Learning XNA 3.0 phần 6 ppsx

230 | Chapter 11: Creating a First-Person Camera } } Notice that you have specified two variables at the class level: one for the view matrix and one for the projection matrix. The camera represented by this class was designed to be stationary. As mentioned previously, the view matrix represents the location and rotation of the camera. If you’re going to create a camera that can be moved through 3D space, you’re going to have to be able to modify the view matrix. In fact, rather than modifying the view matrix, you’ll be rebuilding the matrix every frame. Remember that the view matrix is composed of a position vector, a direction vector, and an up vector. By creating class-level variables for each of those vectors, you can modify things like the position of the camera, the direction of the camera, and which direction is up for the camera by simply modifying the appropriate vector variable and then rebuilding the view matrix of the camera with the new vectors. To move and rotate your camera, you’ll need to add the following three class-level variables to your Camera class: // Camera vectors public Vector3 cameraPosition { get; protected set; } Vector3 cameraDirection; Vector3 cameraUp; These three variables will be used to re-create your camera’s view matrix each frame. Note that the cameraDirection variable is not the same as the camera’s target (or the actual point at which the camera is looking). The view matrix is created by the Matrix.CreateLookAt method, which takes three parameters: the position, target, and up vectors for the camera. The second parameter, the camera’s target, represents the actual point at which your camera will be looking. In contrast, the cameraDirection variable represents a relative direction in which your camera is facing, rather than a target at which the camera is looking. In order to determine the actual target point that the camera is looking at, you need to add your cameraPosition and cameraDirection vectors together (see Figure 11-1). This means two important things in relation to your cameraDirection vector. First, instead of passing cameraDirection as the second parameter of your Matrix. CreateLookAt method, you’ll need to pass cameraPosition + cameraDirection. Sec- ond, your cameraDirection vector cannot be (0, 0, 0)—the variable must contain a value representing something other than the origin because it represents the direc- tion in which your camera is looking, and the vector (0, 0, 0) has no direction. OK, now that that’s cleared up, go ahead and create a method in your Camera class that will define your new view matrix using your three new camera vectors: private void CreateLookAt( ) { view = Matrix.CreateLookAt(cameraPosition, cameraPosition + cameraDirection, cameraUp); } Components of a Moving 3D Camera | 231 Next, you’ll want to set your camera’s Vector3 variables in your constructor. You’re already accepting those three parameters in the constructor, but currently you’re not storing them separately; you’re just using them in the constructor to create a view matrix with the call to Matrix.CreateLookAt. Remove the line of code in the construc- tor that builds your view matrix: view = Matrix.CreateLookAt(pos, target, up); and replace it with the following code: // Build camera view matrix cameraPosition = pos; cameraDirection = target - pos; cameraDirection.Normalize( ); cameraUp = up; CreateLookAt( ); This code sets the position and up vectors directly from the parameters received. The direction is derived from the target of the camera minus the position of the camera. Why? Because the target parameter that was passed in is currently treated as the Figure 11-1. The camera target (the second parameter you’ll pass to the Matrix.CreateLookAt method) is derived from adding the camera’s direction to the camera’s position cameraDirection (2, -2, 0) cameraPosition (-2, -5, 0) Camera’s Target (cameraPosition + cameraDirection) (0, -7, 0) 232 | Chapter 11: Creating a First-Person Camera actual camera target, whereas your direction variable represents the general direction in which the camera is facing. If you derive the target from the camera position plus the camera direction, you can also derive the camera direction from the camera tar- get minus the camera position. Notice that a call to Normalize is being used on the cameraDirection. The Normalize method takes any vector and converts it to a vector with a magnitude (or length) of one. Why this is done will become evident shortly. Basically, you’ll be using this vec- tor not only to represent the direction of the camera, but also to move the camera forward. Finally, the call to CreateLookAt creates an initial view matrix based on the vectors specified. Moving in a First-Person Camera With the cameraDirection vector being normalized and representing the direction in which the camera is looking, you can easily move the camera forward by simply add- ing the cameraDirection to the cameraPosition. Doing this will move the camera toward the camera’s target in a straight line. Moving backward is just as easy: simply subtract the cameraDirection from the