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⌂ ∞ ▽ The Sentinel on the BBC Micro

Drawing the landscape: GetTileEdgeToLeft

       Name: GetTileEdgeToLeft                                       [Show more]
       Type: Subroutine
   Category: Drawing the landscape
    Summary: Move one tile to the left along the tile row that we are analysing
             for view edges and tile angles

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GetTileViewEdges calls GetTileEdgeToLeft
             * GetTileEdgeToRight calls via prev1




 Other entry points:

   prev1                Return from the subroutine with the C flag set



.prev1

 SEC                    \ Set the C flag to indicate that we have reached the
                        \ end of the tile row

 RTS                    \ Return from the subroutine

.GetTileEdgeToLeft

 LDY xTile              \ Set Y to the column of the tile corner we are
                        \ currently analysing when looking for the view edges
                        \ and calculating the tile's pitch and yaw angles

 BEQ prev1              \ If Y = 0 then we are already at the left end of the
                        \ tile row and can't move any further left, so jump to
                        \ prev1 to return from the subroutine with the C flag
                        \ set

 DEY                    \ Otherwise decrement Y to move one tile to the left

 JMP GetTileViewAngles  \ Jump to GetTileViewAngles to analyse the new corner to
                        \ look for the view edges and calculate the tile's pitch
                        \ and yaw angles, and return from the subroutine with
                        \ the C flag clear