diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index d57d265..9a4d5d1 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -19,18 +19,19 @@ jobs:
           - linux/arm/v6
           - linux/arm/v7
           - linux/arm64
+        gtk4: [ON, OFF]
 
     steps:
     - uses: actions/checkout@v4
 
     - name: dependencies
-      run: sudo apt install --fix-missing cmake gcc gfortran libx11-dev libglu1-mesa-dev libmotif-dev tcsh libxaw7-dev libglew-dev libdlm-dev libgl2ps-dev libpng-dev
+      run: sudo apt install --fix-missing cmake gcc gfortran libx11-dev libglu1-mesa-dev libmotif-dev tcsh libxaw7-dev libglew-dev libdlm-dev libgl2ps-dev libpng-dev libgtk-4-dev
 
     - name: clean
       run: make clean
 
     - name: configure
-      run: make config
+      run: make config OPENPHIGS_OPTS="-DUSE_GTK4=${{ matrix.gtk4 }}"
 
     - name: Build
       run: make build
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index e89368f..b3a8519 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -7,11 +7,21 @@ FIND_PACKAGE(PNG REQUIRED)
 FIND_PACKAGE(OpenGL REQUIRED)
 FIND_PACKAGE(X11 REQUIRED)
 FIND_PACKAGE(XMU REQUIRED)
-FIND_PACKAGE(Motif REQUIRED)
+FIND_PACKAGE(Motif)
 FIND_PACKAGE(GLEW)
 FIND_PACKAGE(epoxy)
 FIND_LIBRARY(MATH_LIBRARY m)
 
+option(USE_GTK4 "Use GTK4 for widgets instead of Motif/Xaw" OFF)
+if (USE_GTK4)
+  FIND_PACKAGE(PkgConfig REQUIRED)
+  pkg_check_modules(GTK4 REQUIRED gtk4)
+else()
+  if(NOT Motif_FOUND)
+    message(FATAL_ERROR "Motif not found but USE_GTK4 is OFF. You must have Motif installed.")
+  endif()
+endif()
+
 if (APPLE)
   set(CMAKE_C_COMPILER gcc)
   set(CMAKE_C_COMPILER_ID GNU)
@@ -50,8 +60,12 @@ add_definitions(-g)
 # add_definitions(-DDEBUGINP)
 #add_definitions(-DDEBUGCL)
 #
-# use Motif widgets rather than Xaw
-add_definitions(-DMOTIF)
+# use Motif or GTK4 widgets rather than Xaw
+if(USE_GTK4)
+  add_definitions(-DGTK4_EXT)
+else()
+  add_definitions(-DMOTIF)
+endif()
 
 # use GLEW or epoxy
 if (USE_GLEW)
@@ -65,15 +79,23 @@ endif()
 # gl2ps
 FIND_PATH(GL2PS_INCLUDE_DIR gl2ps.h)
 FIND_LIBRARY(GL2PS_LIBRARY NAMES gl2ps)
-add_definitions(-DGLEW)
-set (GL_INCLUDES ${GL2PS_INCLUDE_DIR})
+if (GL2PS_INCLUDE_DIR)
+  list(APPEND GL_INCLUDES ${GL2PS_INCLUDE_DIR})
+endif()
 
 message(STATUS "GL Includes=${GL_INCLUDES}")
-include_directories(${X11_INCLUDE_DIR} ${MOTIF_INCLUDE_DIR} ${GL_INCLUDES})
+if (USE_GTK4)
+  include_directories(${X11_INCLUDE_DIR} ${GTK4_INCLUDE_DIRS} ${GL_INCLUDES})
+  SET(CMAKE_C_FLAGS "$ENV{CFLAGS} -DGTK4_EXT -g -O0 -fPIC -Wno-deprecated-non-prototype")
+  SET(CMAKE_CXX_FLAGS "$ENV{CXXFLAGS} -DGTK4_EXT -g -O0 -fPIC -Wno-deprecated-non-prototype")
+  SET(CMAKE_Fortran_FLAGS "$ENV{FFLAGS} -DGTK4_EXT -finit-local-zero -g -O0 -fPIC")
+else()
+  include_directories(${X11_INCLUDE_DIR} ${MOTIF_INCLUDE_DIR} ${GL_INCLUDES})
+  SET(CMAKE_C_FLAGS "$ENV{CFLAGS} -DMOTIF -g -O0 -fPIC -Wno-deprecated-non-prototype")
+  SET(CMAKE_CXX_FLAGS "$ENV{CXXFLAGS} -DMOTIF -g -O0 -fPIC -Wno-deprecated-non-prototype")
+  SET(CMAKE_Fortran_FLAGS "$ENV{FFLAGS} -DMOTIF -finit-local-zero -g -O0 -fPIC")
+endif()
 
-SET(CMAKE_C_FLAGS  "$ENV{CFLAGS} -DMOTIF -g -O0 -fPIC -Wno-deprecated-non-prototype")
-SET(CMAKE_CXX_FLAGS "$ENV{CXXFLAGS} -DMOTIF -g -O0  -fPIC -Wno-deprecated-non-prototype")
-SET(CMAKE_Fortran_FLAGS "$ENV{FFLAGS} -DMOTIF -finit-local-zero -g -O0 -fPIC")
 
 SET(CMAKE_EXE_LINKER_FLAGS $ENV{LDFLAGS})
 if (OPENPHIGS_FORCE_32)
@@ -98,11 +120,17 @@ INCLUDE_DIRECTORIES(
     set (XLIBS "")
 endif()
 
