// Copyright (C) 2018 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example hellogles3
    \title Hello GLES3 Example
    \ingroup examples-widgets-opengl
    \examplecategory {Graphics & Multimedia}

    \brief Demonstrates OpenGL ES 3.0 functions via QOpenGLExtraFunctions.

    \image hellogles3-example.png
    \section1 Overview

    This example demonstrates easy, cross-platform usage of OpenGL ES 3.0
    functions via QOpenGLExtraFunctions in an application that
    works identically on desktop platforms with OpenGL 3.3 and mobile/embedded
    devices with OpenGL ES 3.0.

    This example has no QWidget dependencies, it uses QOpenGLWindow, a
    convenience subclass of QWindow that allows easy implementation of windows
    that contain OpenGL-rendered content. In this sense it complements the
    \l{OpenGL Window Example}, which shows the implementation of an OpenGL-based
    QWindow without using the convenience subclass.

    The Qt logo shape implementation is included from the \l{Hello GL2 Example}.

    In other aspects pertaining to using OpenGL there are the following
    differences.

    \list
      \li The OpenGL context creation has to have a sufficiently high version
    number for the features that are in use.
      \li The shader's version directive is different.
    \endlist

    \section1 Setting up in main.cpp

    Here we instantiate our QGuiApplication, QSurfaceformat and set its
    \l{QSurfaceFormat::depthBufferSize()}{depth buffer size}:

    \quotefromfile hellogles3/main.cpp
    \skipto int main(int argc, char *argv[])
    \printuntil fmt.setDepthBufferSize(24);

    We request an OpenGL 3.3 core or OpenGL ES 3.0 context, depending on
    QOpenGLContext::openGLModuleType():

    \skipto if (QOpenGLContext::openGLModuleType() == QOpenGLContext::LibGL) {
    \printuntil QSurfaceFormat::setDefaultFormat(fmt);

    We set the default surface format and instantiate our GLWindow \c glWindow.

    \section1 Implementing GLWindow

    This class delivers the features of the example application.

    To start, \c GLWindow is declared by implementing a subclass of
    QOpenGLWindow:

    \quotefromfile hellogles3/glwindow.h
    \skipto class GLWindow : public QOpenGLWindow
    \printto {

    The following properties are declared using Q_PROPERTY:

    \printto public:

    The following public functions are declared:

    \printto private slots:

    The following private objects are declared:

    \printto };

    On the implementation side, those functions that are not declared inline are
    implemented (or re-implemented) in \c{glwindow.cpp}. The following selections
    will cover implementation particulars pertaining to the use of OpenGL ES 3.0.

    \section2 Animations

    The following code pertains to the animations, and won't be explored here:

    \quotefromfile hellogles3/glwindow.cpp
    \skipto GLWindow::GLWindow()
    \printto static const char *vertexShaderSource =

    For more information see the documentation for \l QPropertyAnimation,
    \l QSequentialAnimationGroup.

    \section2 Shaders
    The shaders are defined like so:

    \printto QByteArray versionedShaderCode(const char *src)

    \note These are OpenGL version agnostic. We take this and append
    the version like so:

    \printto void GLWindow::initializeGL()

    \section2 Initializing OpenGL

    Initializing the shader program in handled by \c initializeGL():

    \printuntil   m_program = new QOpenGLShaderProgram;
    \skipto  m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, versionedShaderCode(vertexShaderSource));

    Now the OpenGL version is prepended and the various matrices and light
    position is set:

    \printuntil m_lightPosLoc = m_program->uniformLocation("lightPos");

    While not strictly required for ES 3, a vertex array object is created.

    \skipto delete m_vao;
    \printuntil f->glEnable(GL_CULL_FACE);

    \section2 Resizing the window

    The perspective needs to be aligned with the new window size as so:

    \skipto void GLWindow::resizeGL(int w, int h)
    \printto void GLWindow::paintGL()

    \section2 Painting

    We use QOpenGLExtraFunctions instead of QOpenGLFunctions as we want to
    do more than what GL(ES) 2.0 offers:

    \printuntil QOpenGLExtraFunctions *f = QOpenGLContext::currentContext()->extraFunctions();

    We clear the screen and buffers and bind our shader program and texture:

    \printuntil m_texture->bind();

    Logic for handling an initial \c paintGL() call or a call after a
    \c resizeGL() call is implemented like so:

    \printuntil }

    Last, we demonstrate a function introduced in OpenGL 3.1 or OpenGL ES 3.0:

    \skipto f->glDrawArraysInstanced(GL_TRIANGLES, 0, m_logo.vertexCount(), 32 * 36);
    \printuntil }

    This works because we earlier requested 3.3 or 3.0 context.
*/
