Charts: DrawCrosshairs

       Name: DrawCrosshairs                                          [Show more]
       Type: Subroutine
   Category: Charts
    Summary: Draw a set of moveable crosshairs as a square reticle
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT103 calls DrawCrosshairs


 Arguments:

   QQ19                 The pixel x-coordinate of the centre of the crosshairs

   QQ19+1               The pixel y-coordinate of the centre of the crosshairs


.DrawCrosshairs

 LDA #248               ; Set the pattern number for sprite 15 to 248, which
 STA pattSprite15       ; contains a one-tile square outline that we can use as
                        ; a square reticle to implement crosshairs on the chart

 LDA #%00000001         ; Set the attributes for sprite 15 as follows:
 STA attrSprite15       ;
                        ;   * Bits 0-1    = sprite palette 1
                        ;   * Bit 5 clear = show in front of background
                        ;   * Bit 6 clear = do not flip horizontally
                        ;   * Bit 7 clear = do not flip vertically

 LDA QQ19               ; Set SC2 to the pixel x-coordinate of the centre of the
 STA SC2                ; crosshairs

 LDY QQ19+1             ; Set Y to the pixel y-coordinate of the centre of the
                        ; crosshairs

 LDA #15                ; Set X = 15 * 4, so X is the index of sprite 15 within
 ASL A                  ; the sprite buffer, as each sprite takes up four bytes
 ASL A                  ; (in other words, xSprite0 + X and ySprite0 + X are the
 TAX                    ; addresses of the x- and y-coordinates of sprite 15 in
                        ; the sprite buffer)

 LDA SC2                ; Set the pixel x-coordinate of sprite 15 to SC2 - 4
 SEC                    ;
 SBC #4                 ; So the centre of the square reticle in sprite 15 is at
 STA xSprite0,X         ; x-coordinate SC2, as the reticle is eight pixels wide

 TYA                    ; Set the pixel y-coordinate of sprite 15 to Y + 10
 CLC                    ;
 ADC #10+YPAL           ; So the reticle is drawn 10 pixels below the coordinate
 STA ySprite0,X         ; in the QQ19+1 argument (this takes the 14-pixel high
                        ; chart title into consideration, and ensures that the
                        ; centre of the reticle is over the correct coordinates
                        ; as the sprite is eight pixels high)

 RTS                    ; Return from the subroutine