+if (USE_GTK4)
+  set(UI_LIBRARIES ${GTK4_LIBRARIES})
+else()
+  set(UI_LIBRARIES ${MOTIF_LIBRARIES})
+endif()
+
 if (USE_GLEW)
   SET(PHIGS_LIBRARIES
       phigs
       ${XLIBS}
-      ${MOTIF_LIBRARIES}
+      ${UI_LIBRARIES}
       ${XMU_LIBRARY}
       ${PNG_LIBRARY}
       ${X11_Xaw_LIB}
@@ -117,7 +145,7 @@ else()
   SET(PHIGS_LIBRARIES
       phigs
       ${XLIBS}
-      ${MOTIF_LIBRARIES}
+      ${UI_LIBRARIES}
       ${XMU_LIBRARY}
       ${PNG_LIBRARY}
       ${X11_Xaw_LIB}
diff --git a/src/include/phigs/private/wsglP.h b/src/include/phigs/private/wsglP.h
index d5d6266..b57e85b 100644
--- a/src/include/phigs/private/wsglP.h
+++ b/src/include/phigs/private/wsglP.h
@@ -197,6 +197,17 @@ extern "C" {
    */
   void wsgl_clear_geometry();
 
+  /*******************************************************************************
+  /*******************************************************************************
+   * wsgl_set_transparency_pass
+   *
+   * DESCR:       Set the transparency pass and update HLHSR depth mask.
+   * RETURNS:     N/A
+   */
+  void wsgl_set_transparency_pass(Ws *ws, int pass);
+
+  extern int wsgl_current_transparency_pass;
+
   /*******************************************************************************
    * wsgl_init
    *
diff --git a/src/include/phigs/sin.h b/src/include/phigs/sin.h
index 89fb1a3..49f4b5d 100644
--- a/src/include/phigs/sin.h
+++ b/src/include/phigs/sin.h
@@ -51,7 +51,22 @@ SOFTWARE.
 
 #include <sys/types.h>
 #include <X11/Xlib.h>
+#ifdef GTK4_EXT
+#include <gtk/gtk.h>
+#ifndef OPENPHIGS_GTK4_TYPES_DEFINED
+#define OPENPHIGS_GTK4_TYPES_DEFINED
+typedef void* XtAppContext;
+typedef GtkWidget* Widget;
+typedef void* XtPointer;
+typedef unsigned int Cardinal;
+typedef char* String;
+typedef void* Arg;
+typedef void* ArgList;
+typedef unsigned long XtArgVal;
+#endif
+#else
 #include <X11/Intrinsic.h>
+#endif
 
 #include "phigs.h"
 #include "ws_type.h"
@@ -391,8 +406,10 @@ extern void             phg_sin_sample();
   */
 
 /* Xt action procs for input. */
+#ifndef GTK4_EXT
   extern XtActionProc     phg_sin_xt_request_satisfied(Widget);
   extern XtActionProc     phg_sin_xt_string_event(Widget, XEvent*, String *, Cardinal*);
+#endif
 
 #define SIN_CLASS_INDEX(_class) \
     ((int)(_class))
diff --git a/src/include/phigs/ws.h b/src/include/phigs/ws.h
index caf84ca..737e26e 100644
--- a/src/include/phigs/ws.h
+++ b/src/include/phigs/ws.h
@@ -51,7 +51,22 @@ SOFTWARE.
 
 #include <stdio.h>
 #include <X11/Xlib.h>
+#ifdef GTK4_EXT
+#include <gtk/gtk.h>
+#ifndef OPENPHIGS_GTK4_TYPES_DEFINED
+#define OPENPHIGS_GTK4_TYPES_DEFINED
+typedef void* XtAppContext;
+typedef GtkWidget* Widget;
+typedef void* XtPointer;
+typedef unsigned int Cardinal;
+typedef char* String;
+typedef void* Arg;
+typedef void* ArgList;
+typedef unsigned long XtArgVal;
+#endif
+#else
 #include <X11/Intrinsic.h>
+#endif
 #include <GL/glx.h>
 
 #include "phigs.h"
diff --git a/src/libphigs/cp/cpm_tlkt.c b/src/libphigs/cp/cpm_tlkt.c
index fcfc60a..b21c19d 100644
--- a/src/libphigs/cp/cpm_tlkt.c
+++ b/src/libphigs/cp/cpm_tlkt.c
@@ -48,12 +48,17 @@ SOFTWARE.
 
 ******************************************************************/
 #include <stdio.h>
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
+#ifndef GTK4_EXT
 #include <X11/Shell.h>
+#endif
+#endif
 #include "phg.h"
 #include "ws.h"
 #include "sin.h"
 
+#ifndef GTK4_EXT
 /* TODO: Fill in the real options. */
 static XrmOptionDescRec options[] = {
 {"-horiz",	"scrollbar.orientation", XrmoptionNoArg,  "horizontal"}
@@ -300,6 +305,7 @@ static XtActionsRec	actions[] = {
 };
 
 XContext        phg_sin_device_context_id;
+#endif
 
 /* No need for a "destroy_toolkit function since all the resources are
  * destroyed when the PM terminates.
@@ -307,8 +313,11 @@ XContext        phg_sin_device_context_id;
 
 XtAppContext phg_cpm_init_toolkit( int argc, char **argv )
 {
-  XtAppContext app_con;
+  XtAppContext app_con = NULL;
 
+#ifdef GTK4_EXT
+  gtk_init();
+#else
   XtToolkitInitialize();
 #ifdef DEBUGINP
   printf("Initialising Xt Toolkit\n");
@@ -321,6 +330,7 @@ XtAppContext phg_cpm_init_toolkit( int argc, char **argv )
     XtAppAddActions( app_con, actions, XtNumber(actions) );
     phg_sin_device_context_id = XUniqueContext();
   }
+#endif
   return app_con;
 }
 
