FindOpenGL: Add support for GLVND on Linux

Find GLVND components if available.  Add `GLX` and `EGL` options for
COMPONENTS that allow requesting these libraries explicitly.  Introduce
new import targets for these windowing-system-specific libraries.

On a GLVND system, populate the legacy `OPENGL_LIBRARIES` variable and
the `OpenGL::GL` target using the `OpenGL` and `GLX` components.  On
non-GLVND systems, continue to use the legacy `GL` library and simply do
not provide the GLVND components.  Application code can choose to adapt
based on the availability of GLVND components as imported targets.

Copyright.txt

@@ -76,6 +76,7 @@ The following individuals and institutions are among the Contributors:
 * Nicolas Bock <[email protected]>
 * Nicolas Despres <[email protected]>
 * Nikita Krupen'ko <[email protected]>
+* NVIDIA Corporation <www.nvidia.com>
 * OpenGamma Ltd. <opengamma.com>
 * Per Øyvind Karlsen <[email protected]>
 * Peter Collingbourne <[email protected]>

Help/release/dev/FindOpenGL-glvnd.rst

@@ -0,0 +1,4 @@
+FindOpenGL-glvnd
+----------------
+
+* The :module:`FindOpenGL` module gained support for GLVND on Linux.

Modules/FindOpenGL.cmake

@@ -7,15 +7,28 @@
 #
 # FindModule for OpenGL and GLU.
 #
+# Optional COMPONENTS
+# ^^^^^^^^^^^^^^^^^^^
+#
+# This module respects several optional COMPONENTS: ``EGL``, ``GLX``, and
+# ``OpenGL``.  There are corresponding import targets for each of these flags.
+#
 # IMPORTED Targets
 # ^^^^^^^^^^^^^^^^
 #
 # This module defines the :prop_tgt:`IMPORTED` targets:
 #
 # ``OpenGL::GL``
-#  Defined if the system has OpenGL.
+#  Defined to the platform-specific OpenGL libraries if the system has OpenGL.
+# ``OpenGL::OpenGL``
+#  Defined to libOpenGL if the system is GLVND-based.
+#  ``OpenGL::GL``
 # ``OpenGL::GLU``
 #  Defined if the system has GLU.
+# ``OpenGL::GLX``
+#  Defined if the system has GLX.
+# ``OpenGL::EGL``
+#  Defined if the system has EGL.
 #
 # Result Variables
 # ^^^^^^^^^^^^^^^^
@@ -23,40 +36,87 @@
 # This module sets the following variables:
 #
 # ``OPENGL_FOUND``
-#  True, if the system has OpenGL.
+#  True, if the system has OpenGL and all components are found.
 # ``OPENGL_XMESA_FOUND``
 #  True, if the system has XMESA.
 # ``OPENGL_GLU_FOUND``
 #  True, if the system has GLU.
+# ``OpenGL_OpenGL_FOUND``
+#  True, if the system has an OpenGL library.
+# ``OpenGL_GLX_FOUND``
+#  True, if the system has GLX.
+# ``OpenGL_EGL_FOUND``
+#  True, if the system has EGL.
 # ``OPENGL_INCLUDE_DIR``
 #  Path to the OpenGL include directory.
+# ``OPENGL_EGL_INCLUDE_DIRS``
+#  Path to the EGL include directory.
