Add second of several ray cast/rendering files

5 files changed, 651 insertions, 11 deletions

diff --git a/apps/graphics/traveler/Makefile b/apps/graphics/traveler/Makefile
index c9c4c096e..6e1c49d58 100644
--- a/apps/graphics/traveler/Makefile
+++ b/apps/graphics/traveler/Makefile
@@ -47,7 +47,7 @@ STACKSIZE = 2048
 
 ASRCS   =
 CSRCS   = trv_color.c trv_doors.c trv_graphics.c trv_input.c trv_mem.c trv_pov.c
-CSRCS  += trv_rayrend.c trv_trigtbl.c
+CSRCS  += trv_rayavoid.c trv_rayrend.c trv_trigtbl.c
 MAINSRC = trv_main.c
 
 ifeq ($(CONFIG_NX),y)
diff --git a/apps/graphics/traveler/include/trv_plane.h b/apps/graphics/traveler/include/trv_plane.h
index 60da752d7..b6f718bf9 100644
--- a/apps/graphics/traveler/include/trv_plane.h
+++ b/apps/graphics/traveler/include/trv_plane.h
@@ -119,6 +119,22 @@ struct trv_planefile_header_s
 #define SIZEOF_TRVPLANEFILEHEADER_T 6
 
 /****************************************************************************
+ * Public Data
+ ****************************************************************************/
+
+/* The is the world!!! The world is described by lists of rectangles, one
+ * for each of the X, Y, and Z planes.
+ */
+
+extern struct trv_rect_head_s g_xplane;  /* list of X=plane rectangles */
+extern struct trv_rect_head_s g_yplane;  /* list of Y=plane rectangles */
+extern struct trv_rect_head_s g_zplane;  /* list of Z=plane rectangles */
+
+/* "Deallocated" planes are retained in a free list */
+
+extern struct trv_rect_list_s *g_rect_freelist;
+
+/****************************************************************************
  * Public Function Prototypes
  ****************************************************************************/
 
diff --git a/apps/graphics/traveler/include/trv_rayavoid.h b/apps/graphics/traveler/include/trv_rayavoid.h
index 7993cd111..08214cbc8 100644
--- a/apps/graphics/traveler/include/trv_rayavoid.h
+++ b/apps/graphics/traveler/include/trv_rayavoid.h
@@ -63,11 +63,11 @@ trv_coord_t trv_rayclip_player_ymotion(FAR struct trv_camera_s *pov,
                                        trv_coord_t height);
 trv_coord_t trv_ray_adjust_zpos(FAR struct trv_camera_s *pov,
                                 trv_coord_t height);
-FAR struct trv_rect_data_s *trv_test_xplane(FAR struct trv_camera_s *pov,
-                                            trv_coord_t dist, int16_t yaw,
-                                            trv_coord_t height);
-FAR struct trv_rect_data_s *trv_test_yplane(FAR struct trv_camera_s *pov,
-                                            trv_coord_t dist, int16_t yaw,
-                                            trv_coord_t height);
+FAR struct trv_rect_data_s *trv_ray_test_xplane(FAR struct trv_camera_s *pov,
+                                                trv_coord_t dist, int16_t yaw,
+                                                trv_coord_t height);
+FAR struct trv_rect_data_s *trv_ray_test_yplane(FAR struct trv_camera_s *pov,
+                                                trv_coord_t dist, int16_t yaw,
+                                                trv_coord_t height);
 
 #endif /* __APPS_GRAPHICS_TRAVELER_INCLUDE_TRV_RAYAVOID_H */
diff --git a/apps/graphics/traveler/src/trv_doors.c b/apps/graphics/traveler/src/trv_doors.c
index 7ae726ba7..75a11c538 100755
--- a/apps/graphics/traveler/src/trv_doors.c
+++ b/apps/graphics/traveler/src/trv_doors.c
@@ -124,8 +124,8 @@ static void trv_door_startopen (void)
       /* Test if there is a door within three steps in front of the player */
       /* Try the X planes first */
 