@@ -328,13 +338,20 @@ Widget phg_cpm_toolkit_add_connection( XtAppContext context, Display* display, P
 {
   int    argc = 0, status = 0;
   char   **argv = (char **)NULL;
-  Widget top_level;
+  Widget top_level = NULL;
 
+  *err = 0;
+#ifdef GTK4_EXT
+  /* In GTK4, Display connection is handled by gtk_init() or implicitly. */
+  /* We can create a dummy or invisible top-level window if needed, but 
+   * OpenPHIGS handles GLX on X11 windows directly. We just need to return 
+   * a valid pointer to indicate success if the caller checks it. */
+  top_level = (Widget)gtk_window_new();
+#else
   /* TODO: XtDisplayInitialize destroys the arg vector, but we need to
    * keep it for the next display initialized.  So copy it to scratch
    * space then pass the copy.
    */
-  *err = 0;
   XtDisplayInitialize( context, display,
         "Workstation", NULL, NULL, 0,
         &argc, argv );
@@ -342,6 +359,7 @@ Widget phg_cpm_toolkit_add_connection( XtAppContext context, Display* display, P
 				  display, NULL );
   if (top_level == NULL)
     *err = ERRN204;
+#endif
   return top_level;
 }
 
@@ -354,16 +372,30 @@ int phg_cpm_toolkit_open_ws( Ws *ws ) {
 #endif
     status = 0;
     sprintf( buf, "workstation%d", ws->id );
+#ifdef GTK4_EXT
+    ws->shell = (Widget)gtk_window_new();
+    if (ws->shell == NULL){
+      status = 1;
+    }
+#else
     ws->shell = XtVaCreatePopupShell( buf, applicationShellWidgetClass,
 				      ws->top_level, NULL);
     if (ws->shell == NULL){
       printf("Failed to create shell for %s! Top_level was %ld\n", buf, (long)ws->top_level);
       status = 1;
     }
+#endif
 
     return status;
 }
 
 void phg_cpm_toolkit_close_ws( Ws *ws ) {
+#ifdef GTK4_EXT
+    if (ws->shell) {
+      gtk_window_destroy(GTK_WINDOW(ws->shell));
+      ws->shell = NULL;
+    }
+#else
     XtDestroyWidget( ws->shell );
+#endif
 }
diff --git a/src/libphigs/input/sin_cho.c b/src/libphigs/input/sin_cho.c
index 856f549..d95fea1 100644
--- a/src/libphigs/input/sin_cho.c
+++ b/src/libphigs/input/sin_cho.c
@@ -59,17 +59,21 @@ SOFTWARE.
 #include "sin.h"
 #include "private/sinP.h"
 
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #ifdef MOTIF
 #include <stdbool.h>
 #include <Xm/Frame.h>
 #include <Xm/ToggleB.h>
 #include <Xm/RowColumn.h>
 #else
+#ifndef GTK4_EXT
 #include <X11/Xaw/Viewport.h>
 #include <X11/Xaw/List.h>
 #endif
+#endif
 /*******************************************************************************
  * choice_notify
  *
@@ -167,7 +171,19 @@ static void enable_choice(
     sprintf( buf, "choice%d", device->num );
     sprintf( title, "Options:");
     /* Create the containing shell. */
-#ifdef MOTIF
+#ifdef GTK4_EXT
+    if (device->item_handle.choice.shell) {
+        gtk_window_destroy(GTK_WINDOW(device->item_handle.choice.shell));
+        device->item_handle.choice.shell = NULL;
+    }
+    device->item_handle.choice.shell = (Widget)gtk_window_new();
+    gtk_window_set_title(GTK_WINDOW(device->item_handle.choice.shell), title);
+    GtkWidget *box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
+    gtk_window_set_child(GTK_WINDOW(device->item_handle.choice.shell), box);
+    device->item_handle.choice.list = (Widget)gtk_list_box_new();
+    gtk_box_append(GTK_BOX(box), (GtkWidget*)device->item_handle.choice.list);
+    w = device->item_handle.choice.list;
+#elif defined(MOTIF)
     device->item_handle.choice.shell =
       XtVaCreatePopupShell( buf, applicationShellWidgetClass,
 			    parent,
@@ -230,10 +246,14 @@ static void enable_choice(
 	XtNcallback, choice_notify, (XtPointer)device );
 #endif
 
+#ifdef GTK4_EXT
+    gtk_window_present(GTK_WINDOW(device->item_handle.choice.shell));
+#else
     XtPopup( device->item_handle.choice.shell, XtGrabNone );
     if ( device->mode == SIN_REQUEST_PENDING )
 	XSaveContext( XtDisplay(w), XtWindow(w), phg_sin_device_context_id,
 	    (caddr_t)device );
+#endif
 #ifdef DEBUGINP
     printf("Enabling choice finished\n");
 #endif
diff --git a/src/libphigs/input/sin_str.c b/src/libphigs/input/sin_str.c
index 983fdc1..6f41c1b 100644
--- a/src/libphigs/input/sin_str.c
+++ b/src/libphigs/input/sin_str.c
@@ -59,15 +59,19 @@ SOFTWARE.
 #include "sin.h"
 #include "private/sinP.h"
 
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #ifdef MOTIF
 #include <Xm/Frame.h>
 #include <Xm/Text.h>
 #else
+#ifndef GTK4_EXT
 #include <X11/Xaw/Viewport.h>
 #include <X11/Xaw/AsciiText.h>
 #endif
+#endif
 
