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Third-Person Camera
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In this section you ll extend the BaseCamera class, created in the previous section, to create a more specific type of camera: a third-person camera. For this type of camera, you ll create a class named ThirdPersonCamera, which extends the BaseCamera class. The thirdperson camera s goal is to follow an object while it moves, and the distance in which the camera follows an object must be variable. Otherwise, it would appear that the object is bound to the camera. To make the camera follow an object, for example the player-controlled character, you need to define some parameters, such as chase position (the position the camera must follow); chase direction (the direction used to follow the chase position); chase speed; and minimum, desired, and maximum distances between the camera and the object. Figure 9-2 illustrates some of the parameters that need to be configured.
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Figure 9-2. For the third-person camera, the square is the camera s chase position, and the dots are the camera s maximum, desired, and minimum allowed positions.
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Setting Chase Parameters
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In the ThirdPersonCamera class, you create the SetChaseParameters method to set the camera s chase parameters that are not frequently updated: the chase distances and speed. You can configure the chase position and direction parameters, which are more frequently updated, through properties:
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// Chase parameters float desiredChaseDistance; float minChaseDistance; float maxChaseDistance; float chaseSpeed;
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CHAPTER 9 s LIGHTS, CAMERA, TRANSFORMATIONS!
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Vector3 chasePosition; public Vector3 ChasePosition { get { return chasePosition; } set { chasePosition = value; } } Vector3 chaseDirection; public Vector3 ChaseDirection { get { return chaseDirection; } set { chaseDirection = value; } } public void SetChaseParameters(float chaseSpeed, float desiredChaseDistance, float minChaseDistance, float maxChaseDistance){ this.chaseSpeed = chaseSpeed; this.desiredChaseDistance = desiredChaseDistance; this.minChaseDistance = minChaseDistance; this.maxChaseDistance = maxChaseDistance; }
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Updating the Camera s Position
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Every time the camera is updated, its position needs to be recalculated. The desired camera position is equal to the camera s chase position, minus the chase direction, multiplied by the chase distance (which is the distance between the camera and the chase position), as shown in Figure 9-2. The desired camera position would be the camera s final position if it were placed at a fixed distance from the chase position. However, to allow the camera to move smoothly, the distance between the camera and the chase position may vary between a minimum and maximum range (defined in the attributes minChaseDistance and maxChaseDistance). This way, the new camera position is calculated through a linear interpolation between its current position and its desired position. You can see the Linear Interpolation note for more details.
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Vector3 targetPosition = chasePosition; Vector3 desiredCameraPosition = chasePosition chaseDirection * desiredChaseDistance; float interpolatedSpeed = MathHelper.Clamp(chaseSpeed * elapsedTimeSeconds, 0.0f, 1.0f);
CHAPTER 9 s LIGHTS, CAMERA, TRANSFORMATIONS!
desiredCameraPosition = Vector3.Lerp(position, desiredCameraPosition, interpolatedSpeed);
The weight used to interpolate the camera s position is calculated based on the time elapsed since the last update and the camera speed. However, because the interpolation weight must be between 0 and 1, you need to clamp its value. XNA s Vector3 class has a Lerp method that helps you interpolate vectors.
LINEAR INTERPOLATION
A linear interpolation is an interpolation between two values that varies linearly according to a defined weight, where the weight is usually a float number defined between 0 and 1. For example, a linear interpolation between the numbers 10 and 20 using the weight value 0.5 results in the value 15, while a linear interpolation using the weights 0 and 1 results in the values 10 and 20. Also, a linear interpolation between two 3-D vectors interpolates the value of each component of the vectors (X, Y, Z) linearly.
Create the UpdateFollowPosition method to update the camera s position. Following is the code for the UpdateFollowPosition method:
private void UpdateFollowPosition(float elapsedTimeSeconds, bool interpolate) { Vector3 targetPosition = chasePosition; Vector3 desiredCameraPosition = chasePosition- chaseDirection * desiredChaseDistance; if (interpolate) { float interpolatedSpeed = MathHelper.Clamp( chaseSpeed * elapsedTimeSeconds, 0.0f, 1.0f); desiredCameraPosition = Vector3.Lerp(position, desiredCameraPosition, interpolatedSpeed); // Clamp the min and max follow distances Vector3 targetVector = desiredCameraPosition - targetPosition; float targetLength = targetVector.Length(); targetVector /= targetLength; if (targetLength < minChaseDistance) { desiredCameraPosition = targetPosition + targetVector * minChaseDistance;
CHAPTER 9 s LIGHTS, CAMERA, TRANSFORMATIONS!
} else if (targetLength > maxChaseDistance) { desiredCameraPosition = targetPosition + targetVector * maxChaseDistance; } } // Needed to recalculate heading, strafe, and up vectors SetLookAt(desiredCameraPosition, targetPosition, upVec); }
The UpdateFollowPosition method has the interpolate parameter, which defines whether the camera will be placed at its desired position (if the interpolate value is false), or will be smoothly interpolated to the desired position. When the camera chases an object for the first time, you must set the interpolate value as false, forcing the camera to start at its desired position. When the final camera position is calculated by interpolating its current position with its desired position, you need to check if the distance from the camera to the chase position is between the minimum and maximum chase distances defined, as shown in Figure 9-2. If the distance is smaller than the minimum, you set it to the minimum allowed distance. Otherwise, if the camera distance is greater than the maximum distance, you set it to the maximum allowed distance. These tests are important and they ensure that the camera can chase objects that have a greater speed than the camera.
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