-      rect = trv_test_xplane(&g_trv_player, 3*STEP_DISTANCE,
-                             g_trv_player.yaw, g_player_height);
+      rect = trv_ray_test_xplane(&g_trv_player, 3*STEP_DISTANCE,
+                                 g_trv_player.yaw, g_player_height);
 
       /* If there is no X door in front of the player, then try the Y Planes
        * (it is assumed that there would not be doors this close in both
@@ -134,8 +134,8 @@ static void trv_door_startopen (void)
 
       if (!rect || !IS_DOOR(rect))
         {
-          rect = trv_test_yplane(&g_trv_player, 3*STEP_DISTANCE,
-                                 g_trv_player.yaw, g_player_height);
+          rect = trv_ray_test_yplane(&g_trv_player, 3*STEP_DISTANCE,
+                                     g_trv_player.yaw, g_player_height);
         }
 
       /* Check if we found a door in either the X or Y plane. */
diff --git a/apps/graphics/traveler/src/trv_rayavoid.c b/apps/graphics/traveler/src/trv_rayavoid.c
new file mode 100755
index 000000000..6fc04ddf2
--- /dev/null
+++ b/apps/graphics/traveler/src/trv_rayavoid.c
@@ -0,0 +1,624 @@
+/*******************************************************************************
+ * apps/graphics/traveler/src/trv_rayavoid.c
+ * This file contains the logic which determines if the desired player motion
+ * would cause a collision with various walls or if the motion would cause
+ * the player to change his vertical position in the world.  This collision
+ * avoidance logic is also used to determine if there is a door in front of
+ * the player.
+ *
+ *   Copyright (C) 2014 Gregory Nutt. All rights reserved.
+ *   Author: Gregory Nutt <gnutt@nuttx.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name NuttX nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/*****************************************************************************
+ * Included files
+ ****************************************************************************/
+
+#include "trv_types.h"
+#include "trv_trigtbl.h"
+#include "trv_plane.h"
+#include "trv_world.h"
+#include "trv_rayavoid.h"
+
+/*****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/* This is the closest that we will allow the player to approach a wall */
+
+#define MIN_APPROACH_DISTANCE (64/4) /* One quarter cell */
+
+/*****************************************************************************
+ * Private Data
+ ****************************************************************************/
+
+/* The rayClipPlayX/YMotion functions have the side effect of saving the
+ * pointer to the wall with which the player collided in the following
+ * private variable.  This gives a "back door" mechanism which is used
+ * to handle door processing.
+ */
+
+static struct trv_rect_data_s *g_clip_rect;
+
+/*****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/*****************************************************************************
+ * Name:  trv_ray_test_xplane
+ *
+ * Description:
+ *
+ *   This function tests if there is an X plane within the supplied distance
+ *   from the player at the supplied angle.  This function is used to
+ *   determine in there is a door in front of the player.  NOTE:  This
+ *   function exploits the normal collision detection logic in
+ *   trv_rayclip_player_xmotion and depends on the side-effect setting of
+ *   g_clip_rect.
+ *
+ *****************************************************************************/
+
+FAR struct trv_rect_data_s *trv_ray_test_xplane(FAR struct trv_camera_s *pov,
+                                              trv_coord_t dist, int16_t yaw,
+                                              trv_coord_t height)
+{
+  (void) trv_rayclip_player_xmotion(pov, dist, yaw, height);
+  return g_clip_rect;
+}
+
+/*****************************************************************************
+ * Name:  trv_rayclip_player_xmotion
+ *
+ * Description:
+ *   This function calculates the acceptable change in the player's position
+ *   along the X component of the specified yaw angle which would not cause
+ *   a collision with an X plane.  This logic is essentially a modified X
+ *   ray cast.
+ *
+ *****************************************************************************/
+
+trv_coord_t trv_rayclip_player_xmotion(FAR struct trv_camera_s *pov,
+                                     trv_coord_t dist, int16_t yaw,
+                                     trv_coord_t height)
+{
+  FAR struct trv_rect_list_s *list; /* Points to the current X plane rectangle */
+  FAR struct trv_rect_data_s *rect; /* Points to the rectangle data */
+  trv_coord_t reqdeltax;
+  trv_coord_t standoff;