 # ``OPENGL_LIBRARIES``
-#  Paths to the OpenGL and GLU libraries.
+#  Paths to the OpenGL library, windowing system libraries, and GLU libraries.
+#  On Linux, this assumes glX and is never correct for EGL-based targets.
+#  Clients are encouraged to use the ``OpenGL::*`` import targets instead.
 #
-# If you want to use just GL you can use these values:
+# Cache variables
+# ^^^^^^^^^^^^^^^
 #
-# ``OPENGL_gl_LIBRARY``
-#  Path to the OpenGL library.
+# The following cache variables may also be set:
+#
+# ``OPENGL_egl_LIBRARY``
+#  Path to the EGL library.
 # ``OPENGL_glu_LIBRARY``
 #  Path to the GLU library.
+# ``OPENGL_glx_LIBRARY``
+#  Path to the GLVND 'GLX' library.
+# ``OPENGL_opengl_LIBRARY``
+#  Path to the GLVND 'OpenGL' library
+# ``OPENGL_gl_LIBRARY``
+#  Path to the OpenGL library.  New code should prefer the ``OpenGL::*`` import
+#  targets.
+#
+# Linux-specific
+# ^^^^^^^^^^^^^^
 #
-# OSX Specific
-# ^^^^^^^^^^^^
+# Some Linux systems utilize GLVND as a new ABI for OpenGL.  GLVND separates
+# context libraries from OpenGL itself; OpenGL lives in "libOpenGL", and
+# contexts are defined in "libGLX" or "libEGL".  GLVND is currently the only way
+# to get OpenGL 3+ functionality via EGL in a manner portable across vendors.
 #
-# On OSX default to using the framework version of OpenGL. People will
-# have to change the cache values of OPENGL_glu_LIBRARY and
+# On Linux systems FindOpenGL defaults to using GLVND if available.  Users can
+# utilize GLVND explicitly with targets ``OpenGL::OpenGL``, ``OpenGL::GLX``, and
+# ``OpenGL::EGL``.  Additionally, when GLVND is available the ``OpenGL::GL``
+# target is equivalent to ``OpenGL::OpenGL OpenGL::GLX``.  When the system is
+# not GLVND-based, ``OpenGL::GL`` expands to libGL as it has historically done.
+# Thus, for non-EGL-based Linux targets, the ``OpenGL::GL`` target is most
+# portable.
+#
+# For EGL targets the client must rely on GLVND support on the user's system.
+# Linking should use the ``OpenGL::OpenGL OpenGL::EGL`` targets.  Using GLES*
+# libraries is theoretically possible in place of ``OpenGL::OpenGL``, but this
+# module does not currently support that; contributions welcome.
+#
+# ``OPENGL_egl_LIBRARY`` and ``OPENGL_EGL_INCLUDE_DIRS`` are defined in the case of
+# GLVND.  For non-GLVND Linux and other systems these are left undefined.
+#
+# macOS-Specific
+# ^^^^^^^^^^^^^^
+#
+# On OSX FindOpenGL defaults to using the framework version of OpenGL. People
+# will have to change the cache values of OPENGL_glu_LIBRARY and
 # OPENGL_gl_LIBRARY to use OpenGL with X11 on OSX.
 
