else()
+    set(VTK_RENDERING_BACKEND None)
+  endif()
 endif()
 set(_target_freetypeopengl)
 if(TARGET VTK::RenderingFreeType\${VTK_RENDERING_BACKEND})
@@ -123,7 +127,11 @@ if(NOT ITK_BINARY_DIR)
   set(VTK_DIR \"${VTK_DIR}\")
   find_package(VTK 9.1 NO_MODULE REQUIRED)
   if(NOT VTK_RENDERING_BACKEND)
-    set(VTK_RENDERING_BACKEND OpenGL2)
+    if(TARGET VTK::RenderingOpenGL2)
+      set(VTK_RENDERING_BACKEND OpenGL2)
+    else()
+      set(VTK_RENDERING_BACKEND None)
+    endif()
   endif()
   set(_target_freetypeopengl)
   if(TARGET VTK::RenderingFreeType\${VTK_RENDERING_BACKEND})
diff --git a/Modules/Bridge/VtkGlue/itk-module-init.cmake b/Modules/Bridge/VtkGlue/itk-module-init.cmake
index 550f1dce272..0873413d3a7 100644
--- a/Modules/Bridge/VtkGlue/itk-module-init.cmake
+++ b/Modules/Bridge/VtkGlue/itk-module-init.cmake
@@ -16,8 +16,13 @@ set(VERSION_MIN "9.1.0")
 
 find_package(VTK ${VERSION_MIN} NO_MODULE REQUIRED)
 
+# VTK 9 omits VTK_RENDERING_BACKEND; infer rendering support from its OpenGL2 target.
 if(NOT VTK_RENDERING_BACKEND)
-  set(VTK_RENDERING_BACKEND OpenGL2)
+  if(TARGET VTK::RenderingOpenGL2)
+    set(VTK_RENDERING_BACKEND OpenGL2)
+  else()
+    set(VTK_RENDERING_BACKEND None)
+  endif()
 endif()
 
 set(_target_freetypeopengl)
diff --git a/Modules/Bridge/VtkGlue/wrapping/itkViewImage.wrap b/Modules/Bridge/VtkGlue/wrapping/itkViewImage.wrap
index 0b4aa6cae29..169247ea1ff 100644
--- a/Modules/Bridge/VtkGlue/wrapping/itkViewImage.wrap
+++ b/Modules/Bridge/VtkGlue/wrapping/itkViewImage.wrap
@@ -1,8 +1,11 @@
-itk_wrap_class("itk::ViewImage")
-unique(types "${WRAP_ITK_SCALAR}")
-foreach(d ${ITK_WRAP_IMAGE_DIMS})
-  foreach(t ${types})
-    itk_wrap_template("${ITKM_I${t}${d}}" "${ITKT_I${t}${d}}")
+# itk::ViewImage needs vtkRenderWindow, absent from a rendering-free VTK.
+if(NOT VTK_RENDERING_BACKEND STREQUAL "None")
+  itk_wrap_class("itk::ViewImage")
+  unique(types "${WRAP_ITK_SCALAR}")
+  foreach(d ${ITK_WRAP_IMAGE_DIMS})
+    foreach(t ${types})
+      itk_wrap_template("${ITKM_I${t}${d}}" "${ITKT_I${t}${d}}")
+    endforeach()
   endforeach()
-endforeach()
-itk_end_wrap_class()
+  itk_end_wrap_class()
+endif()