My qtopengl widget not showing up? - c++

Question

main.cpp
#include "QtGuiApplication4.h"
#include <QtWidgets/QApplication>
#include "megui.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
megui test;
test.show();
return a.exec();
}
qtguiapplication4.h
#pragma once
#include <QtWidgets/QMainWindow>
#include "ui_QtGuiApplication4.h"
class QtGuiApplication4 : public QMainWindow
{
Q_OBJECT
public:
QtGuiApplication4(QWidget *parent = Q_NULLPTR);
private:
Ui::QtGuiApplication4Class ui;
};
qtguiapplication4.cpp
#include "QtGuiApplication4.h"
QtGuiApplication4::QtGuiApplication4(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
}
megui.h
#pragma once
#include "QOpenGLWidget"
#include "QOpenGLBuffer"
#include "QOpenGLFunctions"
#include "QOpenGLShaderProgram"
#include <iostream>
class megui:public QOpenGLWidget, protected QOpenGLFunctions
{
public:
QOpenGLFunctions glfunctions;
QOpenGLShaderProgram program;
void initializeGL();
void paintGL();
megui();
~megui();
};
megui.cpp
#include "megui.h"
void megui::initializeGL()
{
makeCurrent();
float vertexpositions[] = {
0.5f,0.5f,
0.0f,-0.5f,
0.5f,-0.5f
};
QOpenGLBuffer buffer1(QOpenGLBuffer::VertexBuffer);
buffer1.create();
buffer1.setUsagePattern(QOpenGLBuffer::StaticDraw);
buffer1.allocate(vertexpositions, sizeof(vertexpositions));
buffer1.bind();
glfunctions.initializeOpenGLFunctions();
glfunctions.glEnableVertexAttribArray(0);
glfunctions.glVertexAttribPointer(0, 3, GL_FLOAT, false, 0, 0);
program.addShaderFromSourceFile(QOpenGLShader::Vertex, "C:/open gl testimg/opengl testing/opengl testing/vertex.glsl");
program.addShaderFromSourceFile(QOpenGLShader::Fragment, "C:/open gl testimg/opengl testing/opengl testing/fragmentshader.glsl");
program.link();
}
void megui::paintGL()
{
makeCurrent();
glfunctions.initializeOpenGLFunctions();
program.bind();
glfunctions.glDrawArrays(GL_TRIANGLES, 0, 3);
}
megui::megui()
{
}
megui::~megui()
{
}
fragmentshader
#version 430
out vec4 inalcolors;
void main() {
inalcolors =vec4( 1.0f, 0.0f, 0.0f,1.0) ;
}
vertexshader
#version 430
layout(location = 0) in vec4 positions;
void main() {
gl_Position =positions;
}
The widget screen is not showing up but when i remove glDrawArrays call then it shows up.I suspect there is some problem in the opengl code which is causing this.I followed a youtube tutorial everything in his and my code is same expect i use qt opengl wrapper class rather than glew library.what would be the reason?

Answer 1

The QOpenGLBuffer buffer1 is a local variable in the method megui::initializeGL(). Qt OpenGL objects uses Resource Acquisition Is Initialization (RAII) technology. This means at the end of the function buffer1 is destructed and the buffer is gone and invalid.
Further the QOpenGLBuffer object has to be bound (bind()) before it can be allocated (allocate()). See the documentation of QOpenGLBuffer::allocate.
By the way, your vertex coordinates consists of 2 components (x, y) and not of 3.
Create a QOpenGLBuffer member in the class megui
class megui:public QOpenGLWidget, protected QOpenGLFunctions
{
public:
QOpenGLFunctions glfunctions;
QOpenGLShaderProgram program;
QOpenGLBuffer buffer1;
.....
}
megui::megui()
: buffer1(QOpenGLBuffer::VertexBuffer)
{}
Bind the buffer before you allocate it:
void megui::initializeGL()
{
.....
buffer1.create();
buffer1.setUsagePattern(QOpenGLBuffer::StaticDraw);
buffer1.bind(); // <-------- bind before allocate
buffer1.allocate(vertexpositions, sizeof(vertexpositions));
float vertexpositions[] = {
0.5f,0.5f,
0.0f,-0.5f,
0.5f,-0.5f
};
glfunctions.glEnableVertexAttribArray(0);
glfunctions.glVertexAttribPointer(
0, 2, // <-------- 2 instead of 3
GL_FLOAT, false, 0, 0);
buffer1.release();
.....
}