Qt OpenGL (QOpenGLWidget) - Simple Triangle

[saket]

As a newbee to QT+OpenGL using QOpenGLWidget, I am unable to color my triangle. Please find my code here, using QMainWindow for GUI ...

// main.cpp
#include "mainwindow.h"
#include <QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

// main.cpp
#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

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Here the GUI - window

// MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
};
 
#endif // MAINWINDOW_H

// MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

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Here implementation - file ...

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

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Here is the widget rendering Opengl-Context.

#ifndef OPENGLWIDGET_H
#define OPENGLWIDGET_H
 
#include <QWidget>
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QOpenGLContext>
#include <QOpenGLShaderProgram>
#include <QOpenGLBuffer>
#include <QOpenGLVertexArrayObject>
#include <QMatrix4x4>
 
class OpenglWidget : public QOpenGLWidget, public QOpenGLFunctions
{
public:
    OpenglWidget(QWidget *parent = 0);
    ~OpenglWidget();
 
protected:
    void initializeGL();
    void resizeGL(int width, int height);
    void paintGL();
    GLuint m_posAttr;
    GLuint m_colAttr;
    GLuint m_matrixUniform;
    QOpenGLShaderProgram *m_program;
};
#endif // OPENGLWIDGET_H

#ifndef OPENGLWIDGET_H
#define OPENGLWIDGET_H

#include <QWidget>
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QOpenGLContext>
#include <QOpenGLShaderProgram>
#include <QOpenGLBuffer>
#include <QOpenGLVertexArrayObject>
#include <QMatrix4x4>

class OpenglWidget : public QOpenGLWidget, public QOpenGLFunctions
{
public:
    OpenglWidget(QWidget *parent = 0);
    ~OpenglWidget();

protected:
    void initializeGL();
    void resizeGL(int width, int height);
    void paintGL();
    GLuint m_posAttr;
    GLuint m_colAttr;
    GLuint m_matrixUniform;
    QOpenGLShaderProgram *m_program;
};
#endif // OPENGLWIDGET_H

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Here is the implementation file ...

#include "openglwidget.h"
OpenglWidget::OpenglWidget(QWidget *parent) :
    QOpenGLWidget(parent)
{
    setFormat(QSurfaceFormat::defaultFormat());
}
 
OpenglWidget::~OpenglWidget()
{
}
 
static const char *vertexShaderSource =
    "attribute highp vec4 posAttr;\n"
    "attribute lowp vec4 colAttr;\n"
    "varying lowp vec4 col;\n"
    "uniform highp mat4 matrix;\n"
    "void main() {\n"
    "   col = vec4(1, 0, 0, 1);\n"
    "   gl_Position = matrix * posAttr;\n"
    "}\n";
 
static const char *fragmentShaderSource =
    "varying lowp vec4 col;\n"
    "void main() {\n"
    "   gl_FragColor = col;\n"
    "}\n";
 
void OpenglWidget::initializeGL()
{
    makeCurrent();
    initializeOpenGLFunctions();
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
 
    // Create Shader (Do not release until VAO is created)
    m_program = new QOpenGLShaderProgram(this);
    m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
    m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
    m_program->link();
    m_posAttr = m_program->attributeLocation("posAttr");
    m_colAttr = m_program->attributeLocation("colAttr");
    m_matrixUniform = m_program->attributeLocation("matrix");
 
    m_program->release();
}
 
void OpenglWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    makeCurrent();
 
    //m_program->bind();
 
    QMatrix4x4 matrix;
    matrix.perspective(60.0f, 4.0f/3.0f, 0.1f, 100.0f);
    matrix.translate(0, 0, -2);
 
    m_program->setUniformValue(m_matrixUniform, matrix);
 
    GLfloat vertices[] = {
        0.0f, 0.707f, 0.0f,
        -0.5f, -0.5f, 0.0f,
        0.5f, -0.5f, 0.0f
    };
 
    GLfloat colors[] = {
        1.0f, 0.0f, 0.0f,
        0.0f, 1.0f, 0.0f,
        0.0f, 0.0f, 1.0f
    };
 
    glVertexAttribPointer(m_posAttr, 3, GL_FLOAT, GL_FALSE, 0, vertices);
    glVertexAttribPointer(m_colAttr, 3, GL_FLOAT, GL_FALSE, 0, colors);
 
    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);
 
    glDrawArrays(GL_TRIANGLES, 0, 3);
 
    glDisableVertexAttribArray(1);
    glDisableVertexAttribArray(0);
 
    m_program->release();
}
 
void OpenglWidget::resizeGL(int width, int height)
{
    glViewport(0, 0, width, height);
}

#include "openglwidget.h"
OpenglWidget::OpenglWidget(QWidget *parent) :
    QOpenGLWidget(parent)
{
    setFormat(QSurfaceFormat::defaultFormat());
}

OpenglWidget::~OpenglWidget()
{
}

static const char *vertexShaderSource =
    "attribute highp vec4 posAttr;\n"
    "attribute lowp vec4 colAttr;\n"
    "varying lowp vec4 col;\n"
    "uniform highp mat4 matrix;\n"
    "void main() {\n"
    "   col = vec4(1, 0, 0, 1);\n"
    "   gl_Position = matrix * posAttr;\n"
    "}\n";

static const char *fragmentShaderSource =
    "varying lowp vec4 col;\n"
    "void main() {\n"
    "   gl_FragColor = col;\n"
    "}\n";

void OpenglWidget::initializeGL()
{
    makeCurrent();
    initializeOpenGLFunctions();
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);

    // Create Shader (Do not release until VAO is created)
    m_program = new QOpenGLShaderProgram(this);
    m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
    m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
    m_program->link();
    m_posAttr = m_program->attributeLocation("posAttr");
    m_colAttr = m_program->attributeLocation("colAttr");
    m_matrixUniform = m_program->attributeLocation("matrix");

    m_program->release();
}

void OpenglWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    makeCurrent();

    //m_program->bind();

    QMatrix4x4 matrix;
    matrix.perspective(60.0f, 4.0f/3.0f, 0.1f, 100.0f);
    matrix.translate(0, 0, -2);

    m_program->setUniformValue(m_matrixUniform, matrix);

    GLfloat vertices[] = {
        0.0f, 0.707f, 0.0f,
        -0.5f, -0.5f, 0.0f,
        0.5f, -0.5f, 0.0f
    };

    GLfloat colors[] = {
        1.0f, 0.0f, 0.0f,
        0.0f, 1.0f, 0.0f,
        0.0f, 0.0f, 1.0f
    };

    glVertexAttribPointer(m_posAttr, 3, GL_FLOAT, GL_FALSE, 0, vertices);
    glVertexAttribPointer(m_colAttr, 3, GL_FLOAT, GL_FALSE, 0, colors);

    glEnableVertexAttribArray(0);
    glEnableVertexAttribArray(1);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glDisableVertexAttribArray(1);
    glDisableVertexAttribArray(0);

    m_program->release();
}

void OpenglWidget::resizeGL(int width, int height)
{
    glViewport(0, 0, width, height);
}

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Here the rendered triangle is just white. I am unable to understand whether the shader is compiled and attributes are linked but still, I am unable to color the triangle.

If I can get any guidance... ?

test_triangle.jpg