5a8a1eb4af
Url for OpenGL Registry Url for Khronos OpenGL ES API Registry Change-Id: I682ddcedf1e06d589e5c44e364936c78fd9219a5 Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
157 lines
7.5 KiB
Plaintext
157 lines
7.5 KiB
Plaintext
/****************************************************************************
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**
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** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
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** Contact: http://www.qt-project.org/legal
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:FDL$
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** Commercial License Usage
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** Licensees holding valid commercial Qt licenses may use this file in
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** accordance with the commercial license agreement provided with the
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** Software or, alternatively, in accordance with the terms contained in
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** a written agreement between you and Digia. For licensing terms and
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** conditions see http://qt.digia.com/licensing. For further information
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** use the contact form at http://qt.digia.com/contact-us.
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**
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** GNU Free Documentation License Usage
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** Alternatively, this file may be used under the terms of the GNU Free
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** Documentation License version 1.3 as published by the Free Software
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** Foundation and appearing in the file included in the packaging of
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** this file. Please review the following information to ensure
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** the GNU Free Documentation License version 1.3 requirements
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** will be met: http://www.gnu.org/copyleft/fdl.html.
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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\example openglwindow
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\title OpenGL Window Example
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\brief This example shows how to create a minimal QWindow based application
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for the purpose of using OpenGL.
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\image openglwindow-example.png Screenshot of the OpenGLWindow example
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\section1 OpenGLWindow super class
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Our OpenGLWindow class acts as an API which is then subclassed to do the
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actual rendering. It has functions to make a request for render() to be
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called, either immediately with renderNow() or as soon as the event loop
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has finished processing the current batch of events with renderLater().
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The OpenGLWindow subclass can either reimplement render() for OpenGL based
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rendering, or render(QPainter *) for rendering with a QPainter. Use
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OpenGLWindow::setAnimating(true) for render() to be called at the vertical
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refresh rate, assuming vertical sync is enabled in the underlying OpenGL
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drivers.
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In the class that does the OpenGL rendering you will typically want to
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inherit from QOpenGLFunctions, as our OpenGLWindow does, in order to get
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platform independent access to OpenGL ES 2.0 functions. By inheriting from
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QOpenGLFunctions the OpenGL functions it contains will get precedence, and
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you will not have to worry about resolving those functions if you want your
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application to work with OpenGL as well as OpenGL ES 2.0.
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\snippet openglwindow/openglwindow.h 1
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The window's surface type must be set to QSurface::OpenGLSurface to
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indicate that the window is to be used for OpenGL rendering and not for
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rendering raster content with QPainter using a QBackingStore.
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\snippet openglwindow/openglwindow.cpp 1
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Any OpenGL initialization needed can be done by overriding the initialize()
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function, which is called once before the first call to render(), with a
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valid current QOpenGLContext. As can be seen in the following code snippet,
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the default render(QPainter *) and initialize() implementations are empty,
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whereas the default render() implementation initializes a
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QOpenGLPaintDevice and then calls into render(QPainter *).
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\snippet openglwindow/openglwindow.cpp 2
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The renderLater() function simply puts an update request event on
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the event loop, which leads to renderNow() being called once the event
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gets processed.
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We also call renderNow() when we get an expose event. The exposeEvent() is
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the notification to the window that its exposure, meaning visibility, on
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the screen has changed. When the expose event is received you can query
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QWindow::isExposed() to find out whether or not the window is currently
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exposed. Do not render to or call QOpenGLContext::swapBuffers() on a window
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before it has received its first expose event, as before then its final
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size might be unknown, and in addition what is rendered might not even end
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up on the screen.
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\snippet openglwindow/openglwindow.cpp 3
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In renderNow() we return if we are not currently exposed, in which case
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rendering is delayed until we actually get an expose event. If we have not
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yet done so, we create the QOpenGLContext with the same QSurfaceFormat as
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was set on the OpenGLWindow, and call initialize() for the sake of the sub
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class, and initializeOpenGLFunctions() in order for the QOpenGLFunctions
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super class to be associated with the correct QOpenGLContext. In any case
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we make the context current by calling QOpenGLContext::makeCurrent(), call
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render() to do the actual rendering, and finally we schedule for the
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rendered contents to be made visible by calling
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QOpenGLContext::swapBuffers() with the OpenGLWindow as parameter.
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Once the rendering of a frame using an OpenGL context is initiated by
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calling QOpenGLContext::makeCurrent(), giving the surface on which to
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render as a parameter, OpenGL commands can be issued. The commands can be
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issued either directly by including <qopengl.h>, which also includes the
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system's OpenGL headers, or as by using QOpenGLFunctions, which can
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either be inherited from for convenience, or accessed using
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QOpenGLContext::functions(). QOpenGLFunctions gives access to all the
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OpenGL ES 2.0 level OpenGL calls that are not already standard in both
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OpenGL ES 2.0 and desktop OpenGL. For more information about the OpenGL and
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OpenGL ES APIs, refer to the official \l{http://www.opengl.org/registry/}{OpenGL Registry} and
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\l {http://www.khronos.org/registry/gles/}{Khronos OpenGL ES API Registry}.
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If animation has been enabled with OpenGLWindow::setAnimating(true), we
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call renderLater() to put another update request on the event loop.
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\snippet openglwindow/openglwindow.cpp 4
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Enabling animation also triggers an update request as shown in the
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following code snippet.
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\snippet openglwindow/openglwindow.cpp 5
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\section1 Example OpenGL rendering sub class
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Here we sub class OpenGLWindow to show how to do OpenGL to render a
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rotating triangle. By indirectly sub classing QOpenGLFunctions we gain
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access to all OpenGL ES 2.0 level functionality.
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\snippet openglwindow/main.cpp 1
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In our main function we initialize QGuiApplication and instantiate our
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TriangleOpenGLWindow. We give it a QSurfaceFormat specifying that we want
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four samples of multisample antialiasing, as well as a default geometry.
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Since we want to have animation we call the above mentioned setAnimating()
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function with an argument of true.
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\snippet openglwindow/main.cpp 2
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The following code snippet shows the OpenGL shader program used in this
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example. The vertex and fragment shaders are relatively simple, doing
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vertex transformation and interpolated vertex coloring.
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\snippet openglwindow/main.cpp 3
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Here is the code that loads the shaders and initializes the shader program
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By using QOpenGLShaderProgram instead of raw OpenGL we get the convenience
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that strips out the highp, mediump, and lowp qualifiers on desktop OpenGL,
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where they are not part of the standard. We store the attribute and uniform
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locations in member variables to avoid having to do the location lookup
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each frame.
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\snippet openglwindow/main.cpp 4
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Finally, here is our render() function, where we use OpenGL to set up the
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viewport, clear the background, and render a rotating triangle.
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\snippet openglwindow/main.cpp 5
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*/
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