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POOpenGL rotation in 2D
 

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    <p>I have a simple OpenGL application, which displays a line. I store <code>x1, y1, x2</code> and <code>y2</code> in global variables. </p> <p>The rotation function uses the <code>feedback</code> functionality, discussed <a href="https://stackoverflow.com/questions/3277519/opengl-retrieve-raw-vertex-and-index-buffer-from-displayed-objects-in-opengl">here</a> and the translation to origin method, suggested <a href="https://stackoverflow.com/questions/3442743/opengl-es-rotating-3d-model-around-itself">here</a>. It's intended to rotate a line around it's center.</p> <p>Here's the code: </p> <pre><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(); } </code></pre> <p>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 ).</p> <p>Why doesn't this code work ? How do I rotate a line around it's center ( or any arbitrary point ) in 2D ?</p>
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