QPixmap runs over my glScissor(...) setting - c++

Question

I apologize if this isn't exact. I'm doing the best I can to copy code by hand from one computer to another, and the destination computer doesn't have a compiler (don't ask).
Header file
#ifndef MYOPENGLWIDGET_H
#define MYOPENGLWIDGET_H
#include <qopenglwidget.h>
class MyOpenGlWidget : public QOpenGLWidget
{
Q_OBJECT
public:
explicit MyOpenGlWidget(QWidget *parent = 0, Qt::WindowFlags f = Qt::WindowFlags());
virtual ~MyOpenGlWidget();
protected:
// these are supposed to be overridden, so use the "override" keyword to compiler check yourself
virtual void initializeGL() override;
virtual void resizeGL(int w, int h) override;
virtual void paintGL() override;
private:
QPixmap *_foregroundPixmap;
}
#endif
Source file
QOpenGLFunctions_2_1 *f = 0;
MyOpenGlWidget::MyOpenGlWidget(QWidget *parent, Qt::WindowFlags f) :
QOpenGLWidget(parent, f)
{
_foregroundPixmap = 0;
QPixmap *p = new QPixmap("beveled_texture.tiff");
if (!p->isNull())
{
_foregroundPixmap = p;
}
}
MyOpenGlWidget::~MyOpenGlWidget()
{
delete _foregroundPixmap;
}
void MyOpenGlWidget::initializeGL()
{
// getting a deprecated set of functions because such is my work environment
// Note: Also, QOpenGLWidget doesn't support these natively.
f = QOpenGLContext::currentContext()->versionFunctions<QOpenGLFunctions_2_1>();
f->glClearColor(0.0f, 1.0f, 0.0f, 1.0f); // clearing to green
f->glEnable(GL_DEPTH_TEST);
f->glEnable(GL_CULL_FACE); // implicitly culling front face
f->glEnable(GL_SCISSOR_TEST);
// it is either copy the matrix and viewport code from resizeGL or just call the method
this->resizeGL(this->width(), this->height());
}
void MyOpenGlWidget::resizeGL(int w, int h)
{
// make the viewport square
int sideLen = qMin(w, h);
int x = (w - side) / 2;
int y = (h - side) / 2;
// the widget is 400x400, so this random demonstration square will show up inside it
f->glViewport(50, 50, 100, 100);
f->glMatrixMode(GL_PROJECTION);
f->glLoadIdentity();
f->glOrtho(-2.0f, +2.0f, -2.0f, +2.0f, 1.0f, 15.0f); // magic numbers left over from a demo
f->glMatrixMode(GL_MODELVIEW);
// queue up a paint event
// Note: QGLWidget used updateGL(), but QOpenGLWidget uses update().
this->update();
}
void MyOpenGlWidget::paintGL()
{
f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// I want to draw a texture with beveled edges the size of this widget, so I can't
// have the background clearing all the way to the edges
f->glScissor(50, 50, 200, 200); // more magic numbers just for demonstration
// clears to green in just scissored area (unless QPainter is created)
f->glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
// loading identity matrix, doing f->glTranslatef(...) and f->glRotatef(...)
// pixmap loaded earlier in another function
if (_foregroundPixmap != 0)
{
// QPixmap apparently draws such that culling the back face will cull the entire
// pixmap, so have to switch culling for duration of pixmap drawing
f->glCullFace(GL_FRONT);
QPainter(this);
painter.drawPixmap(0, 0, _foregroundPixmap->scaled(this->size()));
// done, so switch back to culling the front face
f->glCullFace(GL_BACK);
}
QOpenGLFunctions_2_1 *f = 0;
void MyOpenGlWidget::initializeGL()
{
// getting a deprecated set of functions because such is my work environment
// Note: Also, QOpenGLWidget doesn't support these natively.
f = QOpenGLContext::currentContext()->versionFunctions<QOpenGLFunctions_2_1>();
f->glClearColor(0.0f, 1.0f, 0.0f, 1.0f); // clearing to green
f->glEnable(GL_DEPTH_TEST);
f->glEnable(GL_CULL_FACE); // implicitly culling front face
f->glEnable(GL_SCISSOR_TEST);
// it is either copy the matrix and viewport code from resizeGL or just call it directly
this->resizeGL(this->width(), this->height());
}
void MyOpenGlWidget::resizeGL(int w, int h)
{
// make the viewport square
int sideLen = qMin(w, h);
int x = (w - side) / 2;
int y = (h - side) / 2;
// the widget is 400x400, so this random demonstration square will show up inside it
f->glViewport(50, 50, 100, 100);
f->glMatrixMode(GL_PROJECTION);
f->glLoadIdentity();
f->glOrtho(-2.0f, +2.0f, -2.0f, +2.0f, 1.0f, 15.0f); // magic numbers left over from a demo
f->glMatrixMode(GL_MODELVIEW);
// queue up a paint event
// Note: QGLWidget used updateGL(), but QOpenGLWidget uses update().
this->update();
}
void MyOpenGlWidget::paintGL()
{
f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// I want to draw a texture with beveled edges the size of this widget, so I can't
// have the background clearing all the way to the edges
f->glScissor(50, 50, 200, 200); // more magic numbers just for demonstration
// clears to green in just scissored area (unless QPainter is created)
f->glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
// loading identity matrix, doing f->glTranslatef(...) and f->glRotatef(...), drawing triangles
// done drawing, so now draw the beveled foreground
if (_foregroundPixmap != 0)
{
// QPixmap apparently draws such that culling the back face will cull the entire
// pixmap, so have to switch culling for duration of pixmap drawing
f->glCullFace(GL_FRONT);
QPainter(this);
painter.drawPixmap(0, 0, _foregroundPixmap->scaled(this->size()));
// done, so switch back to culling the front face
f->glCullFace(GL_BACK);
}
}
The problem is this code from paintGL():
QPainter(this);
As soon as a QPainter object is created, the glScissor(...) call that I made earlier in the function is overrun and some kind of glClearColor(...) call is made (possibly from QPainter's constructor) that clears the entire viewport to the background color that I set just after glScissor(...). Then the pixmap draws my beveled texture just fine.
I don't want QPainter to overrun my scissoring.
The closest I got to an explanation was two QPainter methods, beginNativePainting() and endNativePainting(). According to the documentation, scissor testing is disabled between these two, but in their example they re-enable it. I tried using this "native painting" code, but I couldn't stop QPainter's mere existence from ignoring GL's scissoring and clearing my entire viewport.
Why is this happening and how do I stop this?
Note: This work computer has network policies to prevent me from going to entertainment sites like imgur to upload "what I want" and "what I get" pictures, so I have to make due with text.

Answer 1

Why is this happening
The OpenGL context is a shared resource and you have to share it with other players.
and how do I stop this?
You can't. Just do the proper thing and set viewport, scissor rectangle and all the other drawing related state at the right moment: Right before you are going to draw something that relies on these settings. Don't set them aeons (in computer terms) before, somewhere in some "initialization" or a reshape handler. And be expected that in drawing code any function you call that makes use of OpenGL will leave some garbage behind.