 /*******************************************************************************
  * reset_string_measure
@@ -75,7 +79,13 @@ SOFTWARE.
  * DESCR:       Reset string measure helper function
  * RETURNS:     N/A
  */
-#ifdef MOTIF
+#ifdef GTK4_EXT
+static void reset_string_measure(Sin_input_device *device) {
+    if (device->item_handle.string.textw) {
+        gtk_editable_set_text(GTK_EDITABLE(device->item_handle.string.textw), device->data.string.init_string ? device->data.string.init_string : "");
+    }
+}
+#elif defined(MOTIF)
 static void reset_string_measure(
 				 Sin_input_device *device
 				 )
@@ -127,7 +137,16 @@ static void reset_string_measure(
  * RETURNS:     N/A
  */
 
-#ifdef MOTIF
+#ifdef GTK4_EXT
+static void update_string(Sin_input_device *device) {
+    Sin_string_device_data *data = &device->data.string;
+    if (device->item_handle.string.textw) {
+        const char *text = gtk_editable_get_text(GTK_EDITABLE(device->item_handle.string.textw));
+        strncpy(data->string, text, data->buffer_size - 1);
+        data->string[data->buffer_size - 1] = '\0';
+    }
+}
+#elif defined(MOTIF)
 static void update_string(
 			  Sin_input_device *device
 			  )
@@ -183,6 +202,7 @@ static void update_string(
  * RETURNS:     N/A
  */
 
+#ifndef GTK4_EXT
 XtActionProc phg_sin_xt_string_event(
     Widget w,
     XEvent *event,
@@ -210,11 +230,14 @@ XtActionProc phg_sin_xt_string_event(
     }
     return NULL;
 }
+#endif
 
+#ifndef GTK4_EXT
 static XtTranslations		compiled_translations;
 static String translations = "\
           <Key>Return:	newline() StringEvent() RequestSatisfied() \n\
 	  ";
+#endif
 
 /*******************************************************************************
  * create_string
@@ -239,7 +262,15 @@ static void create_string(
 #endif
     /* Create the containing shell. */
     sprintf( buf, "string%d", device->num );
-#ifdef MOTIF
+#ifdef GTK4_EXT
+    widgets->shell = (Widget)gtk_window_new();
+    gtk_window_set_title(GTK_WINDOW(widgets->shell), "String Input");
+    GtkWidget *box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
+    gtk_window_set_child(GTK_WINDOW(widgets->shell), box);
+    widgets->textw = (Widget)gtk_entry_new();
+    gtk_box_append(GTK_BOX(box), (GtkWidget*)widgets->textw);
+    widgets->pane = (Widget)box;
+#elif defined(MOTIF)
     widgets->shell =
       XtVaCreatePopupShell( buf, applicationShellWidgetClass,
 			    parent,
@@ -281,10 +312,12 @@ static void create_string(
 					      NULL );
 #endif
 
+#ifndef GTK4_EXT
     if ( !compiled_translations ) {
       compiled_translations = XtParseTranslationTable( translations );
     }
     XtOverrideTranslations( widgets->textw, compiled_translations );
+#endif
 #ifdef DEBUGINP
     printf("Done with create string.\n");
 #endif
@@ -309,14 +342,20 @@ static void enable_string(
 	create_string( device );
 
     reset_string_measure( device );
+#ifdef GTK4_EXT
+    gtk_window_present(GTK_WINDOW(device->item_handle.string.shell));
+#else
     XtPopup( device->item_handle.string.shell, XtGrabNone );
+#endif
     if ( !device->flags.been_up_yet ) {
 #ifdef DEBUGINP
       printf("Setting flags\n");
 #endif
+#ifndef GTK4_EXT
       XSaveContext( XtDisplay(device->item_handle.string.textw),
 		    XtWindow(device->item_handle.string.textw),
 		    phg_sin_device_context_id, (caddr_t)device );
+#endif
       device->flags.been_up_yet = 1;
     }
 #ifdef DEBUGINP
@@ -357,8 +396,13 @@ static void disable_string(
 #ifdef DEBUGINP
     printf("Disable string.\n");
 #endif
-    if ( device->item_handle.string.shell )
+    if ( device->item_handle.string.shell ) {
+#ifdef GTK4_EXT
+        gtk_window_close(GTK_WINDOW(device->item_handle.string.shell));
+#else
 	XtPopdown( device->item_handle.string.shell );
+#endif
+    }
 }
 
 /*******************************************************************************
@@ -375,8 +419,13 @@ static void destroy_string(
 #ifdef DEBUGINP
      printf("Destroy string.\n");
 #endif
-    if ( device->item_handle.string.shell )
+    if ( device->item_handle.string.shell ) {
+#ifdef GTK4_EXT
+        gtk_window_destroy(GTK_WINDOW(device->item_handle.string.shell));
+#else
 	XtDestroyWidget( device->item_handle.string.shell );
+#endif
+    }
 }
 
 /*******************************************************************************
diff --git a/src/libphigs/input/sin_val.c b/src/libphigs/input/sin_val.c
index 8719f11..dc97304 100644
--- a/src/libphigs/input/sin_val.c
+++ b/src/libphigs/input/sin_val.c
@@ -59,8 +59,10 @@ SOFTWARE.
 #include "sin.h"
 #include "private/sinP.h"
 