-
 set(_OpenGL_REQUIRED_VARS OPENGL_gl_LIBRARY)
 
-if (CYGWIN)
+# Provide OPENGL_USE_<C> variables for each component.
+foreach(component ${OpenGL_FIND_COMPONENTS})
+  string(TOUPPER ${component} _COMPONENT)
+  set(OPENGL_USE_${_COMPONENT} 1)
+endforeach()
 
+if (CYGWIN)
   find_path(OPENGL_INCLUDE_DIR GL/gl.h )
   list(APPEND _OpenGL_REQUIRED_VARS OPENGL_INCLUDE_DIR)
 
   find_library(OPENGL_gl_LIBRARY opengl32 )
-
   find_library(OPENGL_glu_LIBRARY glu32 )
 
 elseif (WIN32)
@@ -70,7 +130,6 @@ elseif (WIN32)
   endif()
 
 elseif (APPLE)
-
   # The OpenGL.framework provides both gl and glu
   find_library(OPENGL_gl_LIBRARY OpenGL DOC "OpenGL library for OS X")
   find_library(OPENGL_glu_LIBRARY OpenGL DOC
@@ -104,15 +163,14 @@ else()
   # fail since the compiler finds the Mesa headers but NVidia's library.
   # Make sure the NVIDIA directory comes BEFORE the others.
   #  - Atanas Georgiev <[email protected]>
-
   find_path(OPENGL_INCLUDE_DIR GL/gl.h
     /usr/share/doc/NVIDIA_GLX-1.0/include
     /usr/openwin/share/include
     /opt/graphics/OpenGL/include /usr/X11R6/include
     ${_OPENGL_INCLUDE_PATH}
   )
-  list(APPEND _OpenGL_REQUIRED_VARS OPENGL_INCLUDE_DIR)
-
+  find_path(OPENGL_GLX_INCLUDE_DIR GL/glx.h ${_OPENGL_INCLUDE_PATH})
+  find_path(OPENGL_EGL_INCLUDE_DIR EGL/egl.h ${_OPENGL_INCLUDE_PATH})
   find_path(OPENGL_xmesa_INCLUDE_DIR GL/xmesa.h
     /usr/share/doc/NVIDIA_GLX-1.0/include
     /usr/openwin/share/include
@@ -126,6 +184,90 @@ else()
           /usr/shlib /usr/X11R6/lib
           ${_OPENGL_LIB_PATH}
   )
+  # Search for the GLVND libraries.  We do this regardless of COMPONENTS; we'll
+  # take into account the COMPONENTS logic later.
+  find_library(OPENGL_opengl_LIBRARY
+    NAMES OpenGL
+    PATHS /usr/X11R6/lib
+          ${_OPENGL_LIB_PATH}
+  )
+
+  find_library(OPENGL_glx_LIBRARY
+    NAMES GLX
+    PATHS /usr/X11R6/lib
+          ${_OPENGL_LIB_PATH}
+  )
+
+  find_library(OPENGL_egl_LIBRARY
+    NAMES EGL
+    PATHS ${_OPENGL_LIB_PATH}
+  )
+
+  find_library(OPENGL_glu_LIBRARY
+    NAMES GLU MesaGLU
+    PATHS ${OPENGL_gl_LIBRARY}
+          /opt/graphics/OpenGL/lib
+          /usr/openwin/lib
+          /usr/shlib /usr/X11R6/lib
+  )
+
+  # FPHSA cannot handle "this OR that is required", so we conditionally set what
+  # it must look for.  First clear any previous config we might have done:
+  set(_OpenGL_REQUIRED_VARS)
+
+  # now we append the libraries as appropriate.  The complicated logic
+  # basically comes down to "use libOpenGL when we can, and add in specific
+  # context mechanisms when requested, or we need them to preserve the previous
+  # default where glx is always available."
+  if((NOT OPENGL_USE_EGL AND
+      NOT OPENGL_opengl_LIBRARY AND
+          OPENGL_glx_LIBRARY AND
+      NOT OPENGL_gl_LIBRARY) OR
+     (NOT OPENGL_USE_EGL AND
+      NOT OPENGL_glx_LIBRARY AND
+      NOT OPENGL_gl_LIBRARY) OR
+     (NOT OPENGL_USE_EGL AND
+          OPENGL_opengl_LIBRARY AND
+          OPENGL_glx_LIBRARY) OR
+     (    OPENGL_USE_EGL))
+    list(APPEND _OpenGL_REQUIRED_VARS OPENGL_opengl_LIBRARY)
+  endif()
+
+  # GLVND GLX library.  Preferred when available.
+  if((NOT OPENGL_USE_OPENGL AND
+      NOT OPENGL_USE_GLX AND
+      NOT OPENGL_USE_EGL AND
+      NOT OPENGL_glx_LIBRARY AND
+      NOT OPENGL_gl_LIBRARY) OR
+     (    OPENGL_USE_GLX AND
+      NOT OPENGL_USE_EGL AND
+      NOT OPENGL_glx_LIBRARY AND
+      NOT OPENGL_gl_LIBRARY) OR
+     (NOT OPENGL_USE_EGL AND
+          OPENGL_opengl_LIBRARY AND
+          OPENGL_glx_LIBRARY) OR
+     (OPENGL_USE_GLX AND OPENGL_USE_EGL))
+    list(APPEND _OpenGL_REQUIRED_VARS OPENGL_glx_LIBRARY)
+  endif()
+
+  # GLVND EGL library.
+  if(OPENGL_USE_EGL)
+    list(APPEND _OpenGL_REQUIRED_VARS OPENGL_egl_LIBRARY)
+  endif()
+
+  # Old-style "libGL" library: used as a fallback when GLVND isn't available.
+  if((NOT OPENGL_USE_EGL AND
+      NOT OPENGL_opengl_LIBRARY AND
+          OPENGL_glx_LIBRARY AND
+          OPENGL_gl_LIBRARY) OR
+     (NOT OPENGL_USE_EGL AND
+      NOT OPENGL_glx_LIBRARY AND
+          OPENGL_gl_LIBRARY))
+    list(APPEND _OpenGL_REQUIRED_VARS OPENGL_gl_LIBRARY)
+  endif()
+
+  # We always need the 'gl.h' include dir.
+  list(APPEND _OpenGL_REQUIRED_VARS OPENGL_INCLUDE_DIR)
 
