doc: Rewrite the drawing model overview

This is a first cut at updating the drawing model chapter
for the way we do things now. It introduces the scene graph and
render nodes, explains node caching and tree diffing, and removes
sections about subwindows.

docs/reference/gtk/drawing-model.xml

@@ -26,56 +26,42 @@
       widgets and windows, you should read this chapter; this will be
       useful to know if you decide to implement your own widgets.  This
       chapter will also clarify the reasons behind the ways certain
-      things are done in GTK; for example, why you cannot change the
+      things are done in GTK.
-      background color of all widgets with the same method.
     </para>
 
     <refsect2 id="drawing model windows">
+
       <title>Windows and events</title>
 
       <para>
-        Programs that run in a windowing system generally create
+        Applications that use a windowing system generally create
-        rectangular regions in the screen called
+        rectangular regions in the screen called <firstterm>surfaces</firstterm>
-        <firstterm>windows</firstterm>.  Traditional windowing systems
+        (GTK is following the Wayland terminology, other windowing systems
-        do not automatically save the graphical content of windows, and
+        such as X11 may call these <firstterm>windows</firstterm>).
-        instead ask client programs to repaint those windows whenever it
+        Traditional windowing systems do not automatically save the
-        is needed.  For example, if a window that is stacked below other
+        graphical content of surfaces, and instead ask applications to
-        windows gets raised to the top, then a client program has to
+        provide new content whenever it is needed.
-        repaint the area that was previously obscured.  When the
+        For example, if a window that is stacked below other
-        windowing system asks a client program to redraw part of a
+        windows gets raised to the top, then the application has to
-        window, it sends an <firstterm>exposure event</firstterm> to the
+        repaint it, so the previously obscured area can be shown.
-        program for that window.
+        When the windowing system asks an application to redraw
+        a window, it sends an <firstterm>frame event</firstterm>
+        (<firstterm>expose event</firstterm> in X11 terminology)
+        for that window.
+      </para>
+      
+      <para>
+        Each GTK toplevel window or dialog is associated with a
+        windowing system surface. Child widgets such as buttons or
+        entries don't have their own surface; they use the surface
+        of their toplevel.
       </para>
 
       <para>
-        Here, "windows" means "rectangular regions with automatic
+        Generally, the drawing cycle begins when GTK receives
-        clipping", instead of "toplevel application windows".  Most
+        a frame event from the underlying windowing system:  if the
-        windowing systems support nested windows, where the contents of
-        child windows get clipped by the boundaries of their parents.
-        Although GTK and GDK in particular may run on a windowing
-        system with no such notion of nested windows, GDK presents the
-        illusion of being under such a system.  A toplevel window may
-        contain many subwindows and sub-subwindows, for example, one for
-        the menu bar, one for the document area, one for each scrollbar,
-        and one for the status bar.  In addition, controls that receive
-        user input, such as clickable buttons, are likely to have their
-        own subwindows as well.
-      </para>
-
-      <para>
-        In practice, most windows in modern GTK application are client-side
-        constructs. Only few windows (in particular toplevel windows) are
-        <emphasis>native</emphasis>, which means that they represent a
-        window from the underlying windowing system on which GTK is running.
-        For example, on X11 it corresponds to a <type>Window</type>; on Win32,
-        it corresponds to a <type>HANDLE</type>.
-      </para>
-
-      <para>
-        Generally, the drawing cycle begins when GTK receives an
-        exposure event from the underlying windowing system:  if the
         user drags a window over another one, the windowing system will
-        tell the underlying window that it needs to repaint itself.  The
+        tell the underlying surface that it needs to repaint itself.  The
         drawing cycle can also be initiated when a widget itself decides
         that it needs to update its display.  For example, when the user
         types a character in a <link
@@ -85,13 +71,10 @@
       </para>
 
       <para>
-        The windowing system generates events for native windows. The GDK
+        The windowing system generates frame events for surfaces. The GDK
-        interface to the windowing system translates such native events into
+        interface to the windowing system translates such events into
-        <link linkend="GdkEvent"><structname>GdkEvent</structname></link>
+        emissions of the #GtkSurface::render signal on the affected surfaces.
-        structures and sends them on to the GTK layer.  In turn, the GTK layer
+        The GTK toplevel window connects to that signal, and reacts appropriately.
-        finds the widget that corresponds to a particular
-        <classname>GdkSurface</classname> and emits the corresponding event
-        signals on that widget.
       </para>
 
