Detect if other GO enter in line

Hi, I want create a C# script for detect if a gameobjact enter in line of 2 gameobject.
For example:
I have GameObject A and GameObject B one at position 1 and B at position 3. GameObject C walk throught position 2, and print collision!

I thought I might take the position of "A", then the position of "B" and then, so that if a GameObject with a collider enters the position between A and B sends a message to the console. What I thought, however, is that it takes too much memory to recalculate every frame, what do you think?

There is it a simple grafic sample.
Thanks in advance
View attachment $example.bmp

 

Check raycast,
http://docs.unity3d.com/Documentation/ScriptReference/Physics.Raycast.html

 

Thanks it was exactly what i was looking

 

Do you know another script? But that detect just first object in ray? I find this link:
http://answers.unity3d.com/questions/54154/raycast-hit-one-object-at-a-time.html
But i have to work to this code:

Code (csharp):

1.  
2.  
3. private int layerMask = 1<<8;
4.  
5. void Update() {
6. //START POINT OF INTEREST
7.         Vector3 fwd = transform.TransformDirection(Vector3.forward);
8.        
9.         if (Physics.Raycast(transform.position, fwd, 10)) {
10.            
11.             if (Physics.Raycast(transform.position, fwd, 10, layerMask))
12.                 print("There is player in front of the object!");
13.             else {
14.                 print("There is something in front of the object!");
15.             }
16.         }
17. //END POINT OF INTEREST
18. if(target != null) {
19.             //se i tasti di input del tastierino numerico sono premuti
20.             if(_rotateCameraPressed) {
21.                 _x += Input.GetAxis("Rotate Camera Horizontal Buttons") * xSpeed * 0.02f;
22.                 _y -= Input.GetAxis("Rotate Camera Vertical Buttons") * ySpeed * 0.02f;
23.            
24.             RotateCamera();
25.                
26.             }
27.             //altrimenti se il tasto dx del mouse è premuto..
28.             else if(_camButtonDown) {          
29.            
30.             _x += Input.GetAxis("Mouse X") * xSpeed * 0.02f;
31.             _y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;
32.            
33.             RotateCamera();
34.         }
35.         //altrimenti...
36.         else {
37.                
38.             //_myTransform.position = new Vector3(target.position.x, target.position.y + height, target.position.z - walkDistance);
39.             //_myTransform.LookAt(target);
40.            
41.            
42.             // assegna alla variabile float la rotazione y del target(player)
43.             float wantedRotationAngle = target.eulerAngles.y;
44. //          prova = wantedRotationAngle;
45.             float wantedHeight = target.position.y + height;
46.                
47.             float currentRotationAngle = _myTransform.eulerAngles.y;
48.             float currentHeight = _myTransform.position.y;
49.            
50.             // Damp the rotation around the y-axis
51.             currentRotationAngle = Mathf.LerpAngle(currentRotationAngle, wantedRotationAngle, rotationDamping * Time.deltaTime);
52.        
53.             // Damp the height
54.             currentHeight = Mathf.Lerp(currentHeight, wantedHeight, heightDamping * Time.deltaTime);
55.        
56.             // Convert the angle into a rotation
57.             Quaternion currentRotation = Quaternion.Euler(0, currentRotationAngle, 0);
58.            
59.             // Set the position of the camera on the x-z plane to:
60.             // distance meters behind the target
61.             _myTransform.position = target.position;
62.             _myTransform.position -= currentRotation * Vector3.forward * walkDistance;
63.        
64.             // Set the height of the camera
65.             _myTransform.position = new Vector3(_myTransform.position.x, currentHeight, _myTransform.position.z);
66.            
67.             // Always look at the target
68.             _myTransform.LookAt(target);
69.            
70.             }
71.         }
72.         else {
73.             GameObject go = GameObject.FindGameObjectWithTag(cameraTagName);
74.            
75.             if(go == null)
76.                 return;
77.            
78.             target = go.transform;
79.         }
80.     }
81.    
82.     private void RotateCamera() {
83.         Quaternion rotation = Quaternion.Euler(_y, _x, 0);
84.         Vector3 position = rotation * new Vector3(0.0f, 0.0f, -walkDistance) + target.position;
85.  
86.         _myTransform.rotation = rotation;
87.         _myTransform.position = position;
88.     }
89.    
90.     public void CameraSetup() {
91.         _myTransform.position = new Vector3(target.position.x, target.position.y + height, target.position.z - walkDistance);
92.         _myTransform.LookAt(target);
93.     }
94. }
95. }
96.  

I attach this script on main camera and target is a simple cube(set layer 8), the camera follows cube and if object enter in ray of camera, print to console message.
But the problem is that not work layer and not layer, if camera detect one, if i move another object in line, the script not detect change, but if i change public variable value called walkDistance > 10 it detect change. Can you help me? Thanks in advance

ps ignore multiple line of comments

 

You just need to miss out the distance and layer mask parameters, so instead of

Code (csharp):

1. if (Physics.Raycast(transform.position, fwd, 10, layerMask)) { ... }

...you should use

Code (csharp):

1. if (Physics.Raycast(transform.position, fwd)) { ... }

 

Thanks, i try immediatly ^^

 

I follow your istructions, but still to not working. I repost the code, please tell me what do you think

Code (csharp):

1.  
2. Vector3 fwd = transform.TransformDirection(Vector3.forward);
3.        
4.             if (Physics.Raycast(transform.position, fwd, layerMask))
5.                 print("There is player in front of the object!");
6.             else if (Physics.Raycast(transform.position, fwd, 10))
7.                 print("There is something in front of the object!");
8.  

It continue to print "There is player in front of the object!" :'(