   unset(_OPENGL_INCLUDE_PATH)
   unset(_OPENGL_LIB_PATH)
@@ -137,42 +279,99 @@ else()
           /usr/openwin/lib
           /usr/shlib /usr/X11R6/lib
   )
-
 endif ()
 
-if(OPENGL_gl_LIBRARY)
+if(OPENGL_xmesa_INCLUDE_DIR)
+  set( OPENGL_XMESA_FOUND "YES" )
+else()
+  set( OPENGL_XMESA_FOUND "NO" )
+endif()
 
-    if(OPENGL_xmesa_INCLUDE_DIR)
-      set( OPENGL_XMESA_FOUND "YES" )
-    else()
-      set( OPENGL_XMESA_FOUND "NO" )
-    endif()
+if(OPENGL_glu_LIBRARY)
+  set( OPENGL_GLU_FOUND "YES" )
+else()
+  set( OPENGL_GLU_FOUND "NO" )
+endif()
 
-    set( OPENGL_LIBRARIES  ${OPENGL_gl_LIBRARY} ${OPENGL_LIBRARIES})
-    if(OPENGL_glu_LIBRARY)
-      set( OPENGL_GLU_FOUND "YES" )
-      if(NOT "${OPENGL_glu_LIBRARY}" STREQUAL "${OPENGL_gl_LIBRARY}")
-        set( OPENGL_LIBRARIES ${OPENGL_glu_LIBRARY} ${OPENGL_LIBRARIES} )
-      endif()
-    else()
-      set( OPENGL_GLU_FOUND "NO" )
-    endif()
+# OpenGL_OpenGL_FOUND is a bit unique in that it is okay if /either/ libOpenGL
+# or libGL is found.
+# Using libGL with libEGL is never okay, though; we handle that case later.
+if(NOT OPENGL_opengl_LIBRARY AND NOT OPENGL_gl_LIBRARY)
+  set(OpenGL_OpenGL_FOUND FALSE)
+else()
+  set(OpenGL_OpenGL_FOUND TRUE)
+endif()
 
-    # This deprecated setting is for backward compatibility with CMake1.4
-    set (OPENGL_LIBRARY ${OPENGL_LIBRARIES})
+if(OPENGL_glx_LIBRARY AND OPENGL_GLX_INCLUDE_DIR)
+  set(OpenGL_GLX_FOUND TRUE)
+else()
+  set(OpenGL_GLX_FOUND FALSE)
+endif()
 
+if(OPENGL_egl_LIBRARY AND OPENGL_EGL_INCLUDE_DIR)
+  set(OpenGL_EGL_FOUND TRUE)
+else()
+  set(OpenGL_EGL_FOUND FALSE)
 endif()
 
