Pernicious bug

OpenGL OpenGL: Basic Coding
1

Hi folks,
I’ve got a bug that is rather odd and I’ve spent a day trying to fix it.

My GL screen consist of 2 triangles that display a texture that fills the window.
Should be simple, right?

The problem is that whenever the window is resized, everything falls to pieces.

In pseudo C code, my approach is something like this:


static bool window_resized=false;
static int newwidth=0, newheight=0;
resize_window(int w, int h)
{
  window_resized=true;
  newwidth=w; newheight=h;
  glViewport (0,0,w,h);
}
draw_scene(){
  if (window_resized) {
   resize_texture(newwidth, newheight);
   window_resized=false;
  }
  glViewport (0,0,width, height);
  glMatrixMode(perspective..)
  glLoadIdentity()
  glOrtho(-width/2.,width/2.,height/2.,-height/2.,-1,1); // anchor at center of window
  glBindTexture(...)
  glTexImage2D(...)
  glMatrixMode(model..)
  glLoadIdentity()
  glBegin();
  ...draw texture on a quad...
  glEnd()
  glFlush()
  glutSwapBuffers()
}
void do_idle()
{
   glutPostRedisplay();
}

The bug is that whenever I resize, everything is out of whack until the idle routine can force a redisplay. I’d prefer to have no idle routine at all, and I found that removing it results in the entire texture being moved downward (on Linux and on Mac), or (on the Mac) resized oddly etc.
Anyone know what could cause this?

2

So why does your draw routine use newwidth/height for resizing the texture, but width/height for setting the active viewport for rendering and setting up the projection?

I would think you should use the latest for both.

3

That’s a typo, I was writing it out from memory.
width/height are updated to newwidth & newheight.