Drawing with GtkGLArea

The simplest way to draw using OpenGL commands in a GtkGLArea is to create a widget instance and connect to the “render” signal:

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// create a GtkGLArea instance
GtkWidget *gl_area = gtk_gl_area_new ();

// connect to the "render" signal
g_signal_connect (gl_area, "render", G_CALLBACK (render), NULL);

The render() function will be called when the GtkGLArea is ready for you to draw its content:

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static gboolean
render (GtkGLArea *area, GdkGLContext *context)
{
  // inside this function it's safe to use GL; the given
  // #GdkGLContext has been made current to the drawable
  // surface used by the #GtkGLArea and the viewport has
  // already been set to be the size of the allocation

  // we can start by clearing the buffer
  glClearColor (0, 0, 0, 0);
  glClear (GL_COLOR_BUFFER_BIT);

  // draw your object
  draw_an_object ();

  // we completed our drawing; the draw commands will be
  // flushed at the end of the signal emission chain, and
  // the buffers will be drawn on the window
  return TRUE;
}

If you need to initialize OpenGL state, e.g. buffer objects or shaders, you should use the “realize” signal; you can use the “unrealize” signal to clean up. Since the GdkGLContext creation and initialization may fail, you will need to check for errors, using gtk_gl_area_get_error(). An example of how to safely initialize the GL state is:

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static void
on_realize (GtkGLarea *area)
{
  // We need to make the context current if we want to
  // call GL API
  gtk_gl_area_make_current (area);

  // If there were errors during the initialization or
  // when trying to make the context current, this
  // function will return a #GError for you to catch
  if (gtk_gl_area_get_error (area) != NULL)
    return;

  // You can also use gtk_gl_area_set_error() in order
  // to show eventual initialization errors on the
  // GtkGLArea widget itself
  GError *internal_error = NULL;
  init_buffer_objects (&error);
  if (error != NULL)
    {
      gtk_gl_area_set_error (area, error);
      g_error_free (error);
      return;
    }

  init_shaders (&error);
  if (error != NULL)
    {
      gtk_gl_area_set_error (area, error);
      g_error_free (error);
      return;
    }
}

If you need to change the options for creating the GdkGLContext you should use the “create-context” signal.

gtk_gl_area_new ()

GtkWidget *
gtk_gl_area_new (void);

Creates a new GtkGLArea widget.

Since: 3.16

gtk_gl_area_get_context ()

GdkGLContext *
gtk_gl_area_get_context (GtkGLArea *area);

Retrieves the GdkGLContext used by area .

Since: 3.16

gtk_gl_area_make_current ()

void
gtk_gl_area_make_current (GtkGLArea *area);

Ensures that the GdkGLContext used by area is associated with the GtkGLArea.

This function is automatically called before emitting the “render” signal, and doesn't normally need to be called by application code.

Since: 3.16

gtk_gl_area_queue_render ()

void
gtk_gl_area_queue_render (GtkGLArea *area);

Marks the currently rendered data (if any) as invalid, and queues a redraw of the widget, ensuring that the “render” signal is emitted during the draw.

This is only needed when the gtk_gl_area_set_auto_render() has been called with a FALSE value. The default behaviour is to emit “render” on each draw.

Since: 3.16

gtk_gl_area_attach_buffers ()

void
gtk_gl_area_attach_buffers (GtkGLArea *area);

Ensures that the area framebuffer object is made the current draw and read target, and that all the required buffers for the area are created and bound to the frambuffer.

This function is automatically called before emitting the “render” signal, and doesn't normally need to be called by application code.

Since: 3.16

gtk_gl_area_set_error ()

void
gtk_gl_area_set_error (GtkGLArea *area,
                       const GError *error);

Sets an error on the area which will be shown instead of the GL rendering. This is useful in the “create-context” signal if GL context creation fails.

Since: 3.16

gtk_gl_area_get_error ()

GError *
gtk_gl_area_get_error (GtkGLArea *area);

Gets the current error set on the area .

Since: 3.16

gtk_gl_area_set_has_alpha ()

void
gtk_gl_area_set_has_alpha (GtkGLArea *area,
                           gboolean has_alpha);

If has_alpha is TRUE the buffer allocated by the widget will have an alpha channel component, and when rendering to the window the result will be composited over whatever is below the widget.

If has_alpha is FALSE there will be no alpha channel, and the buffer will fully replace anything below the widget.

Since: 3.16

gtk_gl_area_get_has_alpha ()

gboolean
gtk_gl_area_get_has_alpha (GtkGLArea *area);

Returns whether the area has an alpha component.

Since: 3.16

gtk_gl_area_set_has_depth_buffer ()

void
gtk_gl_area_set_has_depth_buffer (GtkGLArea *area,
                                  gboolean has_depth_buffer);

If has_depth_buffer is TRUE the widget will allocate and enable a depth buffer for the target framebuffer. Otherwise there will be none.

Since: 3.16

gtk_gl_area_get_has_depth_buffer ()

gboolean
gtk_gl_area_get_has_depth_buffer (GtkGLArea *area);

Returns whether the area has a depth buffer.

