Detect face collision

Hi,

I am a beginner scripter and i am trying to solve one problem.
My idea is that the "phantom prim" only shout when an object (other prim) collisions with one of the 6 faces of the prim(one in particular), entering by one side and crossing this face, and not backwards (crossing other face of the prim and going out by the right face of the prim).
So i want that the phantom prim shout onlye when another object crosses it by one direction.
I think that if i can force that the phantom prim shout when another object start the collision crossing one face and end the collision crossing the oposite face, i will have it.

Is it possible?

Thank you and Happy New Year.

If you don't mind wasting an extra prim, why not make your detector out of two prims sandwiched together? If the colliding object hits prim A before prim B, you know it was going one direction; if it hits B before A, you know it was going in the opposite direction. That kind of detector would be easy to construct, although you'd have to do some experimentation to see how badly lag might mess it up. Worth a try anyway.

I don't know that it's possible as you outline it, Redlag, but I do something similar by getting the collider's velocity and using that to determine which way he's going.

ETA edited for important clarification
This is how i would detect someone walking into face 2 (and only face 2) of a flattened cube like a door:

default
{
    state_entry()
    {
   
     llVolumeDetect(TRUE);
      
    }

    collision_start(integer num_detected)
    {
        
        integer i;
        key collider = llDetectedKey(i);
        vector pos = (llDetectedPos(i) - llGetPos()) / llGetRot();
        if ( pos.x > 0){ // play with this line to use different faces
        llSay(0,"ouch");
        }
    }
}

As is, it would also detect collisions on the half of each face that has a positive x coordinate.

if the object is not moving quickly you can use volume detect and get the objects position as reported in the collision, apply the rot of the prim and you can tell which face it's striking simply by the the signs of the returned vector.

//-- default cube shape
string uGetDirection( float vFltVal ){
  integer vIdxVal = (integer)(vFltVal > 0.0);
  return llGetSubString( "+-", vIdxVal, vIdxVal );
}

default{
  state_entry(){
    llVolumeDetect( TRUE );
  }

  collision_start( integer vIntCollides ){
    string vStrRpt = "impact on ";
    vector vIntTst = (llGetPos - llDetectedPos( 0 )) * llGetRot();
    if (llAbs( vIntTst.x ) > llAbs( vIntTst.y )){
      if (llAbs( vIntTst.x ) > llAbs( vIntTst.z )){
        vStrRpt += uGetDirection( vIntTst.x ) + "X";
      }else{
        vStrRpt += uGetDirection( vIntTst.z ) + "Z";
      }
    }else if (llAbs( vIntTst.y ) > llAbs( vIntTst.z )){
      vStrRpt += uGetDirection( vIntTst.y ) + "Y";
    }else{
      vStrRpt += uGetDirection( vIntTst.z ) + "Z";
    }
    llOwnerSay( vStrRpt + " face" );
  }
}

written on the fly, may contain errors

ETA:
the bigger the internal volume and the slower the collider the better, bullets tend to get the position reorted a bit slo and may have moved far enough that they report the wrong face...

LOL .... My name invites creative misspelling, but that's a lovely one. Thank you, Jack. You made my day.

then I hope looking at the OP's name doesn't unmake your day =)

here's the above outlined as OP described, using the x axis faces of a cube.

key gKeyCld;
vector gSizObj;

default{
  state_entry(){
    gSizObj = llGetScale() / 2.0;
  }

  collision_start( integer vIntCld ){
    if (gKeyCld == ""
      vector vPosCld = (llGetPos() - llDetectedPos( 0 )) * llGetRot();
       //-- these 3 lines allow for non cubic shape
      vPosCld.x /= gSizObj.x;
      vPosCld.y /= gSizObj.y;
      vPosCld.z /= gSizObj.z;
      if (vPosCld.x > vPosCldy.y && vPosCld.x > vPosCld.z && vPosCld.x > 0.0){
        gKeyCld = llDetectedKey( 0 );
      }
    }
  }
  
  collision_end( integer vIntCld ){
    if (llDetectedKey( 0 ) == gKeyCld){
      vector vPosCld = (llGetPos() - llDetectedPos( 0 )) * llGetRot();
      vPosCld.x /= gSizObj.x;
      vPosCld.y /= gSizObj.y;
      vPosCld.z /= gSizObj.z;
      if (vPosCld.x < vPosCldy.y && vPosCld.x < vPosCld.z && vPosCld.x < 0.0){
        llOwnerSay( "Achievement!" );
        gKeyCld = "";
      }
    }
  }
}

vector gvFacing = <1.0,0.0,0.0>; // Set this to the direction to detect

default {
    state_entry() {
        llVolumeDetect(TRUE);
    }
    collision_start(integer piDetectedCount) {
        vector lvVelocity;
        float  lfAngle;
        
        lvVelocity = llDetectedVel(0);
        
        // Compute the angle between two vectors
        lfAngle = llFabs(llAcos(llVecNorm(gvFacing) * llVecNorm(lvVelocity)));
        
        llOwnerSay((string)(lfAngle * RAD_TO_DEG));
        if (lfAngle < PI_BY_TWO)
            llOwnerSay(llDetectedName(0)+" passed through!");
    }
}

Set the gvFacing variable to the direction you want to detect (<1.0,0.0,0.0> is East through the prim) . You can also use the prim's facing/rotation instead of hard-coding it, but it will depend on which exact face you want (use llRot2Fwd(llGetRot()) for positive-X, for example).

Oh, dang ...... There goes the day.
/me goes back to her corner, sheepishly

aww we still love ya rollegg =)

Hi again,

thank you all for your advices, especially to Talarus Luan, which code helped me.