Extra tracks: HookFixHorizon

       Name: HookFixHorizon                                          [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Apply the horizon line in A instead of horizonLine
  Deep dive: Secrets of the extra tracks
             Code hooks in the extra tracks
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * newContentHi calls HookFixHorizon
             * newContentLo calls HookFixHorizon


 This routine is called from GetTrackAndMarkers. It does the following:

  * Cut objects off at the track line in A rather than horizonLine

  * Collapse the left verge of the track into the right verge, but only for a
    few entries just in front of the horizon section, i.e. for the track
    section list and the first three entries in the track segment list

  * If the track section at the horizon is one of sections 0 to 5, zero the
    vergeDataRight and vergeDataLeft entries for the horizon section index to
    hide the verge


 Arguments:

   A                    The updated value of horizonLine

   Y                    The horizon section index in the verge buffer from
                        horizonListIndex


.HookFixHorizon

 STA &1FEA              \ Modify the DrawObject routine at dobj3 instruction #6
                        \ so that objects get cut off at the track line number
                        \ in A instead of horizonLine when they are hidden
                        \ behind a hill

 STA yVergeLeft,Y       \ Set the pitch angle for the left side of the horizon
                        \ line in the track verge buffer to the updated value of
                        \ horizonLine (this is the instruction that we overwrote
                        \ with the call to the hook routine, so this makes sure
                        \ we still do this)

                        \ We now work through the verge buffer from index Y up
                        \ to index 8, and do the following for each entry:
                        \
                        \   * If xVergeRight < xVergeLeft, set
                        \     xVergeRight= xVergeLeft
                        \
                        \   * Set yVergeRight= yVergeLeft
                        \
                        \ This appears to squeeze the left verge of the track
                        \ into the right verge, but only for a few entries just
                        \ in front of the horizon section, i.e. for the track
                        \ section list and the first three entries in the track
                        \ segment list

.coll1

 LDA xVergeRightLo,Y    \ Set A= xVergeRight - xVergeLeft for the horizon
 SEC                    \
 SBC xVergeLeftLo,Y     \ starting with the low bytes

 LDA xVergeRightHi,Y    \ And then the high bytes
 SBC xVergeLeftHi,Y

 BPL coll2              \ If the result is positive, jump to coll2 to skip the
                        \ following

                        \ If we get here then the result is negative, so
                        \ xVergeRight < xVergeLeft

 LDA xVergeRightLo,Y    \ Set xVergeRight= xVergeLeft
 STA xVergeLeftLo,Y
 LDA xVergeRightHi,Y
 STA xVergeLeftHi,Y

.coll2

 LDA yVergeRight,Y      \ Set yVergeRight= yVergeLeft
 STA yVergeLeft,Y

 CPY #6                 \ If the horizon section index >= 6, jump to coll3
 BCS coll3

                        \ If we get here then the track section at the horizon
                        \ is one of sections 0 to 5

 LDA #0                 \ Zero the vergeDataRight and vergeDataLeft entries for
 STA vergeDataRight,Y   \ the horizon section index
 STA vergeDataLeft,Y

.coll3

 INY                    \ Increment the verge buffer index

 CPY #9                 \ Loop back until we have processed up to index 8
 BCC coll1

 LDY horizonListIndex   \ Restore the value of Y that we had on entering the
                        \ hook routine

 RTS                    \ Return from the subroutine