LlGetBoundingBox

Returns a list that is the bounding box of object relative to its root prim.
Format: [ (vector) min_corner, (vector) max_corner ]

default//An avatar bounding box ruler thingy
{
    state_entry()
    {
        llSetStatus(STATUS_PHANTOM, TRUE);
    }
 
    touch_start(integer total_number)
    {
        key target = llDetectedKey(0);
        list box = llGetBoundingBox(target);
        vector center = (llList2Vector(box, 0) + llList2Vector(box, 1)) * 0.5;
        vector size = llList2Vector(box, 1) - llList2Vector(box, 0);
        llSetPrimitiveParams([PRIM_POSITION, center, PRIM_SIZE, size]);
        llSetText("Name: " + llDetectedName(0) + ", UUID: " + (string)target + 
                "\nBounding Box Size: " + (string)size, <1.0, 1.0, 1.0>, 1.0);
    }
}

Functions