Draw with OPenGL inside QMAinWindow

[wysota]

Remember if we have a window in Win system we can get it device context and draw inside any window.

I don't see how that's relevant.

Here in Qt we are creating additional window. That is a bit strange to me.

"In Qt we have a backing store which does double buffering for us by default. In WinAPI we get flicker instead. That is a bit strange to me".

I was thinking that QGLWidget is just a wrapper which encapsulates access to the QMainWindow device context.

Regular widgets are raster based, they don't have a GL context (at least until Qt 5.4). This is really nothing you should worry about in normal conditions.

[Radek]

QGLWidget need not be a "new window". It's a widget. Period. Therefore, create a main window, create a widget as a part of its central widget, and promote the widget to a QGLWidget - or rather to a class derived from the QGLWidget. You have an OpenGL window as a part of your main window. The OpenGL window can be the whole client area of the main window or you can create additional control widgets around.

Now, use OpenGL for drawing in your OpenGL winwow.

[wysota]

QGLWidget need not be a "new window". It's a widget. Period.

Unfortunately it is not so simple. QGLWidget generates an invisible window (or even a couple of windows on Windows) in the window system. So logically (from the program point of view) there is no additional window (as in something you can drag around) but physically using QGLWidget requires a native window handle which forces the parent widget to also have a native window handle and so on.

But from programmer's point of view there is nothing to worry about in normal conditions. Before Qt 5.4 there is nothing one can do about it anyway (well, you can render to FBO, convert that to image and then draw the image on the widget but that has its downsides too).

[zulunation]

I have successfully set up Opengl on a QGLWidget. It really creates additional window on Windows OS.
That's ok for me.
There is one thing i can't understand.
I'm drawing in the paintGL function. I do not call makeCurrent() swapBuffers() functions at the beginning and at the end respectively.
Everything is fine. When i do that i get:

QOpenGLContext::swapBuffers() called without corresponding makeCurrent()

message in the debugger output window when i call swapBuffers().
Why does that happen?

[wysota]

Why does that happen?

Because you are not calling makeCurrent()

[zulunation]

I do.
Here is my paintGL function

void paintGL()
	 {
		 makeCurrent();
		glViewport(0,0,500,500);
 
		glMatrixMode(GL_PROJECTION);
		glLoadIdentity();
		glOrtho(0,500,0,500,0,-100);
		//glOrtho(-1,1,1,1,0,-1);
 
		glMatrixMode(GL_MODELVIEW);
 
		glLoadIdentity();
		glClear(GL_COLOR_BUFFER_BIT);
		glClearColor(0,1,0,1);
		glColor3f(1,0,0);
 
		const int x = 100;
		 int y = 100;
 
		const int dx = 26;
		const int dy = 16;
 
		glBegin(GL_LINES);
		for(int i = 0;i<10;++i)
		{
		glVertex3f(x,y,0);
		glVertex3f(x+dx,y,0);
 
		glVertex3f(x+dx,y,0);
		glVertex3f(x+dx,y+dy,0);
 
		glVertex3f(x+dx,y+dy,0);
		glVertex3f(x,y+dy,0);
 
		glVertex3f(x,y+dy,0);
		glVertex3f(x,y,0);
 
		y+=18;
		}
		//glVertex3f(90,90,0);
		glEnd();
 
		swapBuffers();
	 }

void paintGL()
	 {
		 makeCurrent();
		glViewport(0,0,500,500);

		glMatrixMode(GL_PROJECTION);
		glLoadIdentity();
		glOrtho(0,500,0,500,0,-100);
		//glOrtho(-1,1,1,1,0,-1);
		
		glMatrixMode(GL_MODELVIEW);
		
		glLoadIdentity();
		glClear(GL_COLOR_BUFFER_BIT);
		glClearColor(0,1,0,1);
		glColor3f(1,0,0);

		const int x = 100;
		 int y = 100;

		const int dx = 26;
		const int dy = 16;
		
		glBegin(GL_LINES);
		for(int i = 0;i<10;++i)
		{
		glVertex3f(x,y,0);
		glVertex3f(x+dx,y,0);

		glVertex3f(x+dx,y,0);
		glVertex3f(x+dx,y+dy,0);

		glVertex3f(x+dx,y+dy,0);
		glVertex3f(x,y+dy,0);

		glVertex3f(x,y+dy,0);
		glVertex3f(x,y,0);

		y+=18;
		}
		//glVertex3f(90,90,0);
		glEnd();

		swapBuffers();
	 }

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I assume makeCurrent/SwapBuffers are not necessary if we are dealing with one widget. They are called by paintGL implicitly. But not sure about that.