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diff --git a/doc/src/examples/cube.qdoc b/doc/src/examples/cube.qdoc new file mode 100644 index 0000000000..0603941c7b --- /dev/null +++ b/doc/src/examples/cube.qdoc @@ -0,0 +1,178 @@ +/**************************************************************************** +** +** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). +** All rights reserved. +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:LGPL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain additional +** rights. These rights are described in the Nokia Qt LGPL Exception +** version 1.1, included in the file LGPL_EXCEPTION.txt in this package. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** +** +** +** +** +** +** +** +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \example opengl/cube + \group all-examples + \title Cube OpenGL ES 2.0 example + + The Cube OpenGL ES 2.0 example shows how to write mouse rotateable + textured 3D cube using OpenGL ES 2.0 with Qt. It shows how to handle + polygon geometries efficiently and how to write simple vertex and + fragment shader for programmable graphics pipeline. In addition it + shows how to use quaternions for representing 3D object orientation. + + This example has been written for OpenGL ES 2.0 but it works also on + desktop OpenGL because this example is simple enough and for the + most parts desktop OpenGL API is same. It compiles also without OpenGL + support but then it just shows a label stating that OpenGL support is + required. + + \image cube.png Screenshot of the Cube example running on N900 + + The example consist of two classes: + + \list + \o \c MainWidget extends QGLWidget and contains OpenGL ES 2.0 + initialization and drawing and mouse and timer event handling + \o \c GeometryEngine handles polygon geometries. Transfers polygon geometry + to vertex buffer objects and draws geometries from vertex buffer objects. + \endlist + + We'll start by initializing OpenGL ES 2.0 in \c MainWidget. + + \tableofcontents + + \section1 Initializing OpenGL ES 2.0 + + Since OpenGL ES 2.0 doesn't support fixed graphics pipeline anymore it has to + be implemented by ourselves. This makes graphics pipeline very flexible but + in the same time it becomes more difficult because user has to implement graphics + pipeline to get even the simplest example running. It also makes graphics pipeline + more efficient because user can decide what kind of pipeline is needed for the + application. + + First we have to implement vertex shader. It gets vertex data and + model-view-projection matrix (MVP) as parameters. It transforms vertex position + using MVP matrix to screen space and passes texture coordinate to + fragment shader. Texture coordinate will be automatically interpolated on polygon + faces. + + \snippet examples/opengl/cube/vshader.glsl 0 + + After that we need to implement second part of the graphics pipeline - fragment + shader. For this exercise we need to implement fragment shader that handles + texturing. It gets interpolated texture coordinate as a parameter and looks up + fragment color from the given texture. + + \snippet examples/opengl/cube/fshader.glsl 0 + + Using \c QGLShaderProgram we can compile, link and bind shader code to + graphics pipeline. This code uses Qt Resource files to access shader source code. + + \snippet examples/opengl/cube/mainwidget.cpp 3 + + The following code enables depth buffering and back face culling. + + \snippet examples/opengl/cube/mainwidget.cpp 2 + + \section1 Loading textures from Qt Resource files + + \c QGLWidget interface implements methods for loading textures from QImage to GL + texture memory. We still need to use OpenGL provided functions for specifying + the GL texture unit and configuring texture filtering options. + + \snippet examples/opengl/cube/mainwidget.cpp 4 + + \section1 Cube Geometry + + There are many ways to render polygons in OpenGL but the most efficient way is + to use only triangle strip primitives and render vertices from graphics hardware + memory. OpenGL has a mechanism to create buffer objects to this memory area and + transfer vertex data to these buffers. In OpenGL terminology these are referred + as Vertex Buffer Objects (VBO). + + \image cube_faces.png Cube faces and vertices + + This is how cube faces break down to triangles. Vertices are ordered this way + to get vertex ordering correct using triangle strips. OpenGL determines triangle + front and back face based on vertex ordering. By default OpenGL uses + counter-clockwise order for front faces. This information is used by back face + culling which improves rendering performance by not rendering back faces of the + triangles. This way graphics pipeline can omit rendering sides of the triangle that + aren't facing towards screen. + + Creating vertex buffer objects and transferring data to them is quite simple using + OpenGL provided functions. + + \snippet examples/opengl/cube/geometryengine.cpp 0 + + \snippet examples/opengl/cube/geometryengine.cpp 1 + + Drawing primitives from VBOs and telling programmable graphics pipeline how to + locate vertex data requires few steps. First we need to bind VBOs to be used. + After that we bind shader program attribute names and configure what + kind of data it has in the bound VBO. Finally we'll draw triangle + strip primitives using indices from the other VBO. + + \snippet examples/opengl/cube/geometryengine.cpp 2 + + \section1 Perspective projection + + Using \c QMatrix4x4 helper methods it's really easy to calculate perpective + projection matrix. This matrix is used to project vertices to screen space. + + \snippet examples/opengl/cube/mainwidget.cpp 5 + + \section1 Orientation of the 3D object + + Quaternions are handy way to represent orientation of the 3D object. Quaternions + involve quite complex mathematics but fortunately all the necessary mathematics + behind quaternions is implemented in \c QQuaternion. That allows us to store + cube orientation in quaternion and rotating cube around given axis is quite + simple. + + The following code calculates rotation axis and angular speed based on mouse events. + + \snippet examples/opengl/cube/mainwidget.cpp 0 + + \c QBasicTimer is used to animate scene and update cube orientation. Rotations + can be concatenated simply by multiplying quaternions. + + \snippet examples/opengl/cube/mainwidget.cpp 1 + + Model-view matrix is calculated using the quaternion and by moving world by Z axis. + This matrix is multiplied with the projection matrix to get MVP matrix for shader + program. + + \snippet examples/opengl/cube/mainwidget.cpp 6 + +*/ |