[Plib-users] R: How to clip an area

[Paolo L. <p.l...@ci...>]

Rather than reasoning in terms of clipping, it seems to me that you have =
to
setup an off-axis projection
(http://www.opengl.org/resources/faq/technical/transformations.htm#tran00=
80)
.

       135=B0
__________________
\45=B0 \  45=B0 /45=B0 /
  \   \    /   /
    \  \  /  /
      \ \/ /
        \/

The left- and right-most are off-axis projections (to be set up through
OpenGL's glFrustum or Plib's ssgSetFrustum), the center one is on-axis =
(i.e.
the classic one set up through OpenGL's gluProjection or Plib's =
ssgSetFOV).

So the sequence of operation would be the following:

glViewport( 0, 0, w/3, h );
ssgSetFrustum( <leftmost frustum params> );
ssgCullAndDraw(...);

glViewport( w/3, 0, w/3, h );
ssgSetFrustum( <center frustum params> );
ssgCullAndDraw(...);

glViewport( 2*w/3, 0, w/3, h );
ssgSetFrustum( <rightmost frustum params> );
ssgCullAndDraw(...);

In order to calculate proper arguments to ssgSetFrustum( left, right,
bottom, top, zNear, zFar) as the given FoV partitioning you could search =
for
keywords glfrustum and off-axis projection, or tiled projection.

If you prefer, just use to code below. You nave to find how to split you
135=B0 FoV screen into 3x45=B0 screens (i.e. the four corners of each =
projection
surface) in terms of (ul,ur,ll,lr) corner positions - and set eyepos as =
to
comprise the 135=B0 screen. It's not important where the coordinate =
system
origin is wrt screen corner and eyepos, nor the units, but, for =
simplicity,
you could just put the coordinate system origin at the center of the =
center
screen and find (ul,ur,ll,lr) and eyepos accordingly, or put it at =
eyepos
(eyepos (0,0,0)) and find (ul,ur,ll,lr) accordingly:

ul ur
+---+------+---+
|   |      |   |
+...+......+...+
|\  |\    /|  /|
+--\+-\--/-+/--+
ll lr\ \/ /
       \/
      eyepos

enum {SC_UL, SC_UR, SC_LL, SC_LR};  // enum for specifying screen corner
                                    // (Ux: upper, Lx: lower, xL: left, =
xR:
right)

void calcOffAxisView(
	// input args
	sgVec3 screen[4],  // the four screen corner positions (x,y,z),
                         // fill entries SC_UL, SC_UR, SC_LL, and SC_LR
	sgVec3 eyepos,     // eye position (x,y,z)
                         // screen and eyepos have to be in the same
coordinate system
                         // (x+ right, y+ up, z+ out of the screen)
	float near_clip, float far_clip,  // near and far clipping planes
	// output args
	float *left, float *right, float *bottom, float *top  // first six
args to ssgSetFrustum,
                                                            // the last =
two
are near_clip and far_clip
)
{
	sgVec3 eyeToScreen, xProj, yProj, zProj;
	float width, height, dist;
	sgVec3 eyePos;

	sgCopyVec3( eyePos, eyepos );
	// Calculate vector from eye to screen origin (lower left corner)
	sgSubVec3(eyeToScreen, eyePos, screen[SC_LL]);
	// Calculate vectors from bottom left corner of screen that form
	// the screen coordinate frame
	sgSubVec3(xProj, screen[SC_LR], screen[SC_LL]);
	width =3D sgLengthVec3(xProj);
	sgScaleVec3(xProj, SG_ONE / width);
	sgSubVec3(yProj, screen[SC_UL], screen[SC_LL]);
	height =3D sgLengthVec3(yProj);
	sgScaleVec3(yProj, SG_ONE / height);
	sgVectorProductVec3(zProj, xProj, yProj);

	// Now, specify an off-axis viewing frustum in order to define the
	// viewing volume with respect to the user's eye position.
	*left =3D sgScalarProductVec3(eyeToScreen, xProj);
	*right =3D width - *left;
	bottom =3D sgScalarProductVec3(eyeToScreen, yProj);
	*top =3D height - *bottom;
	dist =3D sgScalarProductVec3(eyeToScreen, zProj);
	float nc_over_d =3D near_clip / dist;
	*left   *=3D -nc_over_d;
	*right  *=3D  nc_over_d;
	*bottom *=3D -nc_over_d;
	*top    *=3D  nc_over_d;
}

Note that the code above is only valid for the tiling case (i.e. there's =
no
screen rotation) - for arbitrary positioned screens there's an =
additional
matrix multiplication to take into account either in the projection or =
in
the modelview matrix (in this case, its inverse).

Greetings -

Paolo Leoncini


________________________________

	Da: pli...@li...
[mailto:pli...@li...] Per conto di swetha =
korada
	Inviato: luned=EC 15 ottobre 2007 10.35
	A: pli...@li...
	Oggetto: [Plib-users] How to clip an area
=09
=09
	Hi all,
	      =20
	   I am using plib-1.8.2 version.In that, I have set the horizonital
Field Of View(FOV) as 135 degrees.In that I have to render only first 45
degrees area.So I need to clip next 90 degrees.How to achieve this?
	=20
	Hope any one will give reply as soon as possible.
	=20
	Thanks And Regards,
	swetha.
	   =20