Maths (Geometry): FollowGazeVector (Part 3 of 5)

       Name: FollowGazeVector (Part 3 of 5)                          [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: Calculate whether the viewing object's gaze is obstructed by a
             tile of shape 4 or 12 (i.e. a tile with one horizontal edge)
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


                        \ If we get here then the tile shape is 4 or 12, so the
                        \ tile has one horizontal edge with the other two points
                        \ being arbitrary (but not at the same height as the
                        \ horizontal edge)

 LDA yCoordHi           \ If yCoordHi is higher than any of the four tile
 CMP S                  \ corners, then the current position along the viewer's
 BCS gaze7              \ gaze is above the tile surface, so jump to gaze1 via
 CMP T                  \ gaze7 to move along the gaze vector and restart the
 BCS gaze7              \ checks
 CMP U
 BCS gaze7
 CMP V
 BCS gaze7

 JMP gaze4              \ Otherwise the current position along the viewer's
                        \ gaze is below the height of at least one tile corner,
                        \ so we consider this to be enough interference to be
                        \ blocking the viewer's gaze of any tiles that might be
                        \ partially visible beyond (which might be possible if
                        \ the slope is from left to right, for example)
                        \
                        \ So jump to gaze4 to return from the subroutine with
                        \ the C flag set to indicate that the viewer is not
                        \ looking at a tile

.gaze7

 JMP gaze1              \ Jump to gaze1 to move along the gaze vector and
                        \ restart the checks (this jump point is for use by
                        \ branching instructions)