I am experimenting with the new QOpenGLWidget class (note that this is not the QGLWidget class).
I am drawing a triangle. I have a trivial vertex shader which receives coordinates in clip space, so no matrices or projections are involved. One of the vertices has coordinates -1, -1, 0, 1, and another one has coordinates 1, 1, 0, 1.
When I have no call to glViewport whatsoever, the program renders as if I am calling glViewport(0, 0, w, h); in my resizeGL function, which I am not. Namely, the two vertices of the triangle are attached to the lowerleft and upperright corners of the window no matter how I resize the window.
When I actually add a call to glViewport in my resizeGL function, it is apparently ignored - doesn't matter if I pass w/2, h/2 or any other value, the rendering is exactly the same as it would be if I called glViewport(0, 0, w, h); (for instance, I would expect the triangle to appear in the lower-left quarter of the window in case of glViewport(0, 0, w/2, h/2);)
When I call glViewport(0, 0, width()/2, height()/2) in paingGL function, the rendering is as expected - everything is drawn in the lower-left quarter of the window.
So it seems that the glViewport is overridden somewhere between resizeGL and paintGL. What is going on and how do I fix it? Do I have to resort to doing viewport transformations in my paintGL function?
One of the differences between QGLWidget and QOpenGLWidgets listed in the documentation is that the latter renders to a framebuffer rather than directly to the screen. Could this hold the key to the explanation?
Just in case, I'm attaching the complete code for reference.
//triangle.h
#ifndef TRIANGLE_H
#define TRIANGLE_H
#include <QOpenGLBuffer>
#include <QOpenGLFunctions>
class Triangle
{
public:
Triangle();
void render();
void create();
private:
QOpenGLBuffer position_vbo;
QOpenGLFunctions *glFuncs;
};
#endif // TRIANGLE_H
//triangle.cpp
#include "triangle.h"
Triangle::Triangle()
:position_vbo(QOpenGLBuffer::VertexBuffer)
{
}
void Triangle::create()
{
glFuncs = QOpenGLContext::currentContext()->functions();
position_vbo.create();
float val[] = {
-1.0f, -1.0f, 0.0f, 1.0f,
0.0f, -0.366f, 0.0f, 1.0f,
1.0f, 1.0f, 0.0f, 1.0f,
1.0f, 0.0f, 0.0f, 1.0f,
0.0f, 1.0f, 0.0f, 1.0f,
0.0f, 0.0f, 1.0f, 1.0f,
};
position_vbo.setUsagePattern(QOpenGLBuffer::StaticDraw);
position_vbo.bind();
position_vbo.allocate(val, sizeof(val));
position_vbo.release();
}
void Triangle::render()
{
position_vbo.bind();
glFuncs->glEnableVertexAttribArray(0);
glFuncs->glEnableVertexAttribArray(1);
glFuncs->glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
glFuncs->glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, (void*)(3*4*sizeof(float)));
glFuncs->glDrawArrays(GL_TRIANGLES, 0, 3);
glFuncs->glDisableVertexAttribArray(0);
glFuncs->glDisableVertexAttribArray(1);
position_vbo.release();
}
//widget.h
#ifndef WIDGET_H
#define WIDGET_H
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QOpenGLShaderProgram>
#include "triangle.h"
class Widget : public QOpenGLWidget
, protected QOpenGLFunctions
{
Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
protected:
virtual void initializeGL() override;
virtual void paintGL() override;
virtual void resizeGL(int w, int h) override;
private:
QOpenGLShaderProgram* program;
Triangle t;
};
#endif // WIDGET_H
//widget.cpp
#include "widget.h"
#include <exception>
#include <QDebug>
Widget::Widget(QWidget *parent)
: QOpenGLWidget(parent)
{
}
Widget::~Widget()
{
}
void Widget::initializeGL()
{
initializeOpenGLFunctions();
program = new QOpenGLShaderProgram(this);
if(!program->addShaderFromSourceFile(QOpenGLShader::Vertex, ":/shaders/vertexshader.vert"))
{
throw std::exception(("Vertex Shader compilation error: " + program->log()).toLocal8Bit().constData());
}
if(!program->addShaderFromSourceFile(QOpenGLShader::Fragment, ":/shaders/fragmentshader.frag"))
{
throw std::exception(("Fragment Shader compilation error: " + program->log()).toLocal8Bit().constData());
}
if(!program->link())
{
throw std::exception(("Program Link error: " + program->log()).toLocal8Bit().constData());
}
t.create();
}
void Widget::paintGL()
{
glClearColor(0.f, 0.15f, 0.05f, 0.f);
glClear(GL_COLOR_BUFFER_BIT);
//glViewport(0, 0, width()/2, height()/2); //works!!
program->bind();
t.render();
program->release();
}
void Widget::resizeGL(int w, int h)
{
glViewport(0, 0, w/2, h/2); //doesn't work
}
//main.cpp
#include "widget.h"
#include <exception>
#include <QApplication>
#include <QMessageBox>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
try
{
Widget w;
w.show();
return a.exec();
}
catch(std::exception const & e)
{
QMessageBox::warning(nullptr, "Error", e.what());
}
}
//vertex shader
#version 330
layout (location = 0) in vec4 position;
layout (location = 1) in vec4 color;
smooth out vec4 theColor;
void main()
{
gl_Position = position;
theColor = color;
}
//fragment shader
#version 330
out vec4 fragColor;
smooth in vec4 theColor;
void main()
{
fragColor = theColor;
}