-# This deprecated setting is for backward compatibility with CMake1.4
-set(OPENGL_INCLUDE_PATH ${OPENGL_INCLUDE_DIR})
+# User-visible names should be plural.
+if(OPENGL_EGL_INCLUDE_DIR)
+  set(OPENGL_EGL_INCLUDE_DIRS ${OPENGL_EGL_INCLUDE_DIR})
+endif()
 
 include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(OpenGL REQUIRED_VARS ${_OpenGL_REQUIRED_VARS})
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(OpenGL REQUIRED_VARS ${_OpenGL_REQUIRED_VARS}
+                                  HANDLE_COMPONENTS)
 unset(_OpenGL_REQUIRED_VARS)
 
 # OpenGL:: targets
 if(OPENGL_FOUND)
-  if(NOT TARGET OpenGL::GL)
+  # ::OpenGL is a GLVND library, and thus Linux-only: we don't bother checking
+  # for a framework version of this library.
+  if(OPENGL_opengl_LIBRARY AND NOT TARGET OpenGL::OpenGL)
+    if(IS_ABSOLUTE "${OPENGL_opengl_LIBRARY}")
+      add_library(OpenGL::OpenGL UNKNOWN IMPORTED)
+      set_target_properties(OpenGL::OpenGL PROPERTIES IMPORTED_LOCATION
+                            "${OPENGL_opengl_LIBRARY}")
+    else()
+      add_library(OpenGL::OpenGL INTERFACE IMPORTED)
+      set_target_properties(OpenGL::OpenGL PROPERTIES IMPORTED_LIBNAME
+                            "${OPENGL_opengl_LIBRARY}")
+    endif()
+    set_target_properties(OpenGL::OpenGL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_INCLUDE_DIR}")
+  endif()
+
+  # ::GLX is a GLVND library, and thus Linux-only: we don't bother checking
+  # for a framework version of this library.
+  if(OpenGL_GLX_FOUND AND NOT TARGET OpenGL::GLX)
+    if(IS_ABSOLUTE "${OPENGL_glx_LIBRARY}")
+      add_library(OpenGL::GLX UNKNOWN IMPORTED)
+      set_target_properties(OpenGL::GLX PROPERTIES IMPORTED_LOCATION
+                            "${OPENGL_glx_LIBRARY}")
+    else()
+      add_library(OpenGL::GLX INTERFACE IMPORTED)
+      set_target_properties(OpenGL::GLX PROPERTIES IMPORTED_LIBNAME
+                            "${OPENGL_glx_LIBRARY}")
+    endif()
+    set_target_properties(OpenGL::GLX PROPERTIES INTERFACE_LINK_LIBRARIES
+                          OpenGL::OpenGL)
+    set_target_properties(OpenGL::GLX PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_GLX_INCLUDE_DIR}")
+  endif()
+
+  if(TARGET OpenGL::OpenGL AND TARGET OpenGL::GLX AND NOT TARGET OpenGL::GL)
+    # if GLVND with GLX is available, make ::GL a synonym for 'OpenGL::OpenGL
+    # OpenGL::GLX'.
+    add_library(OpenGL::GL INTERFACE IMPORTED)
+    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_LINK_LIBRARIES
+                          OpenGL::OpenGL)
+    set_property(TARGET OpenGL::GL APPEND PROPERTY INTERFACE_LINK_LIBRARIES
+                 OpenGL::GLX)
+    set_target_properties(OpenGL::GL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_INCLUDE_DIR}")
+
+  elseif(NOT TARGET OpenGL::GL)
     if(IS_ABSOLUTE "${OPENGL_gl_LIBRARY}")
       add_library(OpenGL::GL UNKNOWN IMPORTED)
       if(OPENGL_gl_LIBRARY MATCHES "/([^/]+)\\.framework$")
@@ -195,6 +394,28 @@ if(OPENGL_FOUND)
       INTERFACE_INCLUDE_DIRECTORIES "${OPENGL_INCLUDE_DIR}")
   endif()
 