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #ifdef MOTIF
 #include <Xm/Frame.h>
 #include <Xm/Scale.h>
@@ -68,11 +70,13 @@ SOFTWARE.
 #include <Xm/PanedW.h>
 #include <Xm/Label.h>
 #else
+#ifndef GTK4_EXT
 #include <X11/Xaw/Paned.h>
 #include <X11/Xaw/Scrollbar.h>
 #include <X11/Xaw/Label.h>
 #include <X11/Xaw/Box.h>
 #endif
+#endif
 /*******************************************************************************
  * init_valuator
  *
@@ -139,7 +143,9 @@ static void valuator_jump(
     float delta, prec;
     Sin_input_device *device = (Sin_input_device *)client_data;
     Sin_valuator_device_data *data = &device->data.valuator;
-#ifdef MOTIF
+#ifdef GTK4_EXT
+    data->value = gtk_range_get_value(GTK_RANGE(device->item_handle.valuator.scrollbar));
+#elif defined(MOTIF)
     XmScaleCallbackStruct *cbs = (XmScaleCallbackStruct *) call_data;
     delta = data->high - data->low;
     if (delta<=1.0) prec = 100.0;
@@ -220,7 +226,30 @@ static void enable_valuator(
     if (device->item_handle.valuator.pane != NULL){
       return;
     }
-#ifdef MOTIF
+#ifdef GTK4_EXT
+    if (device->item_handle.valuator.shell == NULL) {
+      ws->valuator_shell = (Widget)gtk_window_new();
+      gtk_window_set_title(GTK_WINDOW(ws->valuator_shell), "Valuator");
+      
+      GtkWidget *box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
+      gtk_window_set_child(GTK_WINDOW(ws->valuator_shell), box);
+      ws->valuator_box = (Widget)box;
+
+      if (data->label) {
+          GtkWidget *lbl = gtk_label_new(data->label);
+          gtk_box_append(GTK_BOX(box), lbl);
+      }
+
+      device->item_handle.valuator.scrollbar = (Widget)gtk_scale_new_with_range(GTK_ORIENTATION_HORIZONTAL, data->low, data->high, (data->high - data->low)/100.0);
+      gtk_range_set_value(GTK_RANGE(device->item_handle.valuator.scrollbar), data->init_value);
+      g_signal_connect(device->item_handle.valuator.scrollbar, "value-changed", G_CALLBACK(valuator_jump), device);
+      gtk_box_append(GTK_BOX(box), (GtkWidget*)device->item_handle.valuator.scrollbar);
+      
+      gtk_window_present(GTK_WINDOW(ws->valuator_shell));
+    }
+    device->item_handle.valuator.shell = ws->valuator_shell;
+    device->item_handle.valuator.pane = ws->valuator_box;
+#elif defined(MOTIF)
     XmString label;
     /* Create the containing shell. */
     sprintf( buf, "valuator%d", device->num );
diff --git a/src/libphigs/input/sin_ws.c b/src/libphigs/input/sin_ws.c
index f879e5e..2c2ffea 100644
--- a/src/libphigs/input/sin_ws.c
+++ b/src/libphigs/input/sin_ws.c
@@ -613,6 +613,7 @@ void phg_sin_ws_flush_event_buffer(
     SIN_WS_RESET_EVENT_BUFFER(ev_buf);
 }
 
+#ifndef GTK4_EXT
 XtActionProc phg_sin_xt_request_satisfied(
     Widget  w
     )
@@ -625,3 +626,4 @@ XtActionProc phg_sin_xt_request_satisfied(
     }
     return NULL;
 }
+#endif
diff --git a/src/libphigs/ws/ws_inp.c b/src/libphigs/ws/ws_inp.c
index 83aae05..61d7cff 100644
--- a/src/libphigs/ws/ws_inp.c
+++ b/src/libphigs/ws/ws_inp.c
@@ -50,8 +50,10 @@ SOFTWARE.
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef GTK4_EXT
 #include <X11/Intrinsic.h>
 #include <X11/StringDefs.h>
+#endif
 
 #include "phg.h"
 #include "ws.h"
diff --git a/src/libphigs/ws/ws_pm.c b/src/libphigs/ws/ws_pm.c
index 28f1b8d..fc1859e 100644
--- a/src/libphigs/ws/ws_pm.c
+++ b/src/libphigs/ws/ws_pm.c
@@ -57,47 +57,67 @@ SOFTWARE.
 #include "cp.h"
 #include "ws.h"
 #include "private/wsxP.h"
+#ifndef GTK4_EXT
 #include <X11/IntrinsicI.h>
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #ifdef MOTIF
 #include <Xm/Frame.h>
 #include <Xm/Label.h>
 #include <Xm/PanedW.h>
 #include <Xm/PushB.h>
 #else
+#ifndef GTK4_EXT
 #include <X11/Xaw/Box.h>
 #include <X11/Xaw/Label.h>
 #include <X11/Xaw/Command.h>
 #endif
+#endif
 
 int done_pressed = 0;
 
+#ifdef GTK4_EXT
+static void done_button( GtkWidget *w, gpointer data )
+{
+  Ws *ws = (Ws *)data;
+  done_pressed = 1;
+  gtk_window_destroy(GTK_WINDOW(ws->msg_shell));
+  ws->msg_shell = NULL;
+}
+#else
 void done_button( Widget w,
 		  XtPointer client_data,
 		  XtPointer call_data )
 {
   Ws *ws = (Ws *)client_data;
-#ifdef DEBUGINP
-  printf("Done button pressed\n");
-#endif
   done_pressed = 1;
   XtPopdown( ws->msg_shell );
 }
+#endif
 
 void create_message_win( Ws *ws )
 {
+#ifdef GTK4_EXT
+  ws->msg_shell = (Widget)gtk_window_new();
+  gtk_window_set_title(GTK_WINDOW(ws->msg_shell), "Message");
+  gtk_window_set_default_size(GTK_WINDOW(ws->msg_shell), 400, 100);
+
+  GtkWidget *box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 5);
+  gtk_window_set_child(GTK_WINDOW(ws->msg_shell), box);
+
+  ws->msg_label = (Widget)gtk_label_new("");
+  gtk_box_append(GTK_BOX(box), (GtkWidget*)ws->msg_label);
+
+  GtkWidget *button = gtk_button_new_with_label("OK");
+  g_signal_connect(button, "clicked", G_CALLBACK(done_button), ws);
+  gtk_box_append(GTK_BOX(box), button);
+#else
   Widget frame, pane, box, text, button;
 