cameraPosition. Direction of Movement Think about it—when you walk forward, what direction do you typically move in? Usually, you walk in the direction that you are facing. That is true of most game cam- eras as well. There are a few exceptions that allow the player to look in one direction and move in another, but as a general rule, you move a camera forward in the direction in which it faces. What does that mean to you? Well, because you already have a vector that represents the direction in which the camera is facing, you can use that vector to move your cam- era forward. As you’ll see shortly, you can move a camera forward by simply adding the direction vector to the position vector. This moves the position vector in the direc- tion of the direction vector, and in turn moves the camera forward in the direction in which it’s facing. So, why normalize the direction vector? Remember that normalizing the vector will make it have a length or magnitude of one. Dealing with a vector with a length of one makes it much easier to apply things such as different speed values to the movement of the camera. Moving in a First-Person Camera | 233 Because the cameraDirection vector is normalized (i.e., has a magnitude of one), the camera’s speed will always be one. To allow yourself to change the speed of the cam- era, add a class-level float variable to represent speed: float speed = 3; Next, in your Camera class’s Update method, add code to move the camera forward and backward with the W and S keys: // Move forward/backward if (Keyboard.GetState( ).IsKeyDown(Keys.W)) cameraPosition += cameraDirection * speed; if (Keyboard.GetState( ).IsKeyDown(Keys.S)) cameraPosition -= cameraDirection * speed; At the end of the Update method, you’ll want to call your Camera’s CreateLookAt method to rebuild the camera based on the changes you’ve made to its vectors. Add this line of code just above the call to base.Update: // Recreate the camera view matrix CreateLookAt( ); Compile and run the game at this point, and you’ll see that when you press the W and S keys, the camera moves closer to and farther from the spaceship. It’s impor- tant to note what’s happening here: in our previous 3D examples, you’ve moved objects around by changing the world matrix for those objects, but in this example, instead of moving the object, you’re moving the camera itself. Now that you can move your camera forward and backward, you’ll want to add other movement features. Any good 3D first-person camera also has strafing (or side-to-side movement) support. Your camera vectors are position, direction, and up, so how can you find a vector that will move your camera sideways? We may be able to solve this problem with a little bit of vector math. Think about what you need in order to move sideways: you’ll need a vector that points in the direction that is sideways-on to your camera. If you had that vector, moving sideways would be just as easy as moving for- ward—you could simply add the sideways vector to the camera’s position vector. As shown in Figure 11-2, you currently have a vector for the camera’s up direction as well as a vector for the direction in which the camera is facing, but you don’t have a vector representing the direction that is sideways-on to the camera. Here’s where the vector math can help: a cross product is a binary operation per- formed on two vectors in 3D space that results in another vector that is perpendicu- lar to the two input vectors. Therefore, by taking the cross product of the up and direction vectors of your camera, you will end up with a vector perpendicular to those two vectors (i.e., coming from the side of your camera). This is illustrated in Figure 11-3. The cross product of your negative up and direction vectors will yield a perpendicular vector coming from the other direction (sideways on to the other side of the camera). 234 | Chapter 11: Creating a First-Person Camera It’s not critical that you understand how this vector math works, but if you’re curious, feel free to investigate in some math textbooks. All you need to understand for now is that it does indeed work: the cross prod- uct of any two vectors yields a third vector perpendicular to the other two, and the cross product of the up and direction vectors of your cam- era yields a vector indicating the sideways direction for your camera. It may help to picture a person standing and looking forward. The person’s direc- tion vector would be pointing straight ahead, while his up vector would be pointing Figure 11-2. To move sideways in a strafe, you need a vector representing a sideways direction Figure 11-3. Finding your camera’s sideways vector Up Vector Need a Sideways Vector Direction Vector Up Vector Sideways (Directions x Up) (-Up x Direction) Direction Vector sideways (Up x Direction) Rotations in a First-Person Camera | 235 straight up. The cross product of the up and direction vectors would essentially be the direction the person’s left arm would be pointing in if it were held out perpendic- ular to both his up and direction vectors. The only vector that fits that criterion would be one that was pointing directly