Visible.cpp

/*-----------------------------------------------------------------------------

  Visible.cpp

  2009 Shamus Young

-------------------------------------------------------------------------------

  This module runs the visibility grid, a 2-dimensional array that aids in
  culling objects during rendering. 

  There are many ways this could be refined or sped up, although tests indicate
  it's not a huge drain on performance.

-----------------------------------------------------------------------------*/


#include <windows.h>
#include <math.h>

#include "glTypes.h"
#include "camera.h"
#include "macro.h"
#include "math.h"
#include "visible.h"
#include "world.h"
#include "win.h"

static bool          vis_grid[GRID_SIZE][GRID_SIZE];

/*-----------------------------------------------------------------------------

-----------------------------------------------------------------------------*/

bool Visible (GLvector pos)
{

  return vis_grid[WORLD_TO_GRID(pos.x)][WORLD_TO_GRID(pos.z)];

}


/*-----------------------------------------------------------------------------

-----------------------------------------------------------------------------*/

bool Visible (int x, int z)
{

  return vis_grid[x][z];

}



/*-----------------------------------------------------------------------------

-----------------------------------------------------------------------------*/

void VisibleUpdate (void)
{

  GLvector  angle;
  GLvector  position;
  int       x, y, grid_x, grid_z;
  int       left, right, front, back;
  float     angle_to;
  float     angle_diff;
  float     target_x, target_z;

  //Clear the visibility table
  ZeroMemory (vis_grid, sizeof (vis_grid));
  //Calculate which cell the camera is in
  angle = CameraAngle ();
  position = CameraPosition ();
  grid_x = WORLD_TO_GRID(position.x);
  grid_z = WORLD_TO_GRID(position.z);
  //Cells directly adjactent to the camera might technically fall out of the fov,
  //but still have a few objects poking into screenspace when looking up or down.
  //Rather than obsess over sorting those objects properly, it's more efficient to
  //just mark them visible.
  left = right = front = back = 3;
  //Looking north, can't see south.
  if (angle.y < 45.0f || angle.y > 315.0f)
    front = 0;
  //Looking south, can't see north
  if (angle.y > 135.0f && angle.y < 225.0f)
    back = 0;
  //Looking east, can't see west
  if (angle.y > 45.0f && angle.y < 135.0f)
    left = 0;
  //Looking west, can't see east
  if (angle.y > 225.0f && angle.y < 315.0f)
    right = 0;
  //Now mark the block around us the might be visible
  for (x = grid_x - left; x <= grid_x + right; x++) {
    if (x < 0 || x >= GRID_SIZE) //just in case the camera leaves the world map
      continue;
    for (y = grid_z - back; y <= grid_z + front; y++) {
      if (y < 0 || y >= GRID_SIZE) //just in case the camera leaves the world map
        continue;
      vis_grid[x][y] = true;
    }
  }
  //Doesn't matter where we are facing, objects in current cell are always visible
  vis_grid[grid_x][grid_z] = true;
  //Here, we look at the angle from the current camera position to the cell 
  //on the grid, and how much that angle deviates from the current view angle. 
  for (x = 0; x < GRID_SIZE; x++) {
    for (y = 0; y < GRID_SIZE; y++) {
      //if we marked it visible earlier, skip all this math
      if (vis_grid[x][y])
        continue;
      //if the camera is to the left of this cell, use the left edge
      if (grid_x < x) 
        target_x = (float)x * GRID_RESOLUTION;
      else 
        target_x = (float)(x + 1) * GRID_RESOLUTION;
      if (grid_z < y) 
        target_z = (float)y * GRID_RESOLUTION;
      else
        target_z = (float)(y + 1) * GRID_RESOLUTION;
      angle_to = 180 - MathAngle (target_x, target_z, position.x, position.z);
      //Store how many degrees the cell is to the 
      angle_diff = (float)fabs (MathAngleDifference (angle.y, angle_to));
      vis_grid[x][y] = angle_diff < 45;
    }
  }


}