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⌂ ∞ ▽ The Sentinel on the BBC Micro

Gameplay: CheckEnemyGaze (Part 1 of 2)

       Name: CheckEnemyGaze (Part 1 of 2)                            [Show more]
       Type: Subroutine
   Category: Gameplay
    Summary: Check to see whether the current enemy can see a specific target
             object of a specific type

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyTactics (Part 2 of 8) calls CheckEnemyGaze
             * ApplyTactics (Part 5 of 8) calls CheckEnemyGaze
             * FindObjectToDrain calls CheckEnemyGaze
             * ScanForMeanieTree calls CheckEnemyGaze




 Arguments:

   A                    The type of the object to match against the target (this
                        ensures that we only perform the gaze calculations when
                        the target object is of this type; if it isn't, the
                        routine aborts without doing the calculations)

   Y                    The number of the target object to analyse

   viewingObject        The viewing object (i.e. the enemy doing the scanning)



 Returns:

   targetVisibility     The target visibility result:

                          * If the target object is a robot, then bit 6 will be
                            set if the robot object can be seen, even if the
                            robot's tile can't be seen

                          * Bit 7 will be set if the enemy can see the target
                            object's tile

   treeVisibility       The tree visibility result:

                          * If the target object is a robot, then bit 6 will be
                            set if there is a tree blocking the view of the
                            robot object

                          * Bit 7 will be set if there is a tree blocking the
                            view of the target object's tile

   C flag               Status flag (though note that this is ignored for every
                        call to this routine):

                          * Clear if the target object does not exist, or is not
                            of the specified type, or the object is in the
                            viewing arc

                          * Set if the target object is not within the viewing
                            arc of the enemy

   X                    X is preserved

   Y                    Y is preserved



.CheckEnemyGaze

 STA T                  \ Store the object type in T so we can refer to it later

 STX xStoreEnemyGaze    \ Store X in xStoreEnemyGaze so it can be preserved
                        \ across calls to the routine

 STY targetObject       \ Store the target object in targetObject, so object #Y
                        \ is the target object

 LDA #0                 \ Clear all bits of targetVisibility so we can set them
 STA targetVisibility   \ later with the results of the analysis

 LDA objectFlags,Y      \ If bit 7 is set for object #Y then this object number
 BMI egaz3              \ is not allocated to an object, so jump to egaz3 to
                        \ return from the subroutine with the C flag clear to
                        \ indicate that the target object can't be seen by the
                        \ enemy

 LDA objectTypes,Y      \ If object #Y is not an object of type T (which we set
 CMP T                  \ to the argument A above), then jump to egaz3 to return
 BNE egaz3              \ from the subroutine with the C flag clear to indicate
                        \ that the target object is of the wrong type

 JSR GetObjectAngles    \ Calculate the angles and distances of the vector from
                        \ the viewer to object #Y and put them into the
                        \ following variables:
                        \
                        \   * Set objTypeToAnalyse to the type of object #Y
                        \
                        \   * Set objectViewYaw(Hi Lo) to the yaw angle of the
                        \     viewer's gaze towards the object, relative to the
                        \     screen
                        \
                        \   * Set angle(Hi Lo) to the angle of the hypotenuse of
                        \     the triangle formed by the x-axis and z-axis,
                        \     which is the projection of the 3D vector from the
                        \     viewer to the object down onto the ground plane
                        \     (so imagine a light shining down from above,
                        \     casting the vector's shadow onto the y = 0 plane,
                        \     and that's the hypotenuse)
                        \
                        \   * Set hypotenuse(Hi Lo) to the length of the 3D
                        \     vector from the viewer to the object when
                        \     projected down onto the ground plane
                        \
                        \  * Set yDelta(Hi Lo) to the difference (the delta) in
                        \    the y-axis between the viewer and the object we are
                        \    analysing, so this gives us the altitude difference
                        \    between the viewer and the object

                        \ We now have a very short interlude to set up some of
                        \ the anti-cracker code before continuing in part 2