frustum.cpp

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/*
**  Command & Conquer Generals Zero Hour(tm)
**  Copyright 2025 Electronic Arts Inc.
**
**  This program is free software: you can redistribute it and/or modify
**  it under the terms of the GNU General Public License as published by
**  the Free Software Foundation, either version 3 of the License, or
**  (at your option) any later version.
**
**  This program is distributed in the hope that it will be useful,
**  but WITHOUT ANY WARRANTY; without even the implied warranty of
**  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
**  GNU General Public License for more details.
**
**  You should have received a copy of the GNU General Public License
**  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/
 
/***********************************************************************************************
 ***              C O N F I D E N T I A L  ---  W E S T W O O D  S T U D I O S               ***
 ***********************************************************************************************
 *                                                                                             *
 *                 Project Name : WWMath                                                       *
 *                                                                                             *
 *                     $Archive:: /Commando/Code/wwmath/frustum.cpp                           $*
 *                                                                                             *
 *                       Author:: Greg Hjelstrom                                               *
 *                                                                                             *
 *                     $Modtime:: 4/01/01 12:47p                                              $*
 *                                                                                             *
 *                    $Revision:: 5                                                           $*
 *                                                                                             *
 *---------------------------------------------------------------------------------------------*
 * Functions:                                                                                  *
 *   FrustumClass::Init -- Initialize a frustum object                                         *
 * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
#include "frustum.h"
 
 
/***********************************************************************************************
 * FrustumClass::Init -- Initialize a frustum object                                           *
 *                                                                                             *
 *    This function initializes a frustum from the description of a camera                     *
 *                                                                                             *
 * INPUT:                                                                                      *
 * camera - camera transform, note that the camera looks down the -Z axis                      *
 * vpmin - min corner of the z=-1.0 view plane (not necessarily the near clip plane)           *
 * vpmax - max corner of the z=-1.0 view plane (not necessarily the near clip plane)           *
 * znear - near clip plane (should be negative, negated if it is not)                          *
 * zfar - far clip plane (should be negative, negated if it is not)                            *
 *                                                                                             *
 * OUTPUT:                                                                                     *
 *                                                                                             *
 * WARNINGS:                                                                                   *
 * The vpmin and vpmax variables are the min and max of a view-plane at z=-1.0                 *
 *                                                                                             *
 *                                                                                             *
 * HISTORY:                                                                                    *
 *   2/17/2000  gth : Created.                                                                 *
 *=============================================================================================*/
void FrustumClass::Init
( 
    const Matrix3D &        camera,
    const Vector2 &      vpmin,
    const Vector2 &      vpmax,
    float                       znear,
    float                       zfar
)
{
    int i;
    
    // Store the camera transform
    CameraTransform = camera;
 
   // Forward is negative Z in our viewspace coordinate system.
    // Just flip the sign if the user passed in positive values.
   if ((znear > 0.0f) && (zfar > 0.0f)) {
        znear = -znear;
        zfar = -zfar;
    }
 
    // Calculate the corners of the camera frustum.
    // Generate the camera-space frustum corners by linearly
   // extrapolating the viewplane to the near and far z clipping planes.
 
    // The camera frustum corners are defined in the following order:
    // When looking at the frustum from the position of the camera, the near four corners are
    // numbered: upper left 0, upper right 1, lower left 2, lower right 3. The far plane's
    // Frustum corners are numbered from 4 to 7 in an analogous fashion.
  // (remember: the camera space has x going to the right, y up and z backwards).
 
    //calculate a proper z-vector assuming our right-handed coordinate system
    Vector3  zv;
    Vector3::Cross_Product(CameraTransform.Get_X_Vector(),CameraTransform.Get_Y_Vector(),&zv);
 
    //compare correct z-vector with the one in the camera matrix.  If they point in
    //opposite directions, we have a reflected camera matrix.
    if (Vector3::Dot_Product(CameraTransform.Get_Z_Vector(),zv) < 0)
    {   //flip the frustum corners horizontally for a reflected matrix
      Corners[1].Set(vpmin.X, vpmax.Y, 1.0);
      Corners[5] = Corners[1];
        Corners[1] *= znear;
        Corners[5] *= zfar;
        Corners[0].Set(vpmax.X, vpmax.Y, 1.0);
        Corners[4] = Corners[0];
        Corners[0] *= znear;
        Corners[4] *= zfar;
        Corners[3].Set(vpmin.X, vpmin.Y, 1.0);
        Corners[7] = Corners[3];
        Corners[3] *= znear;
        Corners[7] *= zfar;
        Corners[2].Set(vpmax.X, vpmin.Y, 1.0);
        Corners[6] = Corners[2];
        Corners[2] *= znear;
        Corners[6] *= zfar; 
    }
    else
    {   //normal camera
        Corners[0].Set(vpmin.X, vpmax.Y, 1.0);
        Corners[4] = Corners[0];
        Corners[0] *= znear;
        Corners[4] *= zfar;
        Corners[1].Set(vpmax.X, vpmax.Y, 1.0);
        Corners[5] = Corners[1];
        Corners[1] *= znear;
        Corners[5] *= zfar;
        Corners[2].Set(vpmin.X, vpmin.Y, 1.0);
        Corners[6] = Corners[2];
        Corners[2] *= znear;
        Corners[6] *= zfar;
        Corners[3].Set(vpmax.X, vpmin.Y, 1.0);
        Corners[7] = Corners[3];
        Corners[3] *= znear;
        Corners[7] *= zfar; 
    }
 
 
    // Transform the eight corners of the view frustum from camera space to world space.
    for (i = 0; i < 8; i++) {
        Matrix3D::Transform_Vector(CameraTransform, Corners[i], &(Corners[i]));
    }
 
    // Create the six frustum bounding planes from the eight corner Corners.
    // The bounding planes are oriented so that their normals point outward
    PlaneClass frustum_planes[6];
    Planes[0].Set(Corners[0], Corners[3], Corners[1]);   // near
    Planes[1].Set(Corners[0], Corners[5], Corners[4]);   // bottom
    Planes[2].Set(Corners[0], Corners[6], Corners[2]); // right
    Planes[3].Set(Corners[2], Corners[7], Corners[3]); // top
    Planes[4].Set(Corners[1], Corners[7], Corners[5]); // left
    Planes[5].Set(Corners[4], Corners[7], Corners[6]); // far
 
    // find the bounding box of the entire frustum (may be used for sloppy quick rejection)
    BoundMin = BoundMax = Corners[0];
 
    for (i=1; i<8;i++) {
        if (Corners[i].X < BoundMin.X) BoundMin.X = Corners[i].X;
        if (Corners[i].X > BoundMax.X) BoundMax.X = Corners[i].X;
 
        if (Corners[i].Y < BoundMin.Y) BoundMin.Y = Corners[i].Y;
        if (Corners[i].Y > BoundMax.Y) BoundMax.Y = Corners[i].Y;
 
        if (Corners[i].Z < BoundMin.Z) BoundMin.Z = Corners[i].Z;
        if (Corners[i].Z > BoundMax.Z) BoundMax.Z = Corners[i].Z;
    }
}