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Asked: 2026-05-23T04:04:35+00:00

I have a problem with glViewport. In my liitle programm i have two viewports.

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I have a problem with glViewport. In my liitle programm i have two viewports. I can draw a form (with the motionfunc) in one of those viewports and in the other a line is automatically drawn.
So far so good..
When i try to draw something with mousefunc the viewport is in a total different place. And it is very difficult to find the new correct coordinates for that viewport.
Is there a possibility to reset the coordinates..
I cant use glLoadIdentity in mouse or motion because then nothing is displayed.

I hope you understand what i mean. It is a bit difficult to explain.

OK here a codesnippet….

void mouse (int button, int state, int mx, int my)
{
if (modus == 0 && button==GLUT_LEFT_BUTTON && state != GLUT_DOWN) 
{
    ...
}

else if (modus == 1 && button==GLUT_LEFT_BUTTON && state == GLUT_DOWN) 
{


    **glViewport(10,10 , sw_w1, sw_h1);** 
//the drawing is much higher than in the first viewport in motion. 
//But it should be the same viewport like the first in motion.
    glBegin()...glEnd()
            glFlush();
}   
}   
void motion(int mousex,int mousey)
{
GLdouble h=12;
GLdouble winkel=360/h;
Line liste[num];

liste[++last].x =(mousex)-((sw_w1+2*GAP)/2);
liste[last].y   =(mousey)-((sw_h1+2*GAP)/2);

if (modus==0 && gruppe == 0) {
    if (last>=1)
    {
        glViewport(10, 10, sw_w1, sw_h1); //works fine
        glColor3d(R, G, B);
        for(h+1;h>0;h--){
            glRotated(winkel, 0, 0, 1);
            glBegin(GL_LINE_STRIP);
            for(int i=last-1;i<=last;i++){
                glVertex2i(liste[i].x,liste[i].y);
            }
            glEnd();
        }

        glLineWidth(linewidth+0.5);
        glColor3f(1, 0, 0);
        glBegin(GL_LINE_STRIP);
        for(int i=last-1;i<=last;i++){
            glVertex2i(liste[i].x,liste[i].y);
        }
        glEnd();
        glViewport(1020,10 , sw_w2, sw_h2); //works fine
        glColor3f(1, 0, 0);
        glBegin(GL_LINE_STRIP);
        for(int i=last-1;i<=last;i++){
            glVertex2i(liste[i].x,liste[i].y);
        }
        glEnd();
    }
    glFlush();

}
}

The second and third viewport works fine. The first one is the same as the second but the picture is displayed much higher.Why is that so?
And how could i change it so that i get the same viewport like the second one.
I hope you now understand what i mean.

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1 Answer

  1. Editorial Team

    You should check your modelview/projection matrices and see if they are what you expect them to be in each function.

    Also, as Christian’s commented, it is not necessary, nor recommended, to draw in the motion func. Update your application state per the input and call glutPostRedisplay to signal that you want to redraw your window. That way, your application will have a cleaner design and it will be easier to make it behave consistently.

    (added my comment as answer since that was the problem, and added Christian’s comment, since that is the proper solution. Don’t draw in motionfunc!.)

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