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⌂ ∞ Revs on the BBC Micro

Drawing the track: CheckVergeOnScreen

       Name: CheckVergeOnScreen                                      [Show more]
       Type: Subroutine
   Category: Drawing the track
    Summary: Check whether a verge coordinate is on-screen
  Deep dive: Drawing the track verges

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MapSegmentsToLines calls CheckVergeOnScreen




 This routine tests whether the magnitude of a signed yaw angle is < 20. In
 other words, given a signed yaw angle x, this tests whether |x| < 20.

 As the field of view in Revs is 20 degrees, this tests whether or not a yaw
 angle is visible on-screen.

 It does this by adding 20 and then testing against 40, which gives the result
 we want as the following are all equivalent:

   -20 < x < 20

   -20 + 20 < x < 20 + 20

   0 < x < 40



 Arguments:

   X                    The index within the track verge buffer of the verge to
                        check:

                          * horizonListIndex + 40 for the left verge

                          * horizonListIndex      for the right verge



 Returns:

   V                    The result, where x is the verge yaw angle:

                          * Bit 7 is clear if |x| < 20 (visible on-screen)

                          * Bit 7 is set if |x| >= 20 (not visible)



.CheckVergeOnScreen

 LDA xVergeRightHi,X    \ Set A to the x-coordinate of the X-th entry in the
                        \ track segment list

 CLC                    \ Set A = A + 20
 ADC #20

 CMP #40                \ Set bit 7 if A >= 40, clear it if A < 40
 ROR V

 RTS                    \ Return from the subroutine