outward from the person’s side. XNA provides a method that will create a vector based on the cross product of two other vectors. It’s a static method in the Vector3 class called Cross. Pass in any two vectors to the Cross method, and the resulting vector will be the cross product of the two vectors passed in. To enable the camera to move from side to side using the A and D keys, insert the following code immediately after the code you just added, which moves the camera forward and backward: // Move side to side if (Keyboard.GetState( ).IsKeyDown(Keys.A)) cameraPosition += Vector3.Cross(cameraUp, cameraDirection) * speed; if (Keyboard.GetState( ).IsKeyDown(Keys.D)) cameraPosition -= Vector3.Cross(cameraUp, cameraDirection) * speed; Compile and run the game, and you’ll see that you now have full camera movement with the WASD keys. Very cool! You’re ready to work on rotating your camera in 3D space. Rotations in a First-Person Camera All camera rotations are related to the same rotations that were discussed previously in relation to the rotation of 3D objects in XNA. Essentially, a camera can yaw, pitch, and roll just like an object can. As a reminder, yaw, pitch, and roll rotations are pictured in Figure 11-4. Figure 11-4. Yaw, pitch, and roll apply not only to objects, but to cameras as well Yaw Roll Pitch 236 | Chapter 11: Creating a First-Person Camera In the classes in which I’ve taught XNA, one of the things that has traditionally been difficult for some students to understand is the fact that yaw, pitch, and roll rota- tions when applied to objects or cameras that move and rotate in 3D don’t necessar- ily correspond to rotations around the X-, Y-, and Z-axes. For example, picture the camera you currently have in your game. The camera sits on the Z-axis and faces in the negative Z direction. If you wanted to rotate the cam- era in a roll, you could rotate the camera around the Z-axis. However, what would happen if the camera rotated to turn 90º and was now looking in the direction of positive X? If you performed a rotation around the Z-axis at this point, you’d rotate in a pitch rather than a roll. It’s easier to think of yaw, pitch, and roll as related to the vectors available to you in your camera. For example, a yaw, rather than rotating around the Y-axis, rotates around the camera’s up vector. Similarly, a roll rotates around the camera’s direc- tion vector, and a pitch rotates around a vector coming out of the side of the object, perpendicular to the up and direction vectors. Any idea how you’d get that perpen- dicular vector? That’s right, you’ve used it before to add strafing ability: it’s the cross product of the up and direction vectors. Figure 11-5 illustrates how yaw, pitch, and roll rotations are accomplished in a 3D camera. Which of these rotations you choose to implement in your particular game com- pletely depends on what type of experience you want to give the player. For exam- ple, a typical space simulator will have the ability to yaw, pitch, and roll in an Figure 11-5. Yaw, pitch, and roll rotations using the up and direction vectors of a camera Yaw (Rotate on camera’s up vector) Roll (Rotate on camera’s direction vector) Pitch (Rotate on the cross product of the other camera’s up and direction vectors)) Rotations in a First-Person Camera | 237 unlimited fashion. A helicopter simulator may allow yaw, pitch, and roll to some extent, but might not allow you to perform a complete roll (a fairly difficult task in a helicopter). A land-based shooter may only allow a yaw and a pitch, though some games allow roll rotations for special moves like tilting your head to look around a corner. Once you’ve decided which of these rotations you’ll allow in your camera, the next step is to implement them. Each of these camera rotations can be accomplished by rotating one or more of your camera’s vectors. For a yaw, pitch, or roll, it helps to evaluate the rotation using these steps: first, determine which of the three camera vectors need to rotate; second, figure out what axis you will need to rotate those vec- tors around; and third, determine which methods will be needed to accomplish this. Rotating a Camera in a Yaw Let’s start by creating a yaw for the camera. Of the three camera vectors (position, direction, and up), the only one that changes when performing a yaw is the direction vector. Picture a person standing up and performing a yaw rotation (moving her head from side to side). The person’s up vector doesn’t change, and neither does her position, but the direction vector (the direction in which she is looking) definitely changes. The axis you want to rotate the direction vector around for a yaw is the camera’s up vector. The method used to rotate a Vector3 is Vector3.Transform, which takes two parameters: the source or original vector, and a matrix representing a rotation or translation to apply to the vector. When performing a yaw rotation for a camera, why rotate around the camera’s up vector instead of the Y-axis? The Y-axis may not always be up for a camera. It might be for a land- based shooter game, but consider a flight simulator that freely flies