Track geometry: GetTrackAndMarkers

       Name: GetTrackAndMarkers                                      [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Calculate the 3D coordinates of the track and corner markers
  Deep dive: The track verges
             Corner markers
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MainDrivingLoop (Part 2 of 5) calls GetTrackAndMarkers


 Other entry points:

   gtrm2+6              Calculate the following for segment Y:

                          horizonTrackWidth = |xVergeRightHi - xVergeLeftHi| / 2


.GetTrackAndMarkers

 LDA #0                 \ Set horizonLine = 0, so we can calculate a new pitch
 STA horizonLine        \ angle for the horizon in the following process

 JSR GetSectionAngles   \ Get the yaw and pitch angles for the inner and outer
                        \ track sections in the track section list and store the
                        \ results in xVergeRight/Left and yVergeRight/Left

 LDA #255               \ Set edgeDistanceHi = 255, so GetSegmentAngles can set
 STA edgeDistanceHi     \ it to the distance of the nearest verge

 LDA #13                \ Set edgeSegmentNumber = 13, as the default value for
 STA edgeSegmentNumber  \ the number of the segment within the track segment
                        \ list that is closest to the player's car

 LDA #0                 \ Fetch the index details of the right track segments
 JSR GetSegmentDetails

 LDA #6                 \ Get the yaw and pitch angles for the segments (and the
 JSR GetSegmentAngles   \ verge marks and corner markers) along the right side
                        \ of the track and store the results in xVergeRight,
                        \ yVergeRight, xMarker and vergeDataRight

 LDA segmentListPointer \ Set segmentListRight = segmentListPointer
 STA segmentListRight   \
                        \ So it contains the index of the last entry in the
                        \ track segment list for the right side of the track

 LDA #%10000000         \ Fetch the index details of the left track segments
 JSR GetSegmentDetails

 LDA #46                \ Get the yaw and pitch angles for the segments (and the
 JSR GetSegmentAngles   \ verge marks and corner markers) along the left side
                        \ of the track and store the results in xVergeLeft,
                        \ yVergeLeft, xMarker and vergeDataLeft

 LDA horizonListIndex   \ If horizonListIndex < 40, then this is a valid index
 CMP #40                \ into the track verge buffer so jump to gtrm1 to skip
 BCC gtrm1              \ the following three instructions

 SEC                    \ Set horizonListIndex = horizonListIndex - 40
 SBC #40                \
 STA horizonListIndex   \ so if we set horizonListIndex to the index for the
                        \ outer track coordinates, this corrects the value to
                        \ the index for the inner coordinates

.gtrm1

 TAY                    \ Set Y to the corrected value of horizonListIndex

 STY prevHorizonIndex   \ Store the horizon section index in prevHorizonIndex,
                        \ so we can refer to it in the next call to
                        \ GetTrackAndMarkers

 LDA horizonLine        \ If horizonLine < 79, then the horizon line is a valid
 CMP #79                \ number, so jump to gtrm2 to skip the following two
 BCC gtrm2              \ instructions

 LDA #78                \ Set horizonLine = 78, so the maximum value for the
 STA horizonLine        \ horizon line is 78

.gtrm2

 STA yVergeRight,Y      \ Set the pitch angle for the right side of the horizon
                        \ line in the track verge buffer to the updated value of
                        \ horizonLine

 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

 LDA xVergeRightHi,Y    \ Set A = xVergeRightHi - xVergeLeftHi for the horizon
 SEC                    \ section
 SBC xVergeLeftHi,Y

 JSR Absolute8Bit       \ Set A = |A|, so A contains the arc of the track at
                        \ the horizon (i.e. the track width on the section or
                        \ segment at the horizon) in terms of the high bytes

 LSR A                  \ Set horizonTrackWidth = |A| / 2
 STA horizonTrackWidth  \
                        \ So horizonTrackWidth contains half the width of the
                        \ track on the horizon, in terms of the high bytes

 RTS                    \ Return from the subroutine