Since: 3.16

gtk_gl_area_set_has_stencil_buffer ()

void
gtk_gl_area_set_has_stencil_buffer (GtkGLArea *area,
                                    gboolean has_stencil_buffer);

If has_stencil_buffer is TRUE the widget will allocate and enable a stencil buffer for the target framebuffer. Otherwise there will be none.

Since: 3.16

gtk_gl_area_get_has_stencil_buffer ()

gboolean
gtk_gl_area_get_has_stencil_buffer (GtkGLArea *area);

Returns whether the area has a stencil buffer.

Since: 3.16

gtk_gl_area_set_auto_render ()

void
gtk_gl_area_set_auto_render (GtkGLArea *area,
                             gboolean auto_render);

If auto_render is TRUE the “render” signal will be emitted every time the widget draws. This is the default and is useful if drawing the widget is faster.

If auto_render is FALSE the data from previous rendering is kept around and will be used for drawing the widget the next time, unless the window is resized. In order to force a rendering gtk_gl_area_queue_render() must be called. This mode is useful when the scene changes seldomly, but takes a long time to redraw.

Since: 3.16

gtk_gl_area_get_auto_render ()

gboolean
gtk_gl_area_get_auto_render (GtkGLArea *area);

Returns whether the area is in auto render mode or not.

Since: 3.16

gtk_gl_area_get_required_version ()

void
gtk_gl_area_get_required_version (GtkGLArea *area,
                                  gint *major,
                                  gint *minor);

Retrieves the required version of OpenGL set using gtk_gl_area_set_required_version().

Since: 3.16

gtk_gl_area_set_required_version ()

void
gtk_gl_area_set_required_version (GtkGLArea *area,
                                  gint major,
                                  gint minor);

Sets the required version of OpenGL to be used when creating the context for the widget.

This function must be called before the area has been realized.

Since: 3.16

struct GtkGLArea

struct GtkGLArea;

A GtkWidget used for drawing with OpenGL.

Since: 3.16

struct GtkGLAreaClass

struct GtkGLAreaClass {
  gboolean       (* render)         (GtkGLArea        *area,
                                     GdkGLContext     *context);
  void           (* resize)         (GtkGLArea        *area,
                                     int               width,
                                     int               height);
  GdkGLContext * (* create_context) (GtkGLArea        *area);
};

The GtkGLAreaClass structure contains only private data.

Since: 3.16

The “auto-render” property

  “auto-render”              gboolean

If set to TRUE the “render” signal will be emitted every time the widget draws. This is the default and is useful if drawing the widget is faster.

If set to FALSE the data from previous rendering is kept around and will be used for drawing the widget the next time, unless the window is resized. In order to force a rendering gtk_gl_area_queue_render() must be called. This mode is useful when the scene changes seldomly, but takes a long time to redraw.

Flags: Read / Write

Default value: TRUE

Since: 3.16

The “context” property

  “context”                  GdkGLContext *

The GdkGLContext used by the GtkGLArea widget.

The GtkGLArea widget is responsible for creating the GdkGLContext instance. If you need to render with other kinds of buffers (stencil, depth, etc), use render buffers.

Flags: Read

Since: 3.16

The “has-alpha” property

  “has-alpha”                gboolean

If set to TRUE the buffer allocated by the widget will have an alpha channel component, and when rendering to the window the result will be composited over whatever is below the widget.

If set to FALSE there will be no alpha channel, and the buffer will fully replace anything below the widget.

Flags: Read / Write

Default value: FALSE

Since: 3.16

The “has-depth-buffer” property

  “has-depth-buffer”         gboolean

If set to TRUE the widget will allocate and enable a depth buffer for the target framebuffer.

Flags: Read / Write

Default value: FALSE

Since: 3.16

The “has-stencil-buffer” property

  “has-stencil-buffer”       gboolean

If set to TRUE the widget will allocate and enable a stencil buffer for the target framebuffer.

Flags: Read / Write

Default value: FALSE

Since: 3.16

The “create-context” signal

GdkGLContext*
user_function (GtkGLArea *area,
               gpointer   user_data)

The ::create-context signal is emitted when the widget is being realized, and allows you to override how the GL context is created. This is useful when you want to reuse an existing GL context, or if you want to try creating different kinds of GL options.

If context creation fails then the signal handler can use gtk_gl_area_set_error() to register a more detailed error of how the construction failed.

Flags: Run Last

Since: 3.16

The “render” signal

gboolean
user_function (GtkGLArea    *area,
               GdkGLContext *context,
               gpointer      user_data)

The ::render signal is emitted every time the contents of the GtkGLArea should be redrawn.

The context is bound to the area prior to emitting this function, and the buffers are painted to the window once the emission terminates.

Flags: Run Last

Since: 3.16

The “resize” signal

void
user_function (GtkGLArea *area,
               gint       width,
               gint       height,
               gpointer   user_data)

The ::resize signal is emitted once when the widget is realized, and then each time the widget is changed while realized. This is useful in order to keep GL state up to date with the widget size, like for instance camera properties which may depend on the width/height ratio.

The GL context for the area is guaranteed to be current when this signal is emitted.

The default handler sets up the GL viewport.

Flags: Run Last

Since: 3.16