       <para>
@@ -112,8 +95,13 @@
         it does. On top of this GTK has a frame clock that gives a
         “pulse” to the application. This clock beats at a steady rate,
         which is tied to the framerate of the output (this is synced to
-        the monitor via the window manager/compositor). The clock has
+        the monitor via the window manager/compositor). A typical
-        several phases:
+        refresh rate is 60 frames per second, so a new “pulse” happens
+        roughly every 16 milliseconds.
+      </para>
+
+      <para>
+        The clock has several phases:
         <itemizedlist>
           <listitem><para>Events</para></listitem>
           <listitem><para>Update</para></listitem>
@@ -125,24 +113,24 @@
       </para>
 
       <para>
-        The Events phase is a long stretch of time between each
+        The Events phase is a stretch of time between each redraw where
-        redraw where we get input events from the user and other events
+        GTK processes input events from the user and other events
         (like e.g. network I/O). Some events, like mouse motion are
-        compressed so that we only get a single mouse motion event per
+        compressed so that only a single mouse motion event per clock 
-        clock cycle.
+        cycle needs to be handled.
       </para>
 
       <para>
-        Once the Events phase is over we pause all external events and
+        Once the Events phase is over, external events are paused and
-        run the redraw loop. First is the Update phase, where all
+        the redraw loop is run. First is the Update phase, where all
         animations are run to calculate the new state based on the
         estimated time the next frame will be visible (available via
         the frame clock). This often involves geometry changes which
-        drives the next phase, Layout. If there are any changes in
+        drive the next phase, Layout. If there are any changes in
-        widget size requirements we calculate a new layout for the
+        widget size requirements the new layout is calculated for the
-        widget hierarchy (i.e. we assign sizes and positions). Then
+        widget hierarchy (i.e. sizes and positions for all widgets are
-        we go to the Paint phase where we redraw the regions of the
+        determined). Then comes the Paint phase, where we redraw the
-        window that need redrawing.
+        regions of the window that need redrawing.
       </para>
 
       <para>
@@ -184,162 +172,57 @@
       </para>
     </refsect2>
 
+    <refsect2 id="scene-graph">
+      <title>The scene graph</title>
+
+      <para>
+        The first step in “drawing” a window is that GTK creates
+        <firstterm>render nodes</firstterm> for all the widgets
+        in the window. The render nodes are combined into a tree
+        that you can think of as a <firstterm>scene graph</firstterm>
+        describing your window contents.
+      </para>
+      <para>
+        Render nodes belong to the GSK layer, and there are various kinds
+        of them, for the various kinds of drawing primitives you are likely
+        to need when translating widget content and CSS styling. Typical
+        examples are text nodes, gradient nodes, texture nodes or clip nodes.
+      <para>
+      <para>
+        In the past, all drawing in GTK happened via cairo. It is still possible
+        to use cairo for drawing your custom widget contents, by using a cairo
+        render node.
+      </para>
+      </para>
+        A GSK <firstterm>renderer</firstterm> takes these render nodes, transforms
+        them into rendering commands for the drawing API it targets, and arranges
+        for the resulting drawing to be associated with the right surface. GSK has
+        renderers for OpenGL, Vulkan and cairo.
+      </para>
+    </refsect2>
+
     <refsect2 id="hierarchical-drawing">
       <title>Hierarchical drawing</title>
 
       <para>
-        During the Paint phase we will send a single expose event to
+        During the Paint phase we will send a single ::render signal the toplevel
-        the toplevel window. The event handler will create a cairo
+        window. The signal handler will create a snapshot object (which is a
-        context for the window and emit a GtkWidget::draw() signal
+        helper for creating a scene graph) and emit a GtkWidget::snapshot() signal,
-        on it, which will propagate down the entire widget hierarchy
+        which will propagate down the entire widget hierarchy. This lets each widget
-        in back-to-front order, using the clipping and transform of
+        snapshot its content at the right place and time, correctly handling things
-        the cairo context. This lets each widget draw its content at
+        like partial transparencies and overlapping widgets.
-        the right place and time, correctly handling things like
-        partial transparencies and overlapping widgets.
       </para>
 
       <para>
-        When generating the event, GDK also sets up double buffering to
+        To avoid excessive work when generating scene graphs, GTK caches render nodes.
-        avoid the flickering that would result from each widget drawing
+        GtkWidget keeps a reference to its render node (which in turn, will refer to
-        itself in turn.  <xref linkend="double-buffering"/> describes
+        the render nodes of children, and grandchildren, and so on), and will reuse
-        the double buffering mechanism in detail.
+        that node during the Paint phase. Invalidating a widget (e.g. by calling
-      </para>
+        gtk_widget_queue_draw) discards the cached render node, forcing GTK to
-        
+        regenerate it the next time it needs to snapshot the widget.
-      <para>
-        Normally, there is only a single cairo context which is used in
-        the entire repaint, rather than one per GdkSurface. This means you
-        have to respect (and not reset) existing clip and transformations
-        set on it.
-      </para>
-
-      <para>
-        Most widgets, including those that create their own GdkSurfaces have
-        a transparent background, so they draw on top of whatever widgets
-        are below them. This was not the case in GTK 2 where the theme set
-        the background of most widgets to the default background color. (In
-        fact, transparent GdkSurfaces used to be impossible.)
-      </para>
-
-      <para>
-        The whole rendering hierarchy is captured in the call stack, rather
-        than having multiple separate draw emissions, so you can use effects
-        like e.g. cairo_push/pop_group() which will affect all the widgets
-        below you in the hierarchy. This makes it possible to have e.g.
-        partially transparent containers.
       </para>
     </refsect2>
 