+  # ::EGL is a GLVND library, and thus Linux-only: we don't bother checking
+  # for a framework version of this library.
+  # Note we test for OpenGL::OpenGL as a target.  When this module is updated to
+  # support GLES, we would additionally want to check for the hypothetical GLES
+  # target and enable EGL if either ::GLES or ::OpenGL is created.
+  if(TARGET OpenGL::OpenGL AND OpenGL_EGL_FOUND AND NOT TARGET OpenGL::EGL)
+    if(IS_ABSOLUTE "${OPENGL_egl_LIBRARY}")
+      add_library(OpenGL::EGL UNKNOWN IMPORTED)
+      set_target_properties(OpenGL::EGL PROPERTIES IMPORTED_LOCATION
+                            "${OPENGL_egl_LIBRARY}")
+    else()
+      add_library(OpenGL::EGL INTERFACE IMPORTED)
+      set_target_properties(OpenGL::EGL PROPERTIES IMPORTED_LIBNAME
+                            "${OPENGL_egl_LIBRARY}")
+    endif()
+    set_target_properties(OpenGL::EGL PROPERTIES INTERFACE_LINK_LIBRARIES
+                          OpenGL::OpenGL)
+    # Note that EGL's include directory is different from OpenGL/GLX's!
+    set_target_properties(OpenGL::EGL PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                          "${OPENGL_EGL_INCLUDE_DIR}")
+  endif()
+
   if(OPENGL_GLU_FOUND AND NOT TARGET OpenGL::GLU)
     if(IS_ABSOLUTE "${OPENGL_glu_LIBRARY}")
       add_library(OpenGL::GLU UNKNOWN IMPORTED)
@@ -217,11 +438,31 @@ if(OPENGL_FOUND)
     set_target_properties(OpenGL::GLU PROPERTIES
       INTERFACE_LINK_LIBRARIES OpenGL::GL)
   endif()
+
+  # OPENGL_LIBRARIES mirrors OpenGL::GL's logic ...
+  set(OPENGL_LIBRARIES ${OPENGL_gl_LIBRARY})
+  if(TARGET OpenGL::GLX AND TARGET OpenGL::OpenGL)
+    set(OPENGL_LIBRARIES ${OPENGL_opengl_LIBRARY} ${OPENGL_glx_LIBRARY})
+  endif()
+  # ... and also includes GLU, if available.
+  if(TARGET OpenGL::GLU)
+    list(APPEND OPENGL_LIBRARIES ${OPENGL_glu_LIBRARY})
+  endif()
 endif()
 
+# This deprecated setting is for backward compatibility with CMake1.4
+set(OPENGL_LIBRARY ${OPENGL_LIBRARIES})
+# This deprecated setting is for backward compatibility with CMake1.4
+set(OPENGL_INCLUDE_PATH ${OPENGL_INCLUDE_DIR})
+
 mark_as_advanced(
   OPENGL_INCLUDE_DIR
   OPENGL_xmesa_INCLUDE_DIR
+  OPENGL_egl_LIBRARY
   OPENGL_glu_LIBRARY
+  OPENGL_glx_LIBRARY
   OPENGL_gl_LIBRARY
+  OPENGL_opengl_LIBRARY
+  OPENGL_EGL_INCLUDE_DIR
+  OPENGL_GLX_INCLUDE_DIR
 )

Tests/FindOpenGL/CMakeLists.txt

@@ -1,3 +1,9 @@
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND CMake_TEST_FindOpenGL_VND)
+  set(_vnd_testing TRUE)
+else()
+  set(_vnd_testing FALSE)
+endif()
+
 add_test(NAME FindOpenGL.Test COMMAND
   ${CMAKE_CTEST_COMMAND} -C $<CONFIGURATION>
   --build-and-test
@@ -6,5 +12,6 @@ add_test(NAME FindOpenGL.Test COMMAND
   ${build_generator_args}
   --build-project TestFindOpenGL
   --build-options ${build_options}
+  -DOpenGL_TEST_VND=${_vnd_testing}
   --test-command ${CMAKE_CTEST_COMMAND} -V -C $<CONFIGURATION>
   )

