Maths (Geometry): AddScaledVector

       Name: AddScaledVector                                         [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Add a scaled vector to another vector, one axis at a time
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BuildRoadSign calls AddScaledVector


 This routine does the following calculation, one axis at a time (starting with
 the z-axis, then the y-axis and x-axis):

   [ xRoadSignCoord ]   [ xPlayerCoord ]   [ xTrackSignVector * 2 ^ (8 - Y) ]
   [ yRoadSignCoord ] = [ yPlayerCoord ] - [ yTrackSignVector * 2 ^ (8 - Y) ]
   [ zRoadSignCoord ]   [ zPlayerCoord ]   [ zTrackSignVector * 2 ^ (8 - Y) ]

 The value of Y can be varied between calls to change the scale factor on a
 per-axis basis.


 Arguments:

   A                    The relevant from xTrackSignVector, yTrackSignVector,
                        zTrackSignVector

   W                    Set to 2 for the first call

   Y                    The scale factor for this axis


.AddScaledVector

 PHA                    \ Set (V A T) = (0 A 0)
 LDA #0                 \             = A * 256
 STA T
 STA V
 PLA

 BPL addv1              \ If A is positive then V is already the correct high
                        \ byte for (V A T), so jump to addv1

 DEC V                  \ Otherwise, decrement V to &FF so it's the correct
                        \ high byte for (V A T)

                        \ We now shift (V A T) right by Y places

.addv1

 LSR V                  \ Set (V A T) = (V A T) >> 1
 ROR A
 ROR T

 DEY                    \ Decrement the shift counter

 BNE addv1              \ Loop back until we have right-shifted Y times

 STA U                  \ Set (V U T) = (V A T)
                        \             = A * 256 >> Y
                        \             = A * 2 ^ (8 - Y)
                        \
                        \ We know that V doesn't contain any data, just sign
                        \ bits, so this means:
                        \
                        \   (U T) = A * 2 ^ (8 - Y)
                        \         = xTrackSignVector * 2 ^ (8 - Y)

 LDY W                  \ Fetch the axis index from W

 DEC W                  \ Decrement the axis index in W, ready for the next call
                        \ to AddScaledVector

 LDA xPlayerCoordHi,Y   \ Set xRoadSignCoord = xPlayerCoord - (U T)
 SEC                    \
 SBC T                  \ starting with the low bytes
 STA xRoadSignCoordLo,Y

 LDA xPlayerCoordTop,Y  \ And then the high bytes
 SBC U
 STA xRoadSignCoordHi,Y

 RTS                    \ Return from the subroutine