-#ifdef DEBUGINP
-  printf("Creating message window\n");
-#endif
   /* Create the containing shell. */
   ws->msg_shell = XtVaCreatePopupShell( "message",
 					applicationShellWidgetClass, ws->top_level, NULL );
-#ifdef DEBUGINP
-  printf("Called XtVaCreatePopupShell. msg_shell is %ld\n", (long)ws->msg_shell);
-#endif
 #ifdef MOTIF
   frame =
     XtVaCreateManagedWidget( "frame", xmFrameWidgetClass, ws->msg_shell,
@@ -129,6 +149,7 @@ void create_message_win( Ws *ws )
   ws->msg_label = XtVaCreateManagedWidget( "label", labelWidgetClass, box,
 					   NULL );
 #endif
+#endif
 }
 
 void phg_wsb_message(
@@ -146,12 +167,13 @@ void phg_wsb_message(
   }
 
   if ( args->msg_length > 0 ) {
+#ifdef GTK4_EXT
+    gtk_label_set_text(GTK_LABEL(ws->msg_label), args->msg);
+    gtk_window_present(GTK_WINDOW(ws->msg_shell));
+#else
     /* The unmanage and manage is to get the parent box to allow the
      * label resize.
      */
-#ifdef DEBUGINP
-    printf("Setting message window parameters. Message is %s\n", args->msg);
-#endif
     XtUnmanageChild( ws->msg_label );
 #ifdef MOTIF
     XmString btext = XmStringCreateSimple(args->msg);
@@ -165,15 +187,16 @@ void phg_wsb_message(
 #endif
     XtManageChild( ws->msg_label );
     XtPopup( ws->msg_shell, XtGrabNone );
-#ifdef DEBUGINP
-    printf("Popup done.\n");
 #endif
   }
   phg_wsx_update_ws_rect( ws );
-#ifdef DEBUGINP
-  printf("DEBUG: ws_pm: entering main loop\n");
-#endif
+
   done_pressed = 0;
+#ifdef GTK4_EXT
+  while (done_pressed == 0) {
+    g_main_context_iteration(NULL, TRUE);
+  }
+#else
   XtAppContext app = _XtDefaultAppContext();
   do {
     if (m == 0) {
@@ -187,6 +210,7 @@ void phg_wsb_message(
     }
     m >>= 1;
   } while (done_pressed == 0);
+#endif
   done_pressed = 0;
 }
 
diff --git a/src/libphigs/ws/wsb.c b/src/libphigs/ws/wsb.c
index e6e1edc..84a2be3 100644
--- a/src/libphigs/ws/wsb.c
+++ b/src/libphigs/ws/wsb.c
@@ -55,8 +55,10 @@ SOFTWARE.
 #include <string.h>
 #include <assert.h>
 #include <stdio.h>
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 
 #include "phg.h"
 #include "private/phgP.h"
@@ -854,7 +856,8 @@ void phg_wsb_traverse_all_postings(
 
   WSB_CHECK_POSTED(&owsb->posted);
   if( WSB_SOME_POSTED(&owsb->posted) ) {
-    /* Set up for complete traversal. */
+    /* PASS 1: Opaque elements */
+    wsgl_set_transparency_pass(ws, 0);
     post_str = owsb->posted.lowest.higher;
     end = &(owsb->posted.highest);
     wsgl_begin_rendering(ws);
@@ -863,6 +866,20 @@ void phg_wsb_traverse_all_postings(
       post_str = post_str->higher;
     }
     wsgl_end_rendering(ws);
+
+    /* PASS 2: Transparent elements (drawn with depth write off) */
+    wsgl_set_transparency_pass(ws, 1);
+    post_str = owsb->posted.lowest.higher;
+    wsgl_begin_rendering(ws);
+    while ( post_str != end ) {
+      phg_wsb_traverse_net( ws, post_str->structh );
+      post_str = post_str->higher;
+    }
+    wsgl_end_rendering(ws);
+
+    /* Restore default state (Opaque) */
+    wsgl_set_transparency_pass(ws, 0);
+
     owsb->surf_state = PSURF_NOT_EMPTY;
   }
 }
diff --git a/src/libphigs/ws/wsx.c b/src/libphigs/ws/wsx.c
index e813490..6a049ba 100644
--- a/src/libphigs/ws/wsx.c
+++ b/src/libphigs/ws/wsx.c
@@ -23,8 +23,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #include <X11/Xlib.h>
 
 #ifdef GLEW
@@ -141,7 +143,11 @@ int phg_wsx_setup_tool(
     else {
       /* Initialize rendering context */
       ws->app_context = phg_cpm_init_toolkit(argc, argv);
+#ifdef GTK4_EXT
+      ws->top_level = (Widget)gtk_window_new();
+#else
       ws->top_level = XtInitialize("Workstation", "", NULL, 0, &argc, argv);
+#endif
       /* Create window */
       drawable_id = XCreateWindow(display,
                                   RootWindow(display, best_info->screen),
diff --git a/src/libphigs/ws/wsx_inp.c b/src/libphigs/ws/wsx_inp.c
index 602f3b0..74b6b61 100644
--- a/src/libphigs/ws/wsx_inp.c
+++ b/src/libphigs/ws/wsx_inp.c
@@ -28,8 +28,10 @@
 #include "private/evtP.h"
 #include "private/wsxP.h"
 