Tests/FindOpenGL/Test/CMakeLists.txt

@@ -1,14 +1,70 @@
-cmake_minimum_required(VERSION 3.7)
+cmake_minimum_required(VERSION 3.9)
 project(TestFindOpenGL C)
 include(CTest)
 
 find_package(OpenGL REQUIRED)
 
+# import target for GLU
 add_executable(test_tgt main.c)
 target_link_libraries(test_tgt OpenGL::GLU)
 add_test(NAME test_tgt COMMAND test_tgt)
 
+# OPENGL_LIBRARIES should be whatever libraries are needed to link.
 add_executable(test_var main.c)
 target_include_directories(test_var PRIVATE ${OPENGL_INGLUDE_DIRS})
 target_link_libraries(test_var PRIVATE ${OPENGL_LIBRARIES})
 add_test(NAME test_var COMMAND test_var)
+
+# VND support adds an ::OpenGL import target.  This can be used for OpenGL-only
+# code (code that does not manipulate contexts, like our 'main.c').  Without
+# VND, ::GL can be used for both context and non-context OpenGL code.
+if(OpenGL_TEST_VND)
+  add_executable(test_comp_none main.c)
+  target_link_libraries(test_comp_none PRIVATE OpenGL::OpenGL)
+  add_test(NAME test_comp_none COMMAND test_comp_none)
+else()
+  add_executable(test_comp_none main.c)
+  target_link_libraries(test_comp_none PRIVATE OpenGL::GL)
+  add_test(NAME test_comp_none COMMAND test_comp_none)
+endif()
+
+# GLX
+if(OpenGL_TEST_VND)
+  find_package(OpenGL REQUIRED COMPONENTS OpenGL GLX)
+  add_executable(test_comp_glx main.c)
+  target_link_libraries(test_comp_glx PRIVATE OpenGL::OpenGL OpenGL::GLX)
+  add_test(NAME test_comp_glx COMMAND test_comp_glx)
+else()
+  # non-VND systems won't have it, but an optional search for GLX should still
+  # be okay.
+  find_package(OpenGL COMPONENTS GLX)
+  add_executable(test_comp_glx_novnd main.c)
+  target_link_libraries(test_comp_glx_novnd PRIVATE OpenGL::GL)
+  add_test(NAME test_comp_glx_novnd COMMAND test_comp_glx_novnd)
+endif()
+
+# EGL is only available on Linux+GLVND at present.
+if(OpenGL_TEST_VND)
+  find_package(OpenGL COMPONENTS OpenGL EGL)
+  if(OpenGL_EGL_FOUND)
+    add_executable(test_comp_egl main.c)
+    target_link_libraries(test_comp_egl PRIVATE OpenGL::OpenGL OpenGL::EGL)
+    add_test(NAME test_comp_egl COMMAND test_comp_egl)
+    # EGL-only code should not link to GLX.
+    execute_process(COMMAND ldd test_comp_egl
+                    OUTPUT_VARIABLE LDD_OUT
+                    ERROR_VARIABLE LDD_ERR)
+    if("${LDD_OUT}" MATCHES "GLX")
+      message(FATAL_ERROR "EGL-only code links to GLX!")
+    endif()
+  endif()
+
+  # all three COMPONENTS together.
+  find_package(OpenGL COMPONENTS OpenGL EGL GLX)
+  if(OpenGL_EGL_FOUND AND OpenGL_GLX_FOUND)
+    add_executable(test_comp_both main.c)
+    target_link_libraries(test_comp_both PRIVATE OpenGL::OpenGL OpenGL::EGL
+                          OpenGL::GLX)
+    add_test(NAME test_comp_both COMMAND test_comp_both)
+  endif()
+endif()