+  trv_coord_t footlevel;
+  trv_coord_t relx;                 /* Relative position of the X plane */
+  trv_coord_t absy;                 /* Absolute Y position at relx given yaw */
+  trv_coord_t lastrelx = -1;        /* Last relative X position processed */
+
+  /* Decompose the desired move distance into its X component */
+
+  reqdeltax = tTOs(g_cos_table[ yaw ] * dist);
+
+  /* Assume that no clipping will be performed */
+
+  g_clip_rect = NULL;
+
+  /* Perform X raycasting based on the quadrant of the yaw angle */
+  /* The first and fourth quadrants correspond to the positive X
+   * direction (excluding angles 90 and 270).
+   */
+
+  if (yaw < ANGLE_90 || yaw > ANGLE_270)
+    {
+      /* Calculate the requested distance along the (positive) X axis (adding
+       * a little to how close the play can come to a wall
+       */
+
+      standoff = reqdeltax + MIN_APPROACH_DISTANCE;
+
+      /* This is the position of the player's feet */
+
+      footlevel = pov->z - height;
+
+      /* Look at every rectangle lying in the X plane */
+
+      for (list = g_xplane.head; list; list = list->flink)
+        {
+          rect = &list->d;
+
+          /* Search for a rectangle which lies "beyond" the current camera
+           * position
+           */
+
+          if (rect->plane > pov->x)
+            {
+              /* Get the X distance to the plane.  This is an order list:  if
+               * the distance to the plane is larger then the requested step
+               * then the requested step is acceptable.
+               */
+
+              relx = rect->plane - pov->x;
+              if (relx >= standoff)
+                {
+                  return reqdeltax;
+                }
+
+              /* The distance to the plane is smaller that the requested step.
+               * It may be possible to collide with the plane.  Check if a
+               * collision due to the height of the player is possible first
+               * (its easier to compute)
+               */
+
+              if (footlevel >= rect->vstart && pov->z <= rect->vend)
+                {
+                  /* A collision is possible based on the players height.
+                   * Now, we'll have to check if a collision is possible
+                   * due to the player's Y position.  We can skip this
+                   * step if we are processing another rectangle at the
+                   * same relx distance.
+                   */
+
+                  if (relx != lastrelx)
+                    {
+                      int32_t deltay; /* Scale == "triple" */
+
+                      /* The tangent is equal to the rate of change of Y with
+                       * respect to the X-axis.  The tangent is stored at
+                       * double the the "normal" scaling -- so deltay is
+                       * "triple" precision
+                       */
+
+                      deltay   = TAN(yaw) * ((int32_t)relx);
+                      absy     = tTOs(deltay) + pov->y; /* back to "single" */
+                      lastrelx = relx;
+                    }
+
+                  /* Check if this Y position intersects the rectangle */
+
+                  if (absy >= rect->hstart && absy <= rect->hend)
+                    {
+                      /* It collides -- Check, maybe we can walk through
+                       * this wall
+                       */
+
+                      if (!IS_PASSABLE(rect))
+                        {
+                          /* Nope... return a clipped value for the new
+                           * player position (which includes the a stand-off
+                           * constant)
+                           * NOTE:  This returned value could be negative!
+                           */
+
+                          g_clip_rect = rect;
+                          return relx - MIN_APPROACH_DISTANCE;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+  /* The second and third quadrants correspond to the negative X
+   * direction (excluding angles 90 and 270).
+   */
+
+  else if (yaw > ANGLE_90 && yaw < ANGLE_270)
+    {
+      /* Calculate the requested distance along the (negative) X axis
+       * (adding a little to how close the play can come to a wall)
+       */
+
+      standoff = MIN_APPROACH_DISTANCE - reqdeltax;
+
+      /* This is the position of the player's feet */
+
+      footlevel = pov->z - height;
+
+      /* Look at every rectangle lying in the X plane */
+
+      for (list = g_xplane.tail; list; list = list->blink)
+        {
+          rect = &list->d;
+
+          /* Search for a rectangle which lies "before" the current camera
