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Drawing and painting a 3D image with GL – the Graphic Library – is not as trivial as doing the same on a 2D canvas: no 3D versions of the 2D canvas drawing methods are defined in GL classes. Instead, programs written with GL send data to the GPU (Graphic Processing Unit), and directions for organizing the data in data structures and for the rendition of data to the screen. The data is stored in buffers, and processed by shader programs.

Once you know how to use the OpenGL API, drawing the following image is simple:

The way Qt simplifies the use of GL are as follows:

  • it offers classes instead of integers to wrap GL resources: buffers, textures, vertex array and more.
  • the classes include overloaded functions.
  • the developer can create shaders without specifying the GL version.
  • Qt manages the main loop.

The following sections will describe the process of painting a 3D surface.

Initializing

To start working with GL, create an object of class QOpenGLWidget. Better create a subclass of QOpenGLWidget, if you work with Qt 5.4 or a later version. Don’t perform any GL operations before the method initializeGL is called. If you extend OpenGLWidget, any GL operation should be performed by one of the following methods: initializeGL, paintGL and resizeGL.

In initializeGL place operation to be performed once only at the initialization time. Some examples of initializeGL can be found in

Qt Assistnat->Qt Widgets->C++ Classes->QOpenGLWidget

click the code examples link.

Following is the initialization stage of my program to paint the triangle:

The Class

Following is a definition of the ExtendedOpenGLWidget class:

#include <QtWidgets/QOpenGLWidget>
#include <QOpenGLVertexArrayObject>
#include <QOpenGLBuffer>
#include <QOpenGLShaderProgram>
#include <QOpenGLShader>
#include <QOpenGLContext>


class ExtendedOpenGLWidget:public QOpenGLWidget {
    private:
        QOpenGLVertexArrayObject m_vao;
        QOpenGLBuffer m_vbo;
        QOpenGLShaderProgram *m_program;
        QOpenGLShader *m_fragmentShader, *m_vertexShader;
    public:
        ExtendedOpenGLWidget(QWidget *parent=nullptr);
        ~ExtendedOpenGLWidget();
        void initializeGL();
        void paintGL();
};

The class is defined in file “ui_opengl.h”.

Include Headers

#include <ui_opengl.h>
#include <iostream>

using namespace std;

The file “ui_opengl” is the header file generated by th Qt Designer. The rest can be used for debug printing.

The vertices

Vertices coordinates and colors are defined in my program as follows:

struct vertex {
float coords[3];
float colors[4];
};

struct vertex {
     float coords[3];
     float colors[4];
 };
 vertex vertices[]={
     {{-0.9, -0.9, 0.0}, {0.0, 0.0, 1.0, 1.0}},
     {{ 0.9, -0.9, 0.0}, {0.0, 1.0, 0.0, 1.0}},
     {{ 0.0,  0.9, 0.0}, {1.0, 0.0, 0.0, 1.0}}
 };

The above is a definition of a global variable. If you put it in a class, you will have to specify array sizes.

Constructor

The only thing the constructor does is call the parent constructor:

ExtendedOpenGLWidget::ExtendedOpenGLWidget(QWidget *parent):QOpenGLWidget(parent){
 }

The initializeGL method

This function creates the vertex array object(VAO). If the VAO cannot be created, it cannot be bound to the context. Your program can draw with or without it. In my example, I bind the VAO.

What actually does the drawing is the shader program. For it to draw you should link it at run time with the shaders. If you use Qt 5, the class QOpenGLShaderProgram exists with a method addShaderFromSourceCode, so you can create program in separate text files without the need to recompile your GL program when the shader code changes.

The example in Qt Assistant->Qt GUI->C++ Classes->QOpenGLShaderProgram does not complete the example in Qt Assistnat->Qt Widgets->C++ Classes->QOpenGLWidget; Don’t use it if you bind a buffer, because the coordinates and colors defined by setAttributeArray and setUniformValue will not set the correct locations of the colors of coordinates for the drawing method. In addition, only use coordinates specified in pixel if the GL Widget is of fixed size.

Following is an initializeGL method for coordintes whose values are between -1 and 1, and the color RGBA values are floats between 0 and 1:

void ExtendedOpenGLWidget::initializeGL(){
     glEnable(GL_BLEND);
     glEnable(GL_DEPTH_TEST);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
     m_vao.create();
     if (m_vao.isCreated()){
         cout<<"Created"<<endl;
         m_vao.bind();
     }
     m_vbo.create();
     m_vbo.bind();

     // Allocating buffer memory and populating it
     m_vbo.allocate(vertices, sizeof(vertices));

     m_fragmentShader=new QOpenGLShader(QOpenGLShader::Fragment);
     m_fragmentShader->compileSourceFile("../fragmentshader.txt");
     m_vertexShader=new QOpenGLShader(QOpenGLShader::Vertex);
     m_vertexShader->compileSourceFile("../vertexshader.txt");
     m_program=new QOpenGLShaderProgram();
     m_program->addShader(m_fragmentShader);
     m_program->addShader(m_vertexShader);
     m_program->link();
     m_program->bind();

     // Arguments of setAttributeBuffer in this example:
     //    1: A string, the corresponding attribute in the shaders.
     //    2: the type of each array element.
     //    3: the start point relative to the beginning of the current buffer.
     //    4: the number of elements in each vector.
     //    5: stride: the difference in bytes between two successive vectors
     m_program->setAttributeBuffer("vertex", GL_FLOAT,0,3,sizeof(vertex));
     m_program->setAttributeBuffer("color", GL_FLOAT,sizeof(vertices[0].coords),4,sizeof(vertex));
     m_program->enableAttributeArray("vertex");
     m_program->enableAttributeArray("color");
     m_vao.release();
 }

Shaders

The program uses two shaders:

  • a vertex shader – outputs the coordinates of which point
  • a fragment shader – outputs the color of which point.
  • Following are the files:

Vertex shader:

 attribute highp vec3 vertex;
 attribute highp vec4 color;
 varying highp vec4 f_color;
 void main(void){
     gl_Position=vec4(vertex,1.0);
     f_color=color;
 }

Fragment shader:

 varying mediump vec4 f_color;
 void main(void){
     gl_FragColor = f_color;
 }

Drawing

The method paintGL draws the image. In this example, it uses the function glDrawArrays with the mode GL_TRIANGLES. In 3D the only 3D polygon type a graphical program always knows how to fill because the vertices are on the same plan.

Following is the paintGL method:

void ExtendedOpenGLWidget::paintGL(){
     cout<<"paint"<<endl;
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
     m_vao.bind();
     // Arguments of setAttributeBuffer in this example:
     //    1. mode - how the shaders will draw the array
     //    2. start - first enabled array
     //    3. count - the number of indices.
     glDrawArrays(GL_TRIANGLES,0,3);
     m_vao.release();
}

The main function

An Open GL Widget may be a top-level window or a widget inside another container. In the following main function, the widget is inside a main window:

int main(int argc, char *argv[]){
     QApplication app(argc, argv);
     QMainWindow mainWindow;
     Ui_MainWindow mainObject;
     mainObject.setupUi(&mainWindow);
     mainWindow.show();
     return app.exec();
}

A Destructor

If you want to avoid warnings at the end of the execution, you better define a destructor to clean up the allocated objects.

Following is the code of one:

ExtendedOpenGLWidget::~ExtendedOpenGLWidget(){
     makeCurrent();
     m_vao.destroy();
     m_vbo.destroy();
     delete m_fragmentShader;
     delete m_vertexShader;
     delete m_program;
     doneCurrent();
 }

Learn more about OpenGL here

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