Question

I have a simple OpenGL application, which displays a line. I store x1, y1, x2 and y2 in global variables.

The rotation function uses the feedback functionality, discussed here and the translation to origin method, suggested here. It’s intended to rotate a line around it’s center.

Here’s the code:

void rotate( float theta )
{

    GLfloat* buff = new GLfloat[5];
    glFeedbackBuffer( 5, GL_2D, buff );
    glRenderMode( GL_FEEDBACK );

    int center_x = x1 + ( x2 - x1 )/2;
    int center_y = y1 + ( y2 - y1 )/2;

    glPushMatrix();
        glTranslatef( -center_x, -center_y, 0 );
        glRotatef( theta, 0, 0, 1 );
        glTranslatef( center_x, center_y, 0 );
        line();
    glPopMatrix();

    x1 = (int)buff[1];
    y1 = (int)buff[2];
    x2 = (int)buff[3];
    y2 = (int)buff[4];

    delete[] buff;
    glutPostRedisplay();
}

However, the two translations seem to have little effect – the line still rotates around the lower-left corner of the window. In addition, the line gets clipped if it doesn’t fit on the visible surface ( this doesn’t happen with the translations commented out – the line simply refuses to move if it would fall outside the visible surface ).

Why doesn’t this code work ? How do I rotate a line around it’s center ( or any arbitrary point ) in 2D ?

Answer 1

Looks to me like you’re translating in the wrong direction and the origin is being placed outside your viewport in the lower-left. You want to move the origin to the point you want to translate around–so if the origin is in the lower-left, you need to translate +center_x, +center_y. Rotate, translate back (-center_x, -center_y), then draw.