+           * position
+           */
+
+          if (rect->plane < pov->x)
+            {
+              /* Get the X distance to the plane.  This is an order list:
+               * if the distance to the plane is larger then the requested
+               * step then the requested step is acceptable.
+               */
+
+              relx = pov->x - rect->plane;
+              if (relx >= standoff)
+                {
+                  return reqdeltax;
+                }
+
+              /* The distance to the plane is smaller that the requested
+               * step.  It may be possible to collide with the plane.  Check
+               * if a collision due to the height of the player is possible
+               * first (its easier to compute)
+               */
+
+              if (footlevel >= rect->vstart && pov->z <= rect->vend)
+                {
+                  /* A collision is possible based on the players height.
+                   * Now, we'll have to check if a collision is possible due
+                   * to the player's Y position.  We can skip this step if
+                   * we are processing another rectangle at the same relx
+                   * distance.
+                   */
+
+                  if (relx != lastrelx)
+                    {
+                      int32_t deltay; /* Scale == "triple" */
+
+                      /* The negative tangent is equal to the rate of change
+                       * of Y with respect to the X-axis.The tangent is
+                       * stored at double the the "normal" scaling -- so
+                       * deltay is "triple" precision
+                       */
+
+                      deltay   = -TAN(yaw) * ((int32_t)relx);
+                      absy     = tTOs(deltay) + pov->y; /* back to "single" */
+                      lastrelx = relx;
+                    }
+
+                  /* Check if this Y position intersects the rectangle */
+
+                  if (absy >= rect->hstart && absy <= rect->hend)
+                    {
+                      /* It collides -- Check, maybe we can walk through
+                       * this wall?
+                       */
+
+                      if (!IS_PASSABLE(rect))
+                        {
+                          /* Nope -- return a clipped value for the new
+                           * player position (which includes the a stand-off
+                           * constant)
+                           * NOTE:  This returned value could be positive!
+                           */
+
+                          g_clip_rect = rect;
+                          return MIN_APPROACH_DISTANCE - relx;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+  /* If we got there, then no collisions were found.  Just return the
+   * requested step value
+   */
+
+  return reqdeltax;
+}
+
+/*****************************************************************************
+ * Name:  trv_ray_test_yplane
+ *
+ * Description:
+ *   This function tests if there is an Y plane within the supplied distance
+ *   from the player at the supplied angle.  This function is used to
+ *   determine in there is a door in front of the player.  NOTE:  This
+ *   function exploits the normal collision detection logic in
+ *   trv_rayclip_player_ymotion and depends on the side-effect setting of
+ *   g_clip_rect.
+ *
+ ****************************************************************************/
+
+FAR struct trv_rect_data_s *trv_ray_test_yplane(FAR struct trv_camera_s *pov,
+                                                trv_coord_t dist, int16_t yaw,
+                                                trv_coord_t height)
+{
+  (void)trv_rayclip_player_ymotion(pov, dist, yaw, height);
+  return g_clip_rect;
+}
+
+/*****************************************************************************
+ * Name:  trv_rayclip_player_ymotion
+ *
+ * Description:
+ *   This function calculates the acceptable change in the player's position
+ *   along the Y component of the specified yaw angle which would not cause
+ *   a collision with a Y plane.  This logic is essentially a modified X
+ *   ray cast.
+ *
+ ****************************************************************************/
+
+trv_coord_t trv_rayclip_player_ymotion(FAR struct trv_camera_s *pov,
+                                       trv_coord_t dist, int16_t yaw,
+                                       trv_coord_t height)
+{
+  FAR struct trv_rect_list_s *list; /* Points to the current Y plane rectangle */
+  FAR struct trv_rect_data_s *rect; /* Points to the rectangle data */
