Unexpected projection - opengl

Question

It should be rect, which will takes whole width of window. I am expecting this, because frustum width is -5 to 5 and rect size is 10x10 and rect is on same z-axis position like near plnane of frustum. Result is very small and doesn't look like a rect, i don't know why?
void GLWidget::initializeGL() {
glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
}
void GLWidget::resizeGL(int w, int h) {
glViewport(0,0, w, h);
glMatrixMode(GL_PROJECTION);
double ratio = (double)w/(double)h;
double size = ((10.0/ratio)/2.0);
glFrustum(-5.0, 5.0, -size, size, 10.0, 50.0);
}
void GLWidget::paintGL() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0f, 0.0f, -10.0f);
glColor3f(0.0f, 0.0f, 0.0f);
glBegin(GL_QUADS);
glVertex3f(5.0f, -5.0f, 0.0f);
glVertex3f(5.0f, 5.0f, 0.0f);
glVertex3f(-5.0f, 5.0f, 0.0f);
glVertex3f(-5.0f, -5.0f, 0.0f);
glEnd();
glFlush();
}

Answer 1

It should be rect, which will takes whole width of window.
Well, then switch your projection to something better suited, an ortho projection, for the task when drawing the rect. Also you must reset your projection matrix to identity before applying frustum or ortho.
Hint: All the code in your resizeGL should go into the paintGL method. I see you're using Qt, so you can get the widget's width and height conveniently using the width and height getter functions. Also glClearColor goes into painGL as well:
void GLWidget::initializeGL() {
}
void GLWidget::resizeGL(int w, int h) {
}
void GLWidget::paintGL()
{
double const ratio = (double)width()/(double)height();
double const size = ((10.0/ratio)/2.0);
glClearColor(1.0f, 1.0f, 1.0f, 0.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glViewport(0,0, width(), height());
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-5.0, 5.0, -size, size, 10.0, 50.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0f, 0.0f, -10.0f);
draw_perspective_stuff();
glViewport(0,0, width(), height());
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// using identity projection, this fills the viewport
glColor3f(0.0f, 0.0f, 0.0f);
glBegin(GL_QUADS);
glVertex3f(-1.f, -1.f, 0.0f);
glVertex3f( 1.f, -1.f, 0.0f);
glVertex3f( 1.f, 1.f, 0.0f);
glVertex3f(-1.f, 1.f, 0.0f);
glEnd();
glFlush();
}