+#ifndef GTK4_EXT
 #include <X11/Shell.h>
 #include <X11/StringDefs.h>
+#endif
 
 /*******************************************************************************
  * phg_wsx_input_dispatch_next
@@ -43,8 +45,14 @@ int phg_wsx_input_dispatch_next(
    Phg_sin_evt_tbl *evt_tbl
    )
 {
-   int status;
+   int status = FALSE;
    XEvent event;
+
+#ifdef GTK4_EXT
+   if (g_main_context_iteration(NULL, FALSE)) {
+     status = TRUE;
+   }
+#else
    XtInputMask m;
    XtInputMask t;
 
@@ -56,15 +64,13 @@ int phg_wsx_input_dispatch_next(
      status = TRUE;
      XtAppProcessEvent(ws->app_context, t & m);
    }
+#endif
    if (XCheckWindowEvent(ws->display,
                          ws->input_overlay_window,
                          (unsigned long) 0xffffffffUL,
                          &event) == True) {
       status = TRUE;
    }
-   else {
-      status = FALSE;
-   }
 
    return status;
 }
diff --git a/src/libphigs/wsgl/wsgl.c b/src/libphigs/wsgl/wsgl.c
index 26dd1fd..95bd8ab 100644
--- a/src/libphigs/wsgl/wsgl.c
+++ b/src/libphigs/wsgl/wsgl.c
@@ -49,6 +49,19 @@ short int wsgl_use_shaders = 1;
 short int wsgl_vert_shader_version = 120;
 short int wsgl_frag_shader_version = 120;
 
+int wsgl_current_transparency_pass = 0;
+
+/*******************************************************************************
+ * wsgl_set_transparency_pass
+ *
+ * DESCR:       Set the transparency pass and update HLHSR depth mask.
+ * RETURNS:     N/A
+ ******************************************************************************/
+void wsgl_set_transparency_pass(Ws *ws, int pass) {
+    wsgl_current_transparency_pass = pass;
+    wsgl_update_hlhsr_id(ws);
+}
+
 #define LOG_INT(DATA)                                   \
   css_print_eltype(ELMT_HEAD(DATA)->elementType);       \
   printf(":\tSIZE: %d\t", ELMT_HEAD(DATA)->length);     \
diff --git a/src/libphigs/wsgl/wsgl_attr.c b/src/libphigs/wsgl/wsgl_attr.c
index a7c98b8..7bb21d5 100644
--- a/src/libphigs/wsgl/wsgl_attr.c
+++ b/src/libphigs/wsgl/wsgl_attr.c
@@ -364,7 +364,11 @@ void wsgl_update_hlhsr_id(
 
   case PHIGS_HLHSR_ID_ON:
     glDepthFunc(GL_LESS);
-    glDepthMask(GL_TRUE);
+    if (wsgl_current_transparency_pass == 1) {
+        glDepthMask(GL_FALSE);
+    } else {
+        glDepthMask(GL_TRUE);
+    }
     glEnable(GL_DEPTH_TEST);
     break;
 
diff --git a/src/libphigs/wsgl/wsgl_line.c b/src/libphigs/wsgl/wsgl_line.c
index 5decfb8..d50746c 100644
--- a/src/libphigs/wsgl/wsgl_line.c
+++ b/src/libphigs/wsgl/wsgl_line.c
@@ -51,6 +51,12 @@ void wsgl_polyline(
    point_list.points = (Ppoint *) &data[1];
 
    wsgl_setup_line_attr(ast);
+   {
+       GLfloat cur_color[4];
+       glGetFloatv(GL_CURRENT_COLOR, cur_color);
+       if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+       if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+   }
    glBegin(GL_LINES);
    for (i = 0; i < point_list.num_points; i++) {
       glVertex2f(point_list.points[i].x,
@@ -91,6 +97,12 @@ void wsgl_polyline3(
    point_list.points = (Ppoint3 *) &data[1];
 
    wsgl_setup_line_attr(ast);
+   {
+       GLfloat cur_color[4];
+       glGetFloatv(GL_CURRENT_COLOR, cur_color);
+       if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+       if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+   }
    glBegin(GL_LINES);
    for (i = 0; i < point_list.num_points; i++) {
       glVertex3f(point_list.points[i].x,
diff --git a/src/libphigs/wsgl/wsgl_marker.c b/src/libphigs/wsgl/wsgl_marker.c
index 59a4e9d..4dc7ef5 100644
--- a/src/libphigs/wsgl/wsgl_marker.c
+++ b/src/libphigs/wsgl/wsgl_marker.c
@@ -225,6 +225,12 @@ void wsgl_polymarker(
    point_list.points = (Ppoint *) &data[1];
 
    wsgl_setup_marker_attr(ast, &type, &size);
+   {
+       GLfloat cur_color[4];
+       glGetFloatv(GL_CURRENT_COLOR, cur_color);
+       if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+       if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+   }
    switch (type) {
       case PMARKER_DOT:
 	wsgl_marker_polygon(40, &point_list, size);
@@ -300,6 +306,12 @@ void wsgl_polymarker3(
       }
 
       wsgl_setup_marker_attr(ast, &type, &size);
+      {
+          GLfloat cur_color[4];
+          glGetFloatv(GL_CURRENT_COLOR, cur_color);
+          if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+          if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+      }
       switch (type) {
       case PMARKER_DOT:
 	wsgl_marker_polygon(40, &plist, size);
diff --git a/src/libphigs/wsgl/wsgl_shaders.c b/src/libphigs/wsgl/wsgl_shaders.c
index ca245b6..208344d 100644
--- a/src/libphigs/wsgl/wsgl_shaders.c
+++ b/src/libphigs/wsgl/wsgl_shaders.c
@@ -20,8 +20,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef GTK4_EXT
 #include <X11/StringDefs.h>
 #include <X11/Shell.h>
+#endif
 #include <X11/Xlib.h>
 #ifdef GLEW
 #include <GL/glew.h>
diff --git a/src/libphigs/wsgl/wsgl_tess.c b/src/libphigs/wsgl/wsgl_tess.c
index 2c2fbf5..cbb022f 100644
--- a/src/libphigs/wsgl/wsgl_tess.c
+++ b/src/libphigs/wsgl/wsgl_tess.c
@@ -87,23 +87,9 @@ void wsgl_draw_tess_polygon(Wsgl_tess_vertex *vertices, int num_vertices, int re
     int normal_indices[MAX_VERTICES];
     int n_vertices = 0;
     int n_normals = 0;
-    GLboolean orig_depth_mask;
     GLfloat cur_color[4];
     int has_transparency = 0;
 
