3D geometry: SetObjectCoords (Part 1 of 11)

       Name: SetObjectCoords (Part 1 of 11)                          [Show more]
       Type: Subroutine
   Category: 3D geometry
    Summary: Calculate the coordinates for an object's location
  Deep dive: 3D objects
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ProcessLine (Part 5 of 7) calls SetObjectCoords
             * ProcessRunwayLine (Part 2 of 5) calls SetObjectCoords
             * UpdateBullets calls SetObjectCoords


 Arguments:

   objectId             The ID of the object to process (1 to 39)

   GG                   For bullets only (12, 13, 14 or 15), the point ID for
                        the bullet's anchor point


 Returns:

   objectStatus         The object's status byte: bits 6 and 7 affected

   Flags                Set according to the object's updated status byte

   isObject             The object ID, i.e. a non-zero value so callers of this
                        routine will know we just processed an object

   xObjectHi etc.       Set to the object's coordinates, relative to the plane

   xPointHi etc.        The object's anchor point:

                          * For bullets, point GG is updated with the bullet's
                            anchor point

                          * For other objects, point objectId+216 is updated
                            with the object's anchor point


.SetObjectCoords

 LDY objectId           \ Set Y to the object ID

 STY isObject           \ Store the object ID in isObject, so callers of this
                        \ routine will know we just processed an object

 TYA                    \ Set N = 216 + object ID
 CLC                    \
 ADC #216               \ so N is in the range 217 to 255 and is the point ID
 STA N                  \ that we use for storing the object's anchor point

 LDA #1                 \ Set K = 1, so the CheckObjDistance routine returns
 STA K                  \ the correct 0 or 1 value when checking an object's
                        \ distance, which is the default behaviour (we set K to
                        \ 0 below for bullets only, as we always want them to
                        \ remain visible at distance)

                        \ We now check for some specific objects:
                        \
                        \   * Object groups, i.e. trees (6, 7), hills (8, 9)
                        \   * Bullets (12, 13, 14 or 15)
                        \   * Feeding aliens (object group 30)
                        \   * Flying aliens (31, 32 or 33)
                        \
                        \ so we can pre-process them before moving on to the
                        \ main processing routine in part 8

 CPY #12                \ If the object ID < 12, jump to objc1 to check the next
 BCC objc1              \ range

 CPY #16                \ If the object ID >= 16, jump to objc1 to check the
 BCS objc1              \ next range