and rotates in three dimensions. In that case, you’d always want to yaw around the up vector of the camera. Before you add the Vector3.Transform to perform the yaw, you’ll want to add some code to allow your camera to capture mouse movement. A typical first-person con- figuration uses the WASD keys for movement and the mouse for rotating the cam- era. So, to capture mouse movement, add the following class-level variable to your Camera class: MouseState prevMouseState; Next, in the Initialize method of your Camera class, set the initial position of the mouse cursor to the middle of the screen. Also, add the code to initialize the new variable: 238 | Chapter 11: Creating a First-Person Camera // Set mouse position and do initial get state Mouse.SetPosition(Game.Window.ClientBounds.Width / 2, Game.Window.ClientBounds.Height / 2); prevMouseState = Mouse.GetState( ); Remember that the Mouse.GetState call returns the mouse position. To find out how far the mouse has actually moved, you need to capture the state from the previous frame and compare it to the current state in each frame. You initialize the state vari- able in the Initialize method so that you have something to compare against in the first frame (the first time Update is called). Now you’re ready to code your yaw rotation. In the Update method of your Camera class, add the following code just above the call to CreateLookAt: // Yaw rotation cameraDirection = Vector3.Transform(cameraDirection, Matrix.CreateFromAxisAngle(cameraUp, (-MathHelper.PiOver4 / 150) * (Mouse.GetState( ).X - prevMouseState.X))); // Reset prevMouseState prevMouseState = Mouse.GetState( ); In this code, you’re assigning the cameraDirection vector the value given in the Vector3.Transform call. By passing in cameraDirection as the first parameter, you ensure that the Vector3.Transform method will apply the rotation specified in the sec- ond parameter to the cameraDirection and return the resulting vector. The matrix specified in the second parameter is created from the CreateFromAxisAngle method, which creates a rotation around a specific axis (in this case, the camera’s up vector). The angle of rotation is determined by how much the mouse has moved horizontally. Compile and run the game at this point, and you’ll see that not only can you move in 3D space, but you can now yaw the camera left and right. It may seem a bit awk- ward because you don’t have full rotation of your camera yet, but that will come shortly. If your camera moves backward relative to your mouse (i.e., if you move the mouse right and it rotates the camera left), you’ve probably left off the negative sign in front of the MathHelper.PiOver4 in the code. Add that and it should work properly. Rotating a Camera in a Roll When rotating in a roll, follow the same steps to figure out what to do: ask yourself what vectors rotate when performing a roll, what axis you would rotate those vec- tors on, and what methods need to be used. In a roll, the only camera vector that changes is the camera’s up vector. The vector that you want to rotate your camera’s up vector around is the camera’s direction vec- tor. Add the following code to the Update method of your Camera class, just before the prevMouseState = Mouse.GetState( ) line: Rotations in a First-Person Camera | 239 // Roll rotation if (Mouse.GetState( ).LeftButton == ButtonState.Pressed) { cameraUp = Vector3.Transform(cameraUp, Matrix.CreateFromAxisAngle(cameraDirection, MathHelper.PiOver4 / 45)); } if (Mouse.GetState( ).RightButton == ButtonState.Pressed) { cameraUp = Vector3.Transform(cameraUp, Matrix.CreateFromAxisAngle(cameraDirection, -MathHelper.PiOver4 / 45)); } Run the game now, and you’ll see that with the left and right mouse buttons you can roll your camera left or right. It will probably look a little strange in this example because the only thing that you’re drawing is the spaceship and it is rotating as well, which makes the rotation of your camera seem off. Let’s make the ship not spin anymore, so you can get a better sense of how your camera is working. In the ModelManager’s LoadContent method, change the type of ship that’s being created from a SpinningEnemy to a BasicModel: models.Add(new BasicModel( Game.Content.Load<Model>(@"models\spaceship"))); Run the game again, and your camera rotations and movements should feel more accurate without the ship spinning on its own. Rotating a Camera in a Pitch Coding a pitch is slightly more complicated than coding a yaw or a roll. First, think of what needs to change when you pitch. You may think that just your direction changes, but here’s a question: does your up vector change in a pitch? This is one place where you’ll need to stop and think about what kind of functional- ity you want in your camera. Typically, in a flight simulator, you’d rotate both your direction and your up vector in a pitch. The reason? Remember that in a yaw you rotate around your up vector. In a flight simulator, you’ll want to have your up vec- tor change in a roll and a pitch to make your yaw rotation more realistic. What about pitching in a land-based shooter? Would you want to rotate your up vector in a pitch in that scenario? Again, remember that when you yaw, you do so around the up vector. Imagine hunting down an enemy and looking two or three sto- ries up a wall to see if he’s perched on a ledge. Then, you rotate in a yaw to scan the rest of that level of the building. You’d expect your rotation in that case to be based on the Y-axis—rotating around the up vector (if it was changed by your pitch) would cause an unexpected rotation. [...]