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⌂ ∞ Revs on the BBC Micro

Track geometry: GetSegmentVector

       Name: GetSegmentVector                                        [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Fetch a segment vector from the track data file
  Deep dive: Building a 3D track from sections and segments

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * BuildCarObjects (Part 3 of 3) calls GetSegmentVector
             * GetTrackSegment (Part 1 of 3) calls GetSegmentVector




 Arguments:

   Y                    The index of the coordinate in the track data blocks



 Returns:

   (SS T)               The Y-th entry from xTrackSegmentI as a 16-bit signed
                        integer

   (TT U)               The Y-th entry from yTrackSegmentI as a 16-bit signed
                        integer

   (UU V)               The Y-th entry from zTrackSegmentI as a 16-bit signed
                        integer



.GetSegmentVector

 LDA #0                 \ Zero the high bytes of (SS T), (TT U) and (UU V)
 STA SS
 STA TT
 STA UU

 LDA xTrackSegmentI,Y   \ Set T = the Y-th entry from xTrackSegmentI
 STA T

 BPL coor1              \ If the byte we just fetched is negative, decrement
 DEC SS                 \ the high byte in SS to &FF, so (SS T) has the correct
                        \ sign

.coor1

 LDA yTrackSegmentI,Y   \ Set U = the Y-th entry from yTrackSegmentI
 STA U

 BPL coor2              \ If the byte we just fetched is negative, decrement
 DEC TT                 \ the high byte in TT to &FF, so (TT U) has the correct
                        \ sign

.coor2

 LDA zTrackSegmentI,Y   \ Set V = the Y-th entry from zTrackSegmentI
 STA V

 BPL coor3              \ If the byte we just fetched is negative, decrement
 DEC UU                 \ the high byte in UU to &FF, so (UU V) has the correct
                        \ sign

.coor3

 LDA directionFacing    \ If our car is facing forwards, jump to coor5 to return
 BEQ coor5              \ from the subroutine

                        \ We are facing backwards, so we need to negate the
                        \ coordinate we just fetched

 LDX #2                 \ We are about to negate the following 16-bit variables:
                        \
                        \   (SS T)
                        \   (TT U)
                        \   (UU V)
                        \
                        \ so set a counter in X to use as an index that loops
                        \ through 2, 1 and 0, as:
                        \
                        \   (TT U) = (SS+1 U+1)
                        \   (UU V) = (SS+2 U+2)
                        \
                        \ The following comments are for (SS T), but the same
                        \ process applies for (TT U) and (UU V)

.coor4

 LDA #0                 \ Set (SS T) = 0 - (SS T)
 SEC                    \
 SBC T,X                \ starting with the low bytes
 STA T,X

 LDA #0                 \ And then the high bytes
 SBC SS,X
 STA SS,X

 DEX                    \ Decrement the loop counter to move on to the next
                        \ variable

 BPL coor4              \ Loop back until we have negated all three 16-bit
                        \ variables

.coor5

 RTS                    \ Return from the subroutine