Drawing objects: CheckObjVisibility

       Name: CheckObjVisibility                                      [Show more]
       Type: Subroutine
   Category: Drawing objects
    Summary: Check whether an object is visible on-screen and should therefore
             not be changed if a pan operation is about to happen
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * AbortWhenVisible calls CheckObjVisibility
             * ExpendEnemyEnergy calls CheckObjVisibility
             * ScanForMeanieTree calls CheckObjVisibility
             * AbortWhenVisible calls via cvis1


 Arguments:

   A                    The number of the object to check


 Returns:

   C flag               The object's visibility:

                          * Clear = we are about to repeat a pan and at least
                                    some of the object is visible on-screen (so
                                    the object must not be updated or it could
                                    corrupt the landscape view)

                          * Set = we are about to repeat a pan and the object is
                                  not visible on-screen, or we are not about to
                                  repeat a pan (so the object can be updated)

   bufferColumns        If the object is visible, this is set to the number of
                        character columns in the screen buffer that the object
                        spans

   objYawOffset         If the object is visible, this is set to the yaw offset
                        of the left edge of the object from the left edge of the
                        screen, in character columns

   currentObject        If we are about to repeat a pan, this is set to the
                        object number in A

   X                    X is preserved

   Y                    Y is preserved


 Other entry points:

   cvis1                Contains an RTS


.CheckObjVisibility

 SEC                    \ If bit 6 of samePanKeyPress is clear then the player
 BIT samePanKeyPress    \ if not holding down the same pan key following the
 BPL cvis1              \ completion of a landscape pan, so jump to cvis1 to
                        \ return from the subroutine with the C flag set

                        \ If we get here then we have just completed a pan of
                        \ the landscape view and the player is still holding
                        \ down the same pan key

 STA currentObject      \ Set currentObject to the object in A to pass to the
                        \ GetObjVisibility routine so we check whether the

 LDA playerObject       \ Set viewingObject to the object number of the player,
 STA viewingObject      \ so any future view calculations are done from the
                        \ aspect of the player rather than the enemy object

 TXA                    \ Store X and Y on the stack so they can be preserved
 PHA                    \ across the call to GetObjVisibility
 TYA
 PHA

 JSR GetObjVisibility   \ Calculate whether any part of an object is visible
                        \ on-screen and set the C flag as follows:
                        \
                        \   * Clear = at least some of the object is visible
                        \             on-screen
                        \
                        \   * Set = the object is not visible on-screen
                        \
                        \ If the object is visible, also set the following:
                        \
                        \   * bufferColumns = the number of character columns
                        \     in the screen buffer that the object spans
                        \
                        \   * objYawOffset = the yaw offset of the left edge of
                        \     the object from the left edge of the screen, in
                        \     character columns

 PLA                    \ Retrieve X and Y from the stack so they can be
 TAY                    \ preserved
 PLA
 TAX

.cvis1

 RTS                    \ Return from the subroutine