CODEWITHSUNDEEP
c++opengl
user1418638
user1418638

Reputation: 358

Disappearing object when resizeing screen (OpenGl)

I'm trying to load 3D models from .obj format and it draws the object on the scren without any problem but when I resize the screen everything disappear. Here's the code:

Obj* object = new Obj();
GLuint  texture[1]; 

void handleResize(int w, int h) {
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0,(double)w / (double)h,1.0,200.0);
}
void initRendering() {
object->GetObj("cube.obj");
glShadeModel(GL_LINEAR);                            
glClearColor(0.0f, 0.0f, 0.0f,     0.5f);                           
glEnable(GL_DEPTH_TEST);
}
void handleKeypress(unsigned char key, int x, int y) {
switch (key) {
    case 27:
        {
        exit(0);
        break;
        }
}
}

void drawScene()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW); 
glLoadIdentity();
glPushMatrix();
glRotatef(45.0,0.0,1.0,0.0);
object->DrawObj();
glPopMatrix();
glutSwapBuffers();
glFlush();

}
int _tmain(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(400, 400);


glutCreateWindow("3D");
initRendering();

glutReshapeFunc(handleResize);
glutDisplayFunc(drawScene);
glutKeyboardFunc(handleKeypress);
glutMainLoop();
return 0;
}

And here's the code for Obj.DrawObj():

glBegin(GL_TRIANGLES);
for(int i = 0;i < faces.capacity()-1;i++)
{
        glVertex3f(vertices[faces[i].vertex1].cordinate1,vertices[faces[i].vertex1].cordinate2,vertices[faces[i].vertex1].cordinate3);
    glVertex3f(vertices[faces[i].vertex2].cordinate1,vertices[faces[i].vertex2].cordinate2,vertices[faces[i].vertex2].cordinate3);
      glVertex3f(vertices[faces[i].vertex3].cordinate1,vertices[faces[i].vertex3].cordinate2,vertices[faces[i].vertex3].cordinate3);  
}
glEnd;

Upvotes: 0

Views: 2849

Answers (2)

Tim
Tim

Reputation: 35943

I can see that you're still confused by reading your PasteBin. Let me try to explain:

The reason why you can see your object the first time you draw it is because you have not set a projection matrix. So your object is drawn directly in normalized device coordinates (-1 to 1 range).

When you resize, you're setting the projection matrix for the first time, and this changes what viewing region is drawn to your screen. Your object as it is initially drawn is outside of the viewing region defined by your projection matrix (it is on top of the camera and I guess in front of the near plane. You have to move the object back away from the camera so that it is inside the view frustum. This is what datenwolf was suggesting.

However at the same time you introduced other errors into your code, particularly that you stopped resetting the projection matrix in handleResize. You have to always clear the projection matrix before you call gluPerspective, or else you will get a bogus result.

If you take the exact code from your pastebin, and add a glLoadIdentity to handleResize, I think that should work then:

void handleResize(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);   //<--- add
    glLoadIdentity();              //<--- add
    gluPerspective(45.0,(double)w / (double)h,1.0,200.0);
}

Also, you're still clearing the projection matrix during the drawScene function. When you clear the matrix, you're throwing away the perspective setting that you just set in handleResize, you don't want to do that.

So basically:

  1. Set the Projection matrix in handleResize and on initialization
  2. Don't touch the projection matrix in drawScene
  3. Translate the object so that it fits into the viewing frustum.

Upvotes: 1

datenwolf
datenwolf

Reputation: 162319

In your drawing code you set the projection matrix, which is good. However you set it to identity. In the resize handler you're setting the projection matrix as well, but you shouldn't do it there; yes I know the tutorials have it all there, but this is very bad style. You should move all the code currently in the reshape handler into the drawing handler, replacing the current setting of the projection matrix.

Upvotes: 1

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