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CelticDaddio
05-10-2006, 09:23 PM
I know I am doing something really silly, but I don't see the problem...

I am trying to draw a quad which covers the entire viewport, at some distance tilted at some specified angle to the LOS. For example, a quad at z=-10000, tilted back 30 deg. All is well, until I tilt the quad. I think instead of rotating about teh quads center, I must be rotating about the origin and thus swinging the quad outof the fov. Can someone point out my error? Thanks,
CD


void RenderScene()
{
// Clear the screen and the depth buffer
glClearColor( 0.0f, 0.0f, 1.0f, 1.0f );
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );


// save current matrix mode
GLint matrixMode;
glGetIntegerv(GL_MATRIX_MODE, &matrixMode);


// Switch to Project Matrix Mode
glMatrixMode(GL_PROJECTION);
// save current projection matrix
glPushMatrix();
// reset projection matrix
glLoadIdentity();


// set orthographic projection
glOrtho(-1.0, 1.0, -1.0, 1.0, g_Hither, g_Yon);


// switch to model-view matrix
glMatrixMode(GL_MODELVIEW);
// save current model-view matrix
glPushMatrix();
// reset model-view matrix
glLoadIdentity();


// Draw tilted quad
glLoadIdentity();
glTranslatef(0.0, 0.0, -10000.0);
glRotatef(30.0, 1.0, 0.0, 0.0);
glPushAttrib(GL_COLOR_BUFFER_BIT | GL_LINE_BIT);
glColor3f(1.0, 0.0, 1.0);
glBegin(GL_QUADS);
glVertex3f(-1.0, -1.0, -10000.0);
glVertex3f( 1.0, -1.0, -10000.0);
glVertex3f( 1.0, 1.0, -10000.0);
glVertex3f(-1.0, 1.0, -10000.0);
glEnd();
glPopAttrib();

// restore model-view matrix
glPopMatrix();

// switch to projection mode
glMatrixMode(GL_PROJECTION);
// restore projection matrix
glPopMatrix();

// restore original matrix mode
glMatrixMode(matrixMode);


glFlush();
glutSwapBuffers();
}

Relic
05-11-2006, 01:18 AM
Find my comments in the code:

void RenderScene()
{
// Clear the screen and the depth buffer
glClearColor( 0.0f, 0.0f, 1.0f, 1.0f );
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

// save current matrix mode
GLint matrixMode;
glGetIntegerv(GL_MATRIX_MODE, &matrixMode);

// Switch to Project Matrix Mode
glMatrixMode(GL_PROJECTION);
// save current projection matrix
glPushMatrix();
// reset projection matrix
glLoadIdentity();
// set orthographic projection
glOrtho(-1.0, 1.0, -1.0, 1.0, g_Hither, g_Yon);
// If the rotated quad ends up at z values outside your near and far setting it gets clipped.
// That is, if g_Yon is at 10000 then the below rotation will clip it definitely.
// (Don't get confused by the sign, model space is right-handed, zFar is in the negative direction.
// Screen space is left-handed and zFar needs to be specified bigger than zNear there.)

// switch to model-view matrix
glMatrixMode(GL_MODELVIEW);
// save current model-view matrix
glPushMatrix();
// reset model-view matrix
glLoadIdentity();

// Draw tilted quad
// glLoadIdentity(); // redundant

// Matrices are left-multiplied in OpenGL, the one nearest to the glVertex call is done first.
// You have to read these from the vertex call upwards to understand OpenGL's transformations.
glTranslatef(0.0, 0.0, -10000.0); // Move object back to it's original position.
glRotatef(30.0, 1.0, 0.0, 0.0); // This rotates around the origin but your object was far away before.
// If the object should end up rotated around its local x-axis and positioned at -10000 you need to add this
glTranslatef(0.0, 0.0, 10000.0); // Move the object into the origin.

glPushAttrib(GL_COLOR_BUFFER_BIT | GL_LINE_BIT); // You probably only wanted GL_CURRENT_BIT to save the color?
glColor3f(1.0, 0.0, 1.0);
glBegin(GL_QUADS);
glVertex3f(-1.0, -1.0, -10000.0);
glVertex3f( 1.0, -1.0, -10000.0);
glVertex3f( 1.0, 1.0, -10000.0);
glVertex3f(-1.0, 1.0, -10000.0);
glEnd();
glPopAttrib();

// restore model-view matrix
glPopMatrix();

// switch to projection mode
glMatrixMode(GL_PROJECTION);
// restore projection matrix
glPopMatrix();

// restore original matrix mode
glMatrixMode(matrixMode);

// glFlush(); // Not needed for double buffered OpenGL rendering before swap buffers.
glutSwapBuffers();
}

CelticDaddio
05-11-2006, 07:26 AM
Thanks,
That straightened me out....

CD