Visibility: CheckLineDistance

       Name: CheckLineDistance                                       [Show more]
       Type: Subroutine
   Category: Visibility
    Summary: Check whether a point on a line is within the visible distance for
             the line
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ProcessLine (Part 7 of 7) calls CheckLineDistance
             * ProcessRunwayLine (Part 3 of 5) calls CheckLineDistance
             * UpdateBullets calls CheckLineDistance


 Arguments:

   X                    The line ID of the line to check

   Y                    The point ID of the point to check


 Returns:

   A                    Contains the result as follows:

                          * The current value of showLine if the point is close
                            enough to be visible

                          * 1 if the point is too far away to be visible


.CheckLineDistance

 LDA xPointHi,Y         \ Set A to the high byte of the point's x-coordinate

 BPL dist1              \ If the x-coordinate is positive, skip the following
                        \ instruction

 EOR #&FF               \ Otherwise flip the x-coordinate so it's positive, so:
                        \
                        \   A = |xPointHi|

.dist1

 CMP maxLineDistance,X  \ If A >= this line's visible distance, then the point
 BCS dist4              \ is too far away to be seen in the x-axis, so jump to
                        \ dist4 to return a "not visible" result

 LDA yPointHi,Y         \ Set A to the high byte of the point's y-coordinate

 BPL dist2              \ If the y-coordinate is positive, skip the following
                        \ instruction

 EOR #&FF               \ Otherwise flip the y-coordinate so it's positive, so:
                        \
                        \   A = |yPointHi|

.dist2

 CMP maxLineDistance,X  \ If A >= this line's visible distance, then the point
 BCS dist4              \ is too far away to be seen in the y-axis, so jump to
                        \ dist4 to return a "not visible" result

 LDA zPointHi,Y         \ Set A to the high byte of the point's z-coordinate

 BPL dist3              \ If the z-coordinate is positive, skip the following
                        \ instruction

 EOR #&FF               \ Otherwise flip the z-coordinate so it's positive, so:
                        \
                        \   A = |zPointHi|

.dist3

 CMP maxLineDistance,X  \ If A < this line's visible distance, then the point
 BCC dist5              \ is close enough to be visible in the z-axis, so jump
                        \ to dist5 to return an "is visible" result

.dist4

                        \ If we get here then the point is too far away to be
                        \ visible in at least one axis

 LDA #1                 \ Set A = 1 as the return value for a "not visible"
                        \ result

 RTS                    \ Return from the subroutine

.dist5

 LDA #0                 \ The point is close enough to be visible, so return the
 ORA showLine           \ current value of showLine (this could be achieved by a
                        \ simple LDA showLine instruction, so perhaps this more
                        \ convoluted approach is left over from a different
                        \ version of the routine)

 RTS                    \ Return from the subroutine