Extra tracks: HookFlattenHills (Part 3 of 3)

       Name: HookFlattenHills (Part 3 of 3)                          [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Flatten any hills in the verge buffer, calculate the hill height
             and track width, cut objects off at the hill height
  Deep dive: Secrets of the extra tracks
             Code hooks in the extra tracks
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 This part of the routine flags the verge coordinate at index X as being
 visible on-screen, but only when the track section at the front of the segment
 buffer is one of sections 0 to 5.


.hill6

 LDA objTrackSection+23 \ Set A to the number * 8 of the track section for the
                        \ front segment of the track segment buffer

 CMP #48                \ If A >= 48, i.e. the front segment of the track
 BCS hill7              \ segment buffer is track section 6 or later, jump to
                        \ hill7 to call CheckVergeOnScreen

                        \ Otherwise we replace the call to CheckVergeOnScreen
                        \ with the following, which clears bit 7 of V to flag
                        \ the verge coordinate at index X as being visible
                        \ on-screen

 LSR V                  \ Clear bit 7 of V

 RTS                    \ Return from the subroutine

.hill7

 JMP CheckVergeOnScreen \ Implement the call that we overwrote with the call to
                        \ the hook routine, so we have effectively inserted the
                        \ above code into the main game (the JMP ensures we
                        \ return from the subroutine using a tail call)