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/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: http://www.qt-project.org/
**
** This file is part of the Qt3D documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\example qt3d/nesting
\title Drawing into framebuffer objects
The Nesting example shows how Qt3D can be used to draw into a
framebuffer object and then use the associated texture in
subsequent drawing operations. It is assumed that the reader
is already familar with the following examples:
\list
\o \l{Teapot Example}{Hello Teapot} - drawing a basic object in 3D.
\o \l{Object Effects}{Cube} - applying transformations and materials to objects.
\endlist
In this example we are going to draw two rotating and transparent
cubes. One cube will display a simple texture as in the
\l{Object Effects}{Cube} example,
and the other cube will display a nested scene containing a
rotating teapot.
\image nesting-screenshot.png
As with the other examples, we start by creating the geometric objects
we need in the constructor (we will share the cube geometry between
the two cubes we will be drawing):
\snippet qt3d/nesting/cubeview.cpp 1
We also need three QPropertyAnimation objects to drive our
animations; rotation angle for the teapot, rotation angle for
the cube; orbit angle of the two cubes around each other:
\snippet qt3d/nesting/cubeview.cpp 2
The final step in the constructor is to create a camera for
the nested scene we will be drawing into the framebuffer object.
This is in addition to the default QGLView::camera() object that
provides the camera for the main scene.
\snippet qt3d/nesting/cubeview.cpp 3
To draw the nested scene, we of course need a framebuffer object,
which we create in the \c{initializeGL()} method:
\snippet qt3d/nesting/cubeview.cpp 4
Note that we also set the framebuffer object on an instance of
QGLFramebufferObjectSurface. We will use this fact later when
we render the nested scene.
For the other cube, we need a regular texture. And we'll also
turn on blending to make our cubes transparent:
\snippet qt3d/nesting/cubeview.cpp 5
Now it is time to paint the scene in \c{paintGL()}. The first
thing we want to do is draw the teapot into the framebuffer object.
To do that, we need to establish a nested drawing state:
\snippet qt3d/nesting/cubeview.cpp 6
In the code above, we first push the main scene's modelview and
projection matrices. And then we push a new drawing surface onto
the painter's surface stack. Everything we draw from now on will
be written to \c{fboSurface} and thus the framebuffer object.
To do that, we set the inner camera position, adjust the painter
state, clear the framebuffer object, and draw the teapot:
\snippet qt3d/nesting/cubeview.cpp 7
Now that the nested scene has been drawn to the framebuffer object,
we pop the surface and matrix stacks to return to the main scene:
\snippet qt3d/nesting/cubeview.cpp 8
Because our cubes are transparent, we need to make sure we draw
the objects in the scene from back to front. Otherwise the result
will not look correct. To do this, we check which of the cube
mid-points is furtherest away from the camera and draw that cube first
(the more negative the z value, the further away it is):
\snippet qt3d/nesting/cubeview.cpp 9
Drawing the first cube with the simple texture is very similar to
the \l{Object Effects}{Cube} example:
\snippet qt3d/nesting/cubeview.cpp 10
The main interesting wrinkle is that we draw the cube twice,
once with front faces culled, and the second time with back
faces culled. This effectively causes the cube to be drawn
back to front for proper blending.
The second cube is drawn in a similar way except that we bind
the framebuffer object's texture to the state instead of a static
texture:
\snippet qt3d/nesting/cubeview.cpp 11
Face culling is a simple way to draw transparent objects, but it
really only works on non-intersecting convex objects like our cubes.
For concave objects (like the teapot) or objects that intersect,
we would need to break the objects up into smaller convex pieces
and then order the pieces from back to front.
\l{qt3d-examples.html}{Return to Examples}.
*/
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