Track geometry: SpinTrackSection

       Name: SpinTrackSection                                        [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Apply spin to a section in the track section list
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GetSectionAngles (Part 1 of 3) calls SpinTrackSection


 Arguments:

   Y                    The index of the entry in the track section list:

                        Y = 0 to 5 (update the right verge):

                          * Reset vergeDataRight to zero

                          * Subtract spinYawAngle from the yaw angles in
                            xVergeRightLo, xVergeRightHi

                          * Subtract spinPitchAngle from the pitch angle in
                             yVergeRight

                          * Update horizonListIndex and horizonLine

                        Y = 0 to 5 + 40 (update the left verge):

                          * Reset vergeDataLeft to zero

                          * Subtract spinYawAngle from the yaw angles in
                            xVergeLeftLo, xVergeLeftHi

                          * Subtract spinPitchAngle from the pitch angle in
                             yVergeLeft

                          * Update horizonListIndex and horizonLine


.SpinTrackSection

 LDA #0                 \ Set the Y-th entry in vergeDataRight to 0, to reset
 STA vergeDataRight,Y   \ the colour of the verge mark to black (this is
                        \ recalculated in the GetVergeAndMarkers routine)

 LDA xVergeRightLo,Y    \ Set xVergeRight = xVergeRight - spinYawAngle
 SEC                    \
 SBC spinYawAngleHi     \ starting with the low bytes
 STA xVergeRightLo,Y

 LDA xVergeRightHi,Y    \ And then the high bytes
 SBC spinYawAngleTop
 STA xVergeRightHi,Y

 LDA yVergeRight,Y      \ Set A = Y-th entry in yVergeRight - spinPitchAngle
 SEC
 SBC spinPitchAngle

 STA yVergeRight,Y      \ Store the result in the Y-th entry in yVergeRight

 CMP horizonLine        \ If A < horizonLine, then this track section is lower
 BCC rott1              \ than the current horizon, so jump to rott1 to return
                        \ from the subroutine

 STA horizonLine        \ Otherwise this track section is higher than the
                        \ current horizon pitch angle, so the track obscures the
                        \ horizon and we need to update horizonLine to this
                        \ new pitch angle

 STY horizonListIndex   \ Set horizonListIndex to the track section list index
                        \ number in Y

.rott1

 RTS                    \ Return from the subroutine