Dashboard: SP2

       Name: SP2                                                     [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Draw a dot on the compass, given the planet/station vector
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * COMPAS calls SP2


 Draw a dot on the compass to represent the planet or station, whose normalised
 vector is in XX15.

   XX15 to XX15+2      The normalised vector to the planet or space station,
                       stored as x in XX15, y in XX15+1 and z in XX15+2


.SP2

 LDA XX15               ; Set A to the x-coordinate of the planet or station to
                        ; show on the compass, which will be in the range -96 to
                        ; +96 as the vector has been normalised

 JSR SPS2               ; Set X = A / 16, so X will be from -6 to +6, which
                        ; is the x-offset from the centre of the compass of the
                        ; dot we want to draw. Returns with the C flag clear

 TXA                    ; Set the x-coordinate of sprite 13 (the compass dot) to
 CLC                    ; 220 + X, as 220 is the pixel x-coordinate of the
 ADC #220               ; centre of the compass, and X is in the range -6 to +6,
 STA xSprite13          ; so the dot is in the x-coordinate range 214 to 226

 LDA XX15+1             ; Set A to the y-coordinate of the planet or station to
                        ; show on the compass, which will be in the range -96 to
                        ; +96 as the vector has been normalised

 JSR SPS2               ; Set X = A / 16, so X will be from -6 to +6, which
                        ; is the x-offset from the centre of the compass of the
                        ; dot we want to draw. Returns with the C flag clear

                        ; We now set the y-coordinate of sprite 13 (the compass
                        ; dot) to 186, as 186 is the pixel y-coordinate of the
                        ; centre of the compass, and X is in the range -6 to +6,
                        ; so the dot is in the y-coordinate range 180 to 192

 STX T                  ; Set T = X for use in the calculation below

 LDA #186+YPAL          ; Set A to the pixel y-coordinate of the compass centre

 SEC                    ; Set the y-coordinate of sprite 13 to A - X
 SBC T
 STA ySprite13

 LDA #247               ; Set A to 247, which is the tile number that contains a
                        ; full dot in green, for when the planet or station in
                        ; the compass is in front of us

 LDX XX15+2             ; If the z-coordinate of the XX15 vector is positive,
 BPL P%+4               ; skip the following instruction

 LDA #246               ; The z-coordinate of XX15 is negative, so the planet or
                        ; station is behind us and the compass dot should be
                        ; hollow and yellow, so set A to 246, which is the tile
                        ; number for the hollow yellow dot

 STA pattSprite13       ; Set the pattern number for sprite 13 to A, so we draw
                        ; the compass dot using the correct pattern

 RTS                    ; Return from the subroutine