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            天行健 君子當自強而不息

            Controlling Players and Characters(10)

             

            Controlling Non-Player Characters
             

            As you’ve been able to surmise from the past few sections, controlling the player is
            relatively simple. Now comes the tough part—controlling the game’s NPCs. This
            section shows you the various methods of navigating your game’s NPCs.
             

            Although games might trick you into thinking some elaborate scheme is moving
            the NPCs around the world, that just isn’t the case.

            Do you remember the five general types of NPC movements that I mentioned earlier—
            standing still, wandering around an area, walking along a route, following a character,
            and evading a character? With these in mind, you might want to take a closer look at
            your favorite role-playing games to find out which control schemes they use.

            As for your role-playing game, take a moment to examine the following controls
            and how to implement them.

             

            Standing Still
             

            There’s not much to think about here—just place a character and he stands still
            facing a specific direction. That direction is an angular rotation.

             

            Wandering an Area
             

            Games such as Ultima Online allow NPCs to wander around a set area, whether it is
            the entire level or a section that you define. To keep things simple, you can specify
            the range in which you want a character to wander, within a specific range of coordinates
            (as illustrated in Figure 16.3).

            These coordinates can be stored in variables such as these:

            float WanderMinX, WanderMinY, WanderMinZ;
            float WanderMaxX, WanderMaxY, WanderMaxZ;

            Now, assuming that you are tracking a character’s coordinates in the level in a trio of
            variables, you can move them around randomly and check whether a move is valid:

            float CharXPos, CharYPos, CharZPos; // Character coordinates
            float XMove, ZMove; // Movement amounts - skip YMove movements

            // Distance to move character
            float Distance;

            // Determine a random direction to move - loop until found
            while(1) {
              float Direction = 6.28f / 360.0f * (float)(rand() % 360);

              XMove = cos(Direction) * Distance;
              ZMove = sin(Direction) * Distance;

              // Check if move is valid - ignore height for now
              if(CharXPos+XMove >= WanderMinX && \
                 CharXPos+XMove <= WanderMaxX && \
                 CharZPos+ZMove >= WanderMinZ && \
                 CharZPos+ZMove <= WanderMaxZ) {
                   // Movement allowed, update coordinates
                   CharXPos+=XMove;
                   CharZPos+=ZMove;
                   break; // break out of loop
              }
            }

            CAUTION
            Don’t randomly move a character around at every frame, or you’ll find yourself with characters that look
            like they’re having a conniption fit. Instead, update a character’s direction only every few seconds or so.

             

            Walking a Route
             

            Although NPCs aren’t intelligent enough to know their way around the level, you
            can assign them routes to travel. These routes include coordinates that must be
            reached in order to proceed to the next coordinates. Once the last set of coordinates
            is reached, the character returns to the first set of coordinates and starts the
            path all over again.

             

            Using Route Points
             

            Route points are defined as a set of coordinates, and keeping with the 3-D concept that
            you’re accustomed to, you can use the following structure to store those coordinates:


            typedef struct sRoutePoint {
               float XPos, ZPos;
            } sRoutePoint;

            NOTE
            Note that there’s no need for a Y-coordinate when using a 3-D engine because the height is
            determined by the height of the ground below the character.

            In order to construct a route, you pick the points you want a character to walk and
            construct an array of sRoutePoint structures to store the coordinates. Figure 16.4, for
            example, shows a simple map, with five points marked.

            Because each point in the route is marked with coordinates, you can see how to
            construct the sRoutePoint structures array:

            sRoutePoint Route[5] = {
              { -200.0f, -100.0f },
              { 100.0f, -300.0f },
              { 300.0f, -200.0f },
              { 200.0f, 100.0f },
              { 0.0f, 400.0f }
            };

            long NumRoutePoints = 5; // To make it easier to know # points

             

            Walking from Point to Point
             

            In order to proceed from point to point, a character walking a route needs to compare
            its current coordinates to the point where it’s headed. You use this, combined
            with the character’s walking speed, to compute a pair of movement variables that
            update the character’s position.

            Start by assuming that the character’s coordinates are kept in the following variables
            (along with the character’s walking speed):


            float CharXPos, CharZPos; // No Y-coordinate needed
            float WalkSpeed; // Walking speed per frame
             

            At this point, assume that you’ve already retrieved the coordinates you want the
            character to walk to and placed them into another pair of variables:
             

            float RouteXPos, RouteZPos; // Again, no Y-coordinate

            Now, to start the character moving, you calculate the movement variables:

            // Calculate distance from character to route point
            float XDiff = (float)fabs(RouteXPos - CharXPos);
            float ZDiff = (float)fabs(RouteZPos - CharZPos);
            float Length = sqrt(XDiff*XDiff + ZDiff*ZDiff);

            // Calculate movement towards point
            float MoveX = (RouteXPos - CharXPos) / Length * WalkSpeed;
            float MoveZ = (RouteZPos - CharZPos) / Length * WalkSpeed;

            Whenever you update the character per frame from now on, you’ll need to add
            MoveX and MoveZ to the character’s coordinates, as in the following:

            CharXPos += MoveX;
            CharZPos += MoveZ;

            With that aside, go back and see just how to track which route point a character is
            walking toward. When one route point is reached, the character must walk toward
            the next. To determine when a route point is reached, you check the distance from
            the character to the route point; if the distance is within a certain limit, the character
            has reached the point and is allowed to continue on to the next route point.


            posted on 2007-11-14 15:05 lovedday 閱讀(193) 評論(0)  編輯 收藏 引用

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