... LevelInfo(25, levelInfoList.Add(new LevelInfo(0, 3000, 2800, 260 0, 2400, 2200, 2000, 1800, 160 0, 1400, 1200, 1000, 800, 60 0, 400, 200, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 2, 2, 2, 3, 3, 3, 4, 4, 5, 5, 6, 6, 8, 8, 18, 6, 6, 6, 7, 7, 7, 7, 8, 8, 9, 9, 9, 10, 10, 20, 10)); 9)); 8)); 7)); 6) ); 5)); 4)); 3)); 2)); 1)); 0)); 0)); 0)); 0)); 0)); This code creates 15 different levels,... consider here: typically in a land-based shooter you can’t pitch a full 360 º When looking up, you typically, can’t look straight up; you can pitch your camera until it is maybe 10–15º away from exactly up, but no further One reason for this is that in XNA, if the angle between your up vector and your direction vector is small enough, XNA doesn’t know how to draw what you’re telling it to draw, and it will... LevelInfo (60 0, levelInfoList.Add(new LevelInfo(500, levelInfoList.Add(new LevelInfo(400, levelInfoList.Add(new LevelInfo(300, levelInfoList.Add(new LevelInfo(200, levelInfoList.Add(new LevelInfo(100, levelInfoList.Add(new LevelInfo(50, levelInfoList.Add(new LevelInfo(50, levelInfoList.Add(new LevelInfo(50, levelInfoList.Add(new LevelInfo(25, levelInfoList.Add(new LevelInfo(0, 3000, 2800, 260 0, 2400,... Creating a First-Person Camera • While the rest of us build cameras to rotate and move in 3D space, Chuck Norris doesn’t need to do that when he writes games in XNA Instead, the world and all objects move and rotate around Chuck’s camera in XNA This is done to mimic real life, where Chuck actually never moves or rotates—everything and all of us instead rotate and move around Chuck Norris Test Your Knowledge:... has collided with another object’s sphere When you use models in XNA, bounding spheres are generated for you as part of the model Each model contains one or more ModelMesh objects, and each ModelMesh has a property called BoundingSphere that defines a sphere surrounding that part of the model 3D Collision Detection and Bounding Spheres | 261 Figure 12-2 Dang I’m kind of a bad shot The tricky part of this... really tweak whatever you want to fit your needs 264 | Chapter 12: 3D Collision Detection and Shooting Adding a Crosshair One other thing you’ve probably noticed that your game is missing at this point is some way to make sure you’re aiming at the correct place Typically, this is done by placing a crosshair on the screen Even though you’re working in XNA 3D, you can still draw 2D sprites on the screen,... added to the Game1 class’s constructor, notice the preprocessor directive indicating that the game should only be run in full-screen mode if it is not running in the debug configuration Why is that? In XNA, Visual Studio has an extremely difficult time allowing you to debug and step through breakpoints when a game is running in full-screen mode Because of this, you should always run your game in windowed... book If you want to keep your free-flying camera code, you should make a copy of your project to save the existing code that you have written 240 | Chapter 11: Creating a First-Person Camera Figure 11 -6 Your camera is now freely mobile in 3D space—here’s a view from above the spaceship If you download the source code for this chapter, you’ll find the free-flying camera code in the folder called Flying... -MathHelper.PiOver4 / 45)); } In addition, you can also remove the class-level speed variable from the Camera class, as you won’t be using it anymore What you’re left with is a camera that yaws and pitches 360 º in each direction However, you want to cap that at 22.5º in each direction (for a total of 45º in a yaw and 45º in a pitch) To do this, you’ll need to add four variables at the class level in the Camera... of realism, as seen in Figure 12-1 You’re making some great strides in your game now The next thing to do is add the ability to shoot down the enemies! Figure 12-1 Enemies are coming at us in droves! 2 56 | Chapter 12: 3D Collision Detection and Shooting Firing Shots Many types of games have some form of projectile that is thrown or shot toward other players or enemies How would you go about adding the . cannot be (0, 0, 0) —the variable must contain a value representing something other than the origin because it represents the direc- tion in which your camera is looking, and the vector (0, 0, 0) has. = 0; float pitchAngle = 0; float rollAngle = 0; Vector3 direction; Next, modify the constructor of the SpinningEnemy class as follows: public SpinningEnemy(Model m, Vector3 Position, Vector3. example, if you are running a widescreen monitor at a res- olution of 1, 400 × 900 , you won’t be able to fit a 1,2 80 × 1 ,02 4-sized window on your screen. This will cause issues with gameplay down the