-    tess = gluNewTess();
-    if (!tess) return;
-
-    gluTessCallback(tess, GLU_TESS_BEGIN, (void (CALLBACK *)())tessBeginCB);
-    gluTessCallback(tess, GLU_TESS_END, (void (CALLBACK *)())tessEndCB);
-    gluTessCallback(tess, GLU_TESS_VERTEX, (void (CALLBACK *)())tessVertexCB);
-    gluTessCallback(tess, GLU_TESS_ERROR, (void (CALLBACK *)())tessErrorCB);
-    gluTessCallback(tess, GLU_TESS_COMBINE, (void (CALLBACK *)())tessCombineCB);
-    gluTessCallback(tess, GLU_TESS_EDGE_FLAG, (void (CALLBACK *)())tessEdgeFlagCB);
-
-    /* Determine if depth writing should be disabled for order-independent transparency */
-    glGetBooleanv(GL_DEPTH_WRITEMASK, &orig_depth_mask);
-    
     if (num_vertices > 0 && vertices[0].apply_cb) {
         if (vertices[0].colr_type == PMODEL_RGBA && vertices[0].colr.direct.rgba.alpha < 1.0f) {
             has_transparency = 1;
@@ -114,10 +100,19 @@ void wsgl_draw_tess_polygon(Wsgl_tess_vertex *vertices, int num_vertices, int re
             has_transparency = 1;
         }
     }
-    
-    if (has_transparency && orig_depth_mask == GL_TRUE) {
-        glDepthMask(GL_FALSE);
-    }
+
+    if (wsgl_current_transparency_pass == 0 && has_transparency) return;
+    if (wsgl_current_transparency_pass == 1 && !has_transparency) return;
+
+    tess = gluNewTess();
+    if (!tess) return;
+
+    gluTessCallback(tess, GLU_TESS_BEGIN, (void (CALLBACK *)())tessBeginCB);
+    gluTessCallback(tess, GLU_TESS_END, (void (CALLBACK *)())tessEndCB);
+    gluTessCallback(tess, GLU_TESS_VERTEX, (void (CALLBACK *)())tessVertexCB);
+    gluTessCallback(tess, GLU_TESS_ERROR, (void (CALLBACK *)())tessErrorCB);
+    gluTessCallback(tess, GLU_TESS_COMBINE, (void (CALLBACK *)())tessCombineCB);
+    gluTessCallback(tess, GLU_TESS_EDGE_FLAG, (void (CALLBACK *)())tessEdgeFlagCB);
 
     gluTessBeginPolygon(tess, NULL);
     gluTessBeginContour(tess);
@@ -144,10 +139,6 @@ void wsgl_draw_tess_polygon(Wsgl_tess_vertex *vertices, int num_vertices, int re
     /* Restore default edge flag state for subsequent rendering */
     glEdgeFlag(GL_TRUE);
 
-    if (has_transparency && orig_depth_mask == GL_TRUE) {
-        glDepthMask(GL_TRUE);
-    }
-
     if (record_geom_flag && n_vertices > 0) {
         wsgl_add_geometry(GEOM_FACE, vertex_indices, normal_indices, n_vertices);
     }
diff --git a/src/libphigs/wsgl/wsgl_text.c b/src/libphigs/wsgl/wsgl_text.c
index 79ff1e8..2a97c30 100644
--- a/src/libphigs/wsgl/wsgl_text.c
+++ b/src/libphigs/wsgl/wsgl_text.c
@@ -425,6 +425,12 @@ static void wsgl_text_string(
   int n_vertices = 0;
 
   wsgl_setup_text_attr(ast, &fnt, &char_expan);
+  {
+      GLfloat cur_color[4];
+      glGetFloatv(GL_CURRENT_COLOR, cur_color);
+      if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+      if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+  }
   char_ht = ast->char_ht;
 
   posa.x = text->pos.x;
@@ -496,6 +502,12 @@ static void wsgl_text_string3(
   int n_vertices = 0;
 
   wsgl_setup_text_attr(ast, &fnt, &char_expan);
+  {
+      GLfloat cur_color[4];
+      glGetFloatv(GL_CURRENT_COLOR, cur_color);
+      if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+      if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+  }
   char_ht = ast->char_ht;
 
   posa.x = text->pos.x;
@@ -568,6 +580,12 @@ static void wsgl_anno_text_string3(
   int n_vertices = 0;
 
   wsgl_setup_text_attr(ast, &fnt, &char_expan);
+  {
+      GLfloat cur_color[4];
+      glGetFloatv(GL_CURRENT_COLOR, cur_color);
+      if (wsgl_current_transparency_pass == 0 && cur_color[3] < 1.0f) return;
+      if (wsgl_current_transparency_pass == 1 && cur_color[3] >= 1.0f) return;
+  }
   char_ht = ast->anno_char_ht;
 
   posa.x = text->pos.x;