+  trv_coord_t reqdeltay;
+  trv_coord_t standoff;
+  trv_coord_t footlevel;
+  trv_coord_t rely;                 /* Relative position of the Y plane */
+  trv_coord_t absx;                 /* Absolute X position at rely given yaw */
+  trv_coord_t lastrely = -1;        /* Last relative Y position processed */
+
+  /* Decompose the desired move distance into its Y component */
+
+  reqdeltay = tTOs(g_sin_table[ yaw ] * dist);
+
+  /* Assume that no clipping will be performed */
+
+  g_clip_rect = NULL;
+
+  /* Perform Y raycasting based on the quadrant of the yaw angle */
+  /* The first and second quadrants correspond to the positive Y
+   * direction (excluding angles 0 and 180).
+   */
+
+   if (yaw > ANGLE_0 && yaw < ANGLE_180)
+     {
+       /* Calculate the requested distance along the (positive) X axis
+        * (adding a little to how close the play can come to a wall)
+        */
+
+        standoff = reqdeltay + MIN_APPROACH_DISTANCE;
+
+        /* This is the position of the player's feet */
+
+        footlevel = pov->z - height;
+
+        /* Look at every rectangle lying in a Y plane */
+
+        for (list = g_yplane.head; list; list = list->flink)
+          {
+            rect = &list->d;
+
+           /* Search for a rectangle which lies "beyond" the current camera
+            * position
+            */
+
+           if (rect->plane > pov->y)
+             {
+               /* Get the Y distance to the plane.  This is an order list:
+                * If the distance to the plane is larger then the requested
+                * step then the requested step is acceptable.
+                */
+
+              rely = rect->plane - pov->y;
+              if (rely >= standoff)
+                {
+                  return reqdeltay;
+                }
+
+              /* The distance to the plane is smaller that the requested
+               * step.  It may be possible to collide with the plane.
+               * Check if a collision due to the height of the player is
+               * possible first (its easier to compute)
+               */
+
+              if (footlevel >= rect->vstart && pov->z <= rect->vend)
+                {
+                  /* A collision is possible based on the players height.
+                   * Now, we'll have to check if a collision is possible
+                   * due to the player's X position.  We can skip this
+                   * step if we are processing another rectangle at the
+                   * same relx distance.
+                   */
+
+                  if (rely != lastrely)
+                    {
+                      int32_t deltax; /* Scale == "triple" */
+
+                      /* The inverted tangent is equal to the rate of change
+                       * of X with respect to the Y-axis.  The cotangent is
+                       * stored at double the the "normal" scaling -- so
+                       * deltax is "triple" precision
+                       */
+
+                      deltax   = g_cot_table(yaw) * ((int32_t)rely);
+                      absx     = tTOs(deltax) + pov->x; /* back to "single" */
+                      lastrely = rely;
+                    }
+
+                  /* Check if this X position intersects the rectangle */
+
+                  if (absx >= rect->hstart && absx <= rect->hend)
+                    {
+                      /* It collides -- Check, maybe we can walk through
+                       * this wall?
+                       */
+
+                      if (!IS_PASSABLE(rect))
+                        {
+                          /* Nope -- return a clipped value for the new
+                           * player position (which includes the a stand-off
+                           * constant)
+                           * NOTE:  This returned value could be negative!
+                           */
+
+                          g_clip_rect = rect;
+                          return rely - MIN_APPROACH_DISTANCE;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+  /* The third and fourth quadrants correspond to the negative Y
+   * direction (excluding angles 0 and 180).
+   */
+
+  else if (yaw > ANGLE_180)
+    {
+      /* Calculate the requested distance along the (negative) X axis
+       * (adding a little to how close the play can come to a wall)
+       */
+
+      standoff = MIN_APPROACH_DISTANCE - reqdeltay;
+
+      /* This is the position of the player's feet */
+
+      footlevel = pov->z - height;
+
+      /* Look at every rectangle lying in the X plane */
+
+      for (list = g_yplane.tail; list; list = list->blink)
+        {
