Rendering issue in QOpenGLWindow

Julius Aetherwing

Hello everyone!
I'm pretty new to Qt, so I hope, I posted this to the right subforum.
I am trying to create an application with OpenGL Rendering, so I decided to use QOpenGLWindow for the job. However, I could barely find any tutorials on this, which show a simple example. Additionally, I'm not too familiar with OpenGL, so obviously, things don't work as expected.

I tried to create a simple triangle in the viewport, however it seems that the vertex shader doesn't receive the coordinate data. When I try to render the data as a triangle, I see nothing. When I try to render it as GL_POINTS instead, I see a single pixel at the origin. If I add a constant to the coordinates in the vertex shader, I can move that pixel, so obviously I messed up the data pipeline from the vertex buffer to the shader and only zeroes arrive.

I would appreciate any hints on what might have gone wrong with the way I set up the GL state.

main.cpp:

#include <QApplication>

#include "Window.h"

int main(int argc, char* argv[]) {
  QApplication app{argc, argv};
  
  QSurfaceFormat format;
  format.setDepthBufferSize(4);
  format.setSamples(24);
  format.setVersion(3, 1);
  format.setRenderableType(QSurfaceFormat::OpenGL);
  
  Window w;
  w.show();
  
  return app.exec();
}

Window.h:

#ifndef QTGL__WINDOW_H
#define QTGL__WINDOW_H

#include <QOpenGLFunctions>
#include <QOpenGLWindow>
#include <QOpenGLBuffer>
#include <QOpenGLShaderProgram>
#include <QOpenGLVertexArrayObject>

class Window : public QOpenGLWindow, protected QOpenGLFunctions {
public:
  Window();
  ~Window();

protected:
  void initializeGL() override;
  void resizeGL(int w, int h) override;
  void paintGL() override;
  
private:
  QOpenGLVertexArrayObject vao_;
  QOpenGLBuffer vertices_;
  QOpenGLShaderProgram shader_;
};

#endif // QTGL__WINDOW_H

Window.cpp:

#include "Window.h"

#include <QVector2D>

const char *vertexShader = //
    "#version 130\n"
    "in vec2 pos;\n"
    "void main() {\n"
    "gl_Position = vec4(pos, 0.0, 1.0);\n"
    "}";

const char *fragmentShader = //
    "#version 130\n"
    "out vec4 FragColor;\n"
    "void main() {\n"
    "FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
    "}\n";

Window::Window()
    : vao_{this}, vertices_{QOpenGLBuffer::VertexBuffer}, shader_{this} {}

Window::~Window() {
  vao_.destroy();
  vertices_.destroy();
}

void Window::initializeGL() {
  initializeOpenGLFunctions();
  glClearColor(0.2f, 0.2f, 0.2f, 1.0f);
  
  shader_.addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShader);
  shader_.addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShader);
  shader_.link();
  shader_.bind();
  
  vao_.create();
  vao_.bind();
  
  QVector2D vertices[] = {{-1.0, -1.0}, //
                          {1.0, -1.0},  //
                          {0.0, 1.0}};

  vertices_.create();
  vertices_.setUsagePattern(QOpenGLBuffer::StaticDraw);
  vertices_.allocate(vertices, sizeof(vertices));
  vertices_.bind();
  shader_.enableAttributeArray("pos");
  shader_.setAttributeBuffer("pos", GL_FLOAT, 0, 2);
  
  vertices_.release();
  vao_.release();
  shader_.release();
}

void Window::resizeGL(int w, int h) { glViewport(0, 0, w, h); }

void Window::paintGL() {
  glClear(GL_COLOR_BUFFER_BIT);
  
  shader_.bind();
  vao_.bind();
//  glDrawArrays(GL_POINTS, 0, 3); // renders a single red pixel at 0/0
  glDrawArrays(GL_TRIANGLES, 0, 3); // renders nothing at all
  vao_.release();
  shader_.release();
}

Thank you for your time!

Julius Aetherwing

Anyone? Or could you maybe point me to a proper minimal (!) example using QOpenGLWidget and the QOpenGL helper classes?

Bonnie

@Julius-Aetherwing
Sorry, I'm not familiar with OpenGL, too.
But you can find some example code in Qt install dir, something like "C:\Qt\Examples\Qt-5.12.6\opengl"
The docs of the examples can be found in All Qt Examples