Extra tracks: HookSlopeJump

       Name: HookSlopeJump                                           [Show more]
       Type: Subroutine
   Category: Extra tracks
    Summary: Jump the car when driving fast over sloping segments
  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:
             * ModifyGameCode (Part 3 of 3) calls HookSlopeJump


 This routine is called from part 5 of ApplyElevation to jump the car off the
 ground when driving fast over sloping segments.

 If the car is on the ground, replace the heightAboveTrack * 4 part of the
 car's y-coordinate calculation with playerSpeedHi * yTrackSegmentI * 4, to
 give:

   (yPlayerCoordTop yPlayerCoordHi) =   (ySegmentCoordIHi ySegmentCoordILo)
                                      + carProgress * yTrackSegmentI
                                      + playerSpeedHi * yTrackSegmentI * 4
                                      + 172

 So driving fast over sloping segments can make the car jump.


 Arguments:

   A                    Current value of heightAboveTrack


.HookSlopeJump

 BNE slop1              \ If A is non-zero, skip the following (so the hook has
                        \ no effect when the car is off the ground)

                        \ If we get here then heightAboveTrack = 0, so the car
                        \ is on the ground

 LDA playerSpeedHi      \ Set A = the high byte of the current speed

 JSR MultiplyHeight     \ Set:
                        \
                        \   A = A * yTrackSegmentI
                        \     = playerSpeedHi * yTrackSegmentI
                        \
                        \ The value given in yTrackSegmentI is the y-coordinate
                        \ of the segment vector, i.e. the vector from this
                        \ segment to the next, which is the same as the change
                        \ in height as we move through the segment
                        \
                        \ So this value is higher with greater speed and on
                        \ segments that have higher slopes

 BPL slop1              \ If A is positive, skip the following instruction

 DEC W                  \ Decrement W to &FF, so (W A) has the correct sign

.slop1

 ASL A                  \ Implement the shifts that we overwrote with the call
 ROL W                  \ to the hook routine, so we have effectively inserted
                        \ the above code into the main game

 RTS                    \ Return from the subroutine

 EQUB &00, &00          \ These bytes appear to be unused