Maths (Geometry): RotateCoordToCar

       Name: RotateCoordToCar                                        [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Rotate a vector from the 3D world coordinate system into the frame
             of reference of the player's car
  Deep dive: The core driving model
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyDrivingModel calls RotateCoordToCar


 Calculate the following:

   [ xVelocity ]   [ cosYawAngle   0   -sinYawAngle ]   [ xPlayerSpeed ]
   [     -     ] = [      0        1         0      ] . [ yPlayerSpeed ]
   [ zVelocity ]   [ sinYawAngle   0    cosYawAngle ]   [ zPlayerSpeed ]

 This rotates the player's delta vector from the 3D world coordinate system to
 the frame of reference of the player's car.

 The individual calculations are as follows:

   xVelocity = xPlayerSpeed * cosYawAngle - zPlayerSpeed * sinYawAngle

   zVelocity = xPlayerSpeed * sinYawAngle + zPlayerSpeed * cosYawAngle


.RotateCoordToCar

 LDY #0                 \ Set Y = 0, so in the call to RotateVector, variableY
                        \ is xPlayerSpeed and variableY+1 is zPlayerSpeed

 LDA #8                 \ Set A = 8, so in the call to RotateVector, we store
                        \ the result in xVelocity and zVelocity

 LDX #%11000000         \ Set bits 6 and 7 of X, to set the polarity in the call
                        \ to RotateVector

 BNE RotateVector       \ Jump to RotateVector to calculate the following:
                        \
                        \   xVelocity =   xPlayerSpeed * cosYawAngle
                        \               - zPlayerSpeed * sinYawAngle
                        \
                        \   zVelocity =   xPlayerSpeed * sinYawAngle
                        \               + zPlayerSpeed * cosYawAngle
                        \
                        \ This BNE is effectively a JMP as X is never zero