-    <refsect2 id="scrolling drawing model">
-      <title>Scrolling</title>
-
-      <para>
-        Traditionally, GTK has used self-copy operations to implement
-        scrolling with native windows. With transparent backgrounds, this
-        no longer works. Instead, we just mark the entire affected area for
-        repainting when these operations are used. This allows (partially)
-        transparent backgrounds, and it also more closely models modern
-        hardware where self-copy operations are problematic (they break the
-        rendering pipeline).
-      </para>
-    </refsect2>
-
-  </refsect1>
-
-  <refsect1 id="double-buffering">
-    <title>Double buffering</title>
-
-    <para>
-      If each of the drawing calls made by each subwidget's
-      <literal>draw</literal> handler were sent directly to the
-      windowing system, flicker could result.  This is because areas may get
-      redrawn repeatedly:  the background, then decorative frames, then text
-      labels, etc.  To avoid flicker, GTK employs a <firstterm>double
-	buffering</firstterm> system at the GDK level.  Widgets normally don't
-      know that they are drawing to an off-screen buffer; they just issue their
-      normal drawing commands, and the buffer gets sent to the windowing system
-      when all drawing operations are done.
-    </para>
-
-    <para>
-      Two basic functions in GDK form the core of the double-buffering
-      mechanism:  <link
-      linkend="gdk_surface_begin_paint_region"><function>gdk_surface_begin_paint_region()</function></link>
-      and <link
-      linkend="gdk_surface_end_paint"><function>gdk_surface_end_paint()</function></link>.
-      The first function tells a <classname>GdkSurface</classname> to
-      create a temporary off-screen buffer for drawing.  All
-      subsequent drawing operations to this window get automatically
-      redirected to that buffer.  The second function actually paints
-      the buffer onto the on-screen window, and frees the buffer.
-    </para>
-
-    <refsect2 id="automatic-double-buffering">
-      <title>Automatic double buffering</title>
-
-      <para>
-	It would be inconvenient for all widgets to call
-	<function>gdk_surface_begin_paint_region()</function> and
-	<function>gdk_surface_end_paint()</function> at the beginning
-	and end of their draw handlers.
-      </para>
-
-      <para>
-	To make this easier, GTK normally calls
-        <function>gdk_surface_begin_paint_region()</function>
-        before emitting the #GtkWidget::draw signal, and
-	then it calls <function>gdk_surface_end_paint()</function>
-	after the signal has been emitted. This is convenient for
-	most widgets, as they do not need to worry about creating
-	their own temporary drawing buffers or about calling those
-	functions.
-      </para>
-
-      <para>
-	However, some widgets may prefer to disable this kind of
-	automatic double buffering and do things on their own.
-        To do this, call the
-        <function>gtk_widget_set_double_buffered()</function>
-        function in your widget's constructor. Double buffering
-        can only be turned off for widgets that have a native
-        window.
-      </para>
-
-      <example id="disabling-double-buffering">
-	<title>Disabling automatic double buffering</title>
-
-	<programlisting>
-static void
-my_widget_init (MyWidget *widget)
-{
-  ...
-
-  gtk_widget_set_double_buffered (widget, FALSE);
-
-  ...
-}
-	</programlisting>
-      </example>
-
-      <para>
-	When is it convenient to disable double buffering?  Generally,
-	this is the case only if your widget gets drawn in such a way
-	that the different drawing operations do not overlap each
-	other.  For example, this may be the case for a simple image
-	viewer:  it can just draw the image in a single operation.
-	This would <emphasis>not</emphasis> be the case with a word
-	processor, since it will need to draw and over-draw the page's
-	background, then the background for highlighted text, and then
-	the text itself.
-      </para>
-
-      <para>
-	Even if you turn off double buffering on a widget, you
-	can still call
-	<function>gdk_surface_begin_paint_region()</function> and
-	<function>gdk_surface_end_paint()</function> by hand to use
-	temporary drawing buffers.
-      </para>
-    </refsect2>
   </refsect1>
 
 </refentry>