GLwDrawingArea or GLwMDrawingArea Widget

Purpose

OpenGL Draw Widget Class

Library

OpenGLC bindings library: libXGLW.a

C Syntax

#include </usr/include/GL/GLwDrawA.h>

Widget = XtCreateWidget(Widget, glwDrawingAreaWidgetClass, ...);
ld ... -lXGLW -l<anywidgetlibrary> -lXt -lGL -lX11 ...

#include </usr/include/GL/GLwMDrawA.h>

Widget = XtCreateWidget(Widget, glwMDrawingAreaWidgetClass, ...);
ld ... -lXGLW -lXm -IXt -IGL -IX11 ...

Description

GLwDrawingArea and GLwMDrawingArea are widgets suitable for OpenGL drawing. Based on supplied parameters, these widgets provide a window with the appropriate visual and color maps needed for OpenGL. The GLwDrawingArea and GLwMDrawingArea widgets also provide callbacks for redraw, resize, input, and initialization.

The GLwDrawingArea widget is not a part of any widget set, but depends only on the Intrinsics Library (Xt). GLwDrawingArea can be used with any widget set. The GLwMDrawingArea widget is identical to the GLwDrawingArea widget except that it is a subclass of the Motif XmPrimitive widget class and has resources and defaults suitable for use with Motif. For example, GLwMDrawingArea provides the default Motif background and foreground colors for resources, and handles keyboard traversal more efficiently. Although the GLwDrawingArea widget can be used in a Motif program, it is recommended that GLwMDrawingArea be used instead.

Because both GLwDrawingArea and GLwMDrawingArea widgets behave almost identically, the remainder of this article refers only to GLwDrawingArea, except when the behaviors differ. Unless explicitly stated, all statements about GLwDrawingArea also apply to GLwMDrawingArea.

Among the information the programmer must provide to create a GLwDrawingArea widget is information necessary to determine the visual. The programmer can provide this information through resources by using one of the following methods:

• Supply a specific visualInfo structure. This visualInfo must have been obtained elsewhere; it is the application designer's responsibility to ensure that it is compatible with the OpenGL rendering done by the application.
• Provide an attribute list, which is formatted identically to that used for direct OpenGL programming.
• Specify each attribute as an individual resource. The method is the simplest, and is the only method that works from resource files.

In addition to allocating the visual, the GLwDrawingArea widget also allocates the color map unless one is provided by the application.

Note: If the color map is provided by the application, the application writer is responsible for guaranteeing compatibility between the color map and the visual.

If an application creates multiple GLwDrawingArea widgets in the same visual, the same color map will be used. However, the color map will not be shared among separate applications.

Creating the widget does not actually create the window until it is realized, and consequently, the application should not perform any OpenGL operations to the window immediately after creation. Instead, the application must wait until after it has realized the window. Alternatively, the ginit callback may be used to indicate when the window has been created. Upon receiving this callback, the application can perform all OpenGL initialization for the window, and can subsequently perform other operations on it. The initialization is discussed in more detail in the following sections.

Applications select which GLwDrawingArea they are accessing using either the glXMakeCurrent subroutine or the convenience function GLwDrawingAreaMakeCurrent, which uses a widget instead of a display and window. If there is only one GLwDrawingArea, this need only be called once; however, if there is more than one GLwDrawingArea, the widget should be called at the start of each callback. Callbacks in this case include not only callbacks provided by the widget itself, but any other callback that leads to Graphics Library (GL) activity, such as a timeout or a workproc.

If an application is using double buffering, it may call GLwDrawingAreaSwapBuffers instead of glXSwapBuffers. This allows the use of the widget instead of the display and window.

The GLwDrawingArea widget class is subclassed from the Core class, and inherits behavior and resources from the Core class. The GLwDrawingArea widget has the following class information:

The GLwMDrawingArea widget class is subclassed from the XmPrimitive class, and inherits behavior and resources from the XmPrimitive and Core classes.

New Resources

The following table defines a set of widget resources used by the programmer to specify data. The programmer can also set the resource values for the inherited classes to set attributes for this widget. To reference a resource by name or by class in an .Xdefaults file, remove the GLwN or GLwC prefix and use the remaining letters. There are two tables included. The following table includes resources that correspond directly to the attributes used by the glXChooseVisual subroutine. As with glXChooseVisual, all Boolean resources default to False and all integer resources default to 0. These resources can all be set only at creation time, and are used to determine the visual. If either the GLwNattribList or GLwNvisualInfo resource is set, these resources are ignored. The specific meaning of these resources is discussed in the glXChooseVisual subroutine and will not be discussed here.

The following table lists other resources of the GLwDrawingArea widget. Following the table is a description of each resource. The codes in the access column indicate if the given resource can be set at creation time (C), set by using XtSetValues (S), retrieved by using XtGetValues (G), or is not applicable (N/A).

Inherited Resources

Both GLwDrawingArea and GLwMDrawingArea inherit behavior and resources from the Core superclass. Other than the behavior of the color map and background resources described previously, all defaults are the same as for Core.