+          rect = &list->d;
+
+          /* Search for a rectangle which lies "before" the current camera
+           * position
+           */
+
+          if (rect->plane < pov->y)
+            {
+              /* Get the X distance to the plane.  This is an order list:  if
+               * the distance to the plane is larger then the requested step
+               * then the requested step is acceptable.
+               */
+
+              rely = pov->y - rect->plane;
+              if (rely >= standoff)
+                {
+                  return reqdeltay;
+                }
+
+              /* The distance to the plane is smaller that the requested
+               * step.  It may be possible to collide with the plane.  Check
+               * if a collision due to the height of the player is possible
+               * first (its easier to compute)
+               */
+
+              if (footlevel >= rect->vstart && pov->z <= rect->vend)
+                {
+                  /* A collision is possible based on the players height.
+                   * Now, we'll have to check if a collision is possible due
+                   * to the player's X position.  We can skip this step if
+                   * we are processing another rectangle at the same relx
+                   * distance.
+                   */
+
+                  if (rely != lastrely)
+                    {
+                      int32_t deltax; /* Scale == "triple" */
+
+                      /* The negative inverted tangent is equal to the rate
+                       * of change of X with respect to the Y-axis.  The
+                       * cotangent is stored at double the the "normal"
+                       * scaling -- so deltax is "triple" precision
+                       */
+
+                      deltax   = -g_cot_table(yaw - ANGLE_180) * ((int32_t)rely);
+                      absx     = tTOs(deltax) + pov->x; /* back to "single" */
+                      lastrely = rely;
+                    }
+
+                  /* Check if this X position intersects the rectangle */
+
+                  if (absx >= rect->hstart && absx <= rect->hend)
+                    {
+                      /* It collides -- Check, maybe we can walk through
+                       * this wall
+                       */
+
+                      if (!IS_PASSABLE(rect))
+                        {
+                          /* It collides -- return a clipped value for the
+                           * new player position (which includes the a
+                           * stand-off constant)
+                           * NOTE:  This returned value could be positive!
+                           */
+
+                          g_clip_rect = rect;
+                          return MIN_APPROACH_DISTANCE - rely;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+  /* Return the clipped value */
+
+  return reqdeltay;
+}
+
+/*****************************************************************************
+ * Name:  trv_ray_adjust_zpos
+ *
+ * Description:
+ *   Make sure that the player is standing on something!
+ *
+ ****************************************************************************/
+ 
+trv_coord_t trv_ray_adjust_zpos(FAR struct trv_camera_s *pov,
+                                trv_coord_t height)
+{
+  struct trv_rect_list_s *list; /* Points to the current Z plane rectangle */
+  struct trv_rect_data_s *rect; /* Points to the rectangle data */
+
+  /* We will place the player's feet at the largest Z plane
+   * which is lower the the player's eye level.  We traverse
+   * the the g_zplane list in order of increase Z values
+   */
+
+  for (list = g_zplane.head; list; list = list->flink)
+    {
+      rect = &list->d;
+
+      /* The Z plane list is an ordered list.  If the next
+        * plane is over the player's head, then the player
+        * must be standing at "ground zero."
+        */
+
+      if (rect->plane >= pov->z)
+        {
+          return height - pov->z;
+        }
+
+      /* Check if this plane if under the player */
+
+      if (pov->x >= rect->hstart &&  pov->x <= rect->hend &&
+          pov->y >= rect->vstart &&  pov->y <= rect->vend)
+        {
+          /* We have the the smallest Z plane under the player
+           * which is below the player's eye level (pov->z).
+           * Determine the approach delta Z value to return
+           */
+
+          return height + rect->plane - pov->z;
+        }
+    }
+
+  /* No plane was found under the player.  Set him at his
+   * height above the "bottom" of the world
+   */
+
+  return height - pov->z;
+}