glTranslatef improperly used: nothing displayed

[ramy1989]

I am trying to write a simple application with xcode that displaces a quad.For this I am using glTranslatef.The evidenced part of the code is where I try to translate the points.

#import <OpenGL/OpenGL.h>
#import <GLUT/GLUT.h>
 
 
int width=500, height=500;
 
 
void init()
{
    glViewport(0, 0, width, height);
    glLoadIdentity();
    glOrtho(0, width, height, 0, 0, 1);
}
 
 
void display()
{
    glClearColor(0.9, 0.9, 0.9, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 0.0, 0.0);
     glMatrixMode(GL_MODELVIEW);  /************************/
    glLoadIdentity();[/B]          /**************/
    glTranslatef(100,0,0);    /****** trying to translate the vertexes  */
    glBegin(GL_QUADS);
    glVertex2i(100, 100);
    glVertex2i(300, 100);
    glVertex2i(300, 300);
    glVertex2i(100, 300);
    glEnd();
    glFlush();
}
 
 
int main(int argc, char * argv[])
{
    glutInit(&argc, argv);
    glutInitWindowPosition(100, 100);
    glutInitWindowSize(width, height);
    glutCreateWindow("Test");
    glutDisplayFunc(display);
    init();
    glutMainLoop();
    return 0;
}

The quad should be displaced of 100 units in the widnow.Since in the glOrtho I specify the size of the view, I think that the quad shall be displayed inside the window.But instead I don't see anything: there isn't a quad in the window.So maybe I'm not using properly glTranslatef, but I can't find the error.

[Rosario Leonardi]

In the init you were modifing the model matrix, and then overwriting it in the render.

void init()
{
   glViewport(0, 0, width, height);
   glMatrixMode(GL_PROJECTION); // Add this line
   glLoadIdentity(); 
   glOrtho(0, width, height, 0, 0, 1);
}

Tips: Don't use fixed pipeline anymore is deprecated. Don't use Glut is outdated.

[ramy1989]

Ok but it seems like I'm missing something of how matrixes work, if I add a roratef before the glTranslatef:

glRotatef(0.01, 1.0, 0.0, 0.0);

Again nothing is displayed, what am I missing (I added the glMatrixMode(GL_PROJECTION); line).

[Rosario Leonardi]

However this is a lot trickier.

Now you are doing
This is your rotation matrix A
1 0 0 0
0 c -s 0
0 s c 0
0 0 0 1
where c = cos(0.01) Degree
where s = sin(0.01) Degree
cause sin of 0.01 is really low this is almost an identity

This is your translation matrix B
1 0 0 100
0 1 0 0
0 0 1 0
0 0 0 1

Your transformation is vEye = A* (B * v)

1 0 0 100
0 c -s 0
0 s c 0
0 0 0 1

the first point should be in 200 100*c 100*-s 1

The Z in negative, but your glOrtho specify a near plane in 0. So all de vertexes are culled away.

 
    glOrtho(0, width, height, 0, -1, 1);

should do the trick.

Mmm... btw, why are you doing random transformation? Is not better to read a book or a tutorial to learn?