Drawing circles: TT14

       Name: TT14                                                    [Show more]
       Type: Subroutine
   Category: Drawing circles
    Summary: Draw a circle with crosshairs on a chart
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TT22 calls TT14
             * TT23 calls TT14


 Draw a circle with crosshairs at the current system's galactic coordinates.


.TT126

 LDA #104               ; Set QQ19 = 104, for the x-coordinate of the centre of
 STA QQ19               ; the fixed circle on the Short-range Chart

 LDA #90                ; Set QQ19+1 = 90, for the y-coordinate of the centre of
 STA QQ19+1             ; the fixed circle on the Short-range Chart

 LDA #16                ; Set QQ19+2 = 16, the size of the crosshairs on the
 STA QQ19+2             ; Short-range Chart

 JSR TT15               ; Draw the set of crosshairs defined in QQ19, at the
                        ; exact coordinates as this is the Short-range Chart

 LDA QQ14               ; Set K = QQ14 + (QQ14 / 32)
 LSR A                  ;
 LSR A                  ; So K is the circle's radius, based on the fuel level
 LSR A                  ; in QQ14 (so K is in the range 0 to 73, as the latter
 LSR A                  ; division gets rounded up by the ADC adding in the C
 LSR A                  ; flag, and QQ14 is in the range 0 to 70)
 ADC QQ14
 STA K

 JMP TT128              ; Jump to TT128 to draw a circle with the centre at the
                        ; same coordinates as the crosshairs, (QQ19, QQ19+1),
                        ; and radius K that reflects the current fuel levels,
                        ; returning from the subroutine using a tail call

.TT14

 LDA QQ11               ; If the view type in QQ11 is $9C (Short-range Chart),
 CMP #$9C               ; jump up to TT126 to draw the crosshairs and circle for
 BEQ TT126              ; that view

                        ; Otherwise this is the Long-range Chart, so we draw the
                        ; crosshairs and circle for that view instead

 LDA QQ14               ; Set K = QQ14/4 - QQ14/16
 LSR A                  ;       = 0.1875 * QQ14
 LSR A                  ;
 STA K                  ; So K scales the fuel level in QQ14 to act as the
 LSR A                  ; circle's radius, scaling the fuel level from a range
 LSR A                  ; of 0 to 70 down to a range of 0 to 13, so the fuel
 STA T1                 ; circle has a maximum radius of 13 pixels on the
 LDA K                  ; Long-range Chart
 SEC
 SBC T1
 STA K

                        ; We now set the pixel coordinates of the crosshairs in
                        ; QQ9 and QQ9+1 so they fit into the chart, with a
                        ; 31-pixel margin on the left and an 8-pixel margin at
                        ; the top (to which we will add another 24 pixels below)
                        ;
                        ; The Long-range Chart is twice as wide as it is high,
                        ; so we need to scale the y-coordinate in QQ19+1 by an
                        ; extra division by 2 when compared to the x-coordinate

 LDA QQ0                ; Set QQ19 = 31 + QQ9 - (QQ9 / 4)
 LSR A                  ;          = 31 + 0.75 * QQ9
 LSR A                  ;
 STA T1                 ; So this scales the x-coordinate from a range of 0 to
 LDA QQ0                ; 255 into a range from 31 to 222, so it fits nicely
 SEC                    ; into the Long-range Chart
 SBC T1
 CLC
 ADC #31
 STA QQ19

 LDA QQ1                ; Set QQ19+1 = 8 + (QQ10 - (QQ10 / 4)) / 2
 LSR A                  ;            = 8 + 0.375 * QQ10
 LSR A                  ;
 STA T1                 ; So this scales the y-coordinate from a range of 0 to
 LDA QQ1                ; 255 into a range from 8 to 127, so it