In addition, the Motif version GLwMDrawingArea also inherits from XmPrimitive. The behavior of the color resources has been described previously. The TraversalOn resource is disabled for this widget, but if keyboard input is required it should be enabled. (Also, the application should call XmProcessTraversal(widget, XmTRAVERSE_CURRENT) whenever mouse button 1 is clicked in the widget. This is similar to the requirements of the Motif Drawing area.) Because Motif gets confused by having multiple visuals in one top level shell, XmNhighlightOnEnter has been disabled, and XmNhighlightThickness has been set to 0.

Callback Information

A pointer to the following structure is passed to each callback:

typedef struct
{
   Integer reason;
   XEvent * event;
   Dimension    width, height;
} GLwDrawingAreaCallbackStruct;

Translations

The GLwDrawingArea widget has the following translations:

The GLwMDrawingArea widget has the following additional translation:

An application wishing to catch other events than these defaults can do so by installing a different translation table.

Action Routines

The GLwDrawingArea widget has the following action routine:

Initialization

When the widget is initially created (for example, through XtCreateWidget) the associated window is not actually created. Instead, window creation is delayed until the widget is realized. However, glXchooseVisual is called immediately, so information based on its results is available.

Between the time the widget is created and it is realized, the following apply:

• No OpenGL operations can be done to the window.
• No resize callbacks are generated.
• The normal window is available (XtWindow returns Null).
• The GLwDrawingAreaMakeCurrent function (and glXMakeCurrent subroutine) should not be called.

When the widget is realized, the following actions take place:

• The window is created.
• The ginit callback is called. The user may use this callback to perform any needed OpenGL initialization to the window.

Notes

When using the input callback to receive keyboard input, the keycode in the event must be converted to a keysym. Use XLookupKeysym or XLookupString to do the conversion. Keyboard input can also be dealt using translations, in which case no such conversion is required.

Motif programmers should keep in mind that OSF uses virtual bindings and replaces some of the key bindings. As a common example, if the Esc key is to be used to exit the program (as it often is in GL programs), the translation should specify <key>osfCancel instead of <key>Escape.

Motif programmers may also create a GLwMDrawingArea widget with the Motif style GLwCreateMDrawingArea.

Examples

The following are some code fragments that create a GLwDrawingArea widget and manage the appropriate callbacks:

#include </usr/include/GL/GLwDrawA.h>
   static GLXContext glx_context;  /* assume only one context */
   . . .
   main()
   {
       Arg args[10];
       int n;
       Widget parent;   /* The parent of the gl widget */
       Widget glw;   /* The GLwDrawingArea widget          */
       . . .
       /* Create the widget using RGB mode.  This can also be set
        * in an X Defaults file
        */
       n = 0;
       XtSetArg(args[n], GLwNrgba, TRUE); n++;
       glw = XtCreateManagedWidget("glw",
      GLwDrawingAreaWidgetClass,
      parent, args, n);
       XtAddCallback(glw, GLwNexposeCallback, exposeCB, 0);
       XtAddCallback(glw, GLwNresizeCallback, resizeCB, 0);
       XtAddCallback(glw, GLwNginitCallback, ginitCB, 0);
       /* Also add input callback if needed */
       . . .
   }
   static void
   exposeCB(Widget w, XtPointer client_data,
             GLwDrawingAreaCallbackStruct call_data)
   {
       GLwDrawingAreaMakeCurrent (w, glx_context);
       /* redraw the display */
   }
   static void
   resizeCB(Widget w, XtPointer client_data,
             GLwDrawingAreaCallbackStruct call_data)
   {
       GLwDrawingAreaMakeCurrent (w, glx_context);
       /* perform any resize actions */
        glViewport (0, 0, call_data->width -1, 
         call_data->height -1);
      /* redraw the display */
   }
   static void
   ginitCB(Widget w, XtPointer client_data,
             GLwDrawingAreaCallbackStruct call_data)
   {
       Arg args[1];
       XVisualInfo *vi;
       XtSetArg(args[0], GLwNvisualInfo, &vi);
       XtGetValues(w, args, 1);
       /* create a visual context */
       glx_context = glXCreateContext(XtDisplay(w), vi, 0, GL_FALSE);
       GLwDrawingAreaMakeCurrent (w, glx_context);
       /* Perform any necessary graphics initialization.*

The Motif program need only differ by including GLwMDrawA.h instead of GLwDrawA.h and by creating a widget of type GLwMDrawingAreaWidgetClass instead of GLwDrawingAreaWidgetClass. As an alternative, the Motif program could use GLwCreateMDraw instead.

Attention:

Notes:

1. If a GLwDrawingArea widget is created as a child of an already realized widget, the GLwDrawingArea widget will be created immediately, without giving the user an opportunity to add the ginit callback. In such a case, initialization should be done immediately after creating the widget rather than by using the callback.
2. If the non-Motif GLwDrawingArea widget is used in a Motif program and keyboard traversal is attempted, the behavior is undefined if the user traverses into the GLwDrawingArea widget.

Files

Related Information

The GLwCreateMDrawingArea function, GLwDrawingAreaMakeCurrent function, GLwDrawingAreaSwapBuffers function.

The glXChooseVisual subroutine, glXMakeCurrent subroutine.

OpenGL Overview.