# translation problem need help

Hello

I have a problem when drawing a quad.

But to draw the quad at top left corner of the window I have to do a translation like glTranslatef(-0.55, 0, -1) I do not know why this value 0.55. I calculated this by running the application multiple times and adjusting.

My code

``````gluPerspective(45.0f, 640/480, 0.1f, 5000.0f);
glMatrixMode(GL_MODELVIEW);

glTranslatef(-0.55, 0, -1); // problem here what is this 0.55?

glPushMatrix();
glVertex3f (0.0, 0.0, 0.0);
glVertex3f (0.5, 0.0, 0.0);
glVertex3f (0.5, 0.5, 0.0);
glVertex3f (0.0, 0.5, 0.0);
glEnd();
...
``````

Actually what you need is the world coordinate of the upper left corner of the window.

p = UnprojectPoint(0, windowHeight - 1, 0);

p is the upper left coordinate in world space.

glVertex3f(p.x, p.y, p.z);
glVertex3f(p.x, p.y + 1, p.z);
glVertex3f(p.x + 1, p.y + 1, p.z);
glVertex3f(p.x + 1, p.y, p.z);
glEnd();

Assuming a CCW backface culling mode.

Another way to do it is using the upper left coordinate of the near clipping plane…

There are different ways as well depending what you exactly need to achieve.

The reason why your translation seems so odd is because you are using a perspective projection. By definition this means that the further away from the origin of the space your quad is, the smaller it will look.

If you simply want draw a quad taking the full space of your screen, you’ll be much better off using an ortographic projection or simply no projection at all which by definition will make your clip space go from -1 to 1 in each direction.

Look for information on ortographic projections, perspective and clip space.

Both of you thank you very much for answering.

By using UnprojectPoint some how I managed to draw the quad on the screen.
My code for UnprojectPoint

``````Vector3f getOGLPosition(int x, int y)
{
GLint viewport[4];
GLdouble modelview[16];
GLdouble projection[16];
GLfloat winX, winY, winZ;
GLdouble posX, posY, posZ;

glGetDoublev( GL_MODELVIEW_MATRIX, modelview );
glGetDoublev( GL_PROJECTION_MATRIX, projection );
glGetIntegerv( GL_VIEWPORT, viewport );

winX = (float)x;
winY = (float)viewport[3] - (float)y;
glReadPixels( x, int(winY), 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ );

gluUnProject( winX, winY, winZ, modelview, projection, viewport, &posX, &posY, &posZ);

return Vector3f(posX, posY, posZ);
}
``````

And when drawing

``````Vector3f leftBottom = getOGLPosition(m_Left, m_Bottom);
Vector3f rightBottom = getOGLPosition(m_Right, m_Bottom);
Vector3f rightTop = getOGLPosition(m_Right, m_Top);
Vector3f leftTop = getOGLPosition(m_Left, m_Top);

glPushMatrix();
glDisable(GL_DEPTH_TEST);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, 2);

glTexCoord2f (0.0, 0.0);
glVertex3f (leftBottom.x, leftBottom.y, leftBottom.z +1);

glTexCoord2f (1.0, 0.0);
glVertex3f (rightBottom.x, rightBottom.y, rightBottom.z +1);

glTexCoord2f (1.0, 1.0);
glVertex3f (rightTop.x, rightTop.y, rightTop.z +1);

glTexCoord2f (0.0, 1.0);
glVertex3f (leftTop.x, leftTop.y, leftTop.z +1);
glEnd();

glDisable(GL_TEXTURE_2D);
glEnable(GL_DEPTH_TEST);
glPopMatrix();
``````

The problem I face now is texturing.
I have printed the values of those 4 corners of quad and they are,
(-543, 650, -5917)
(202, 658, -5622)
(-13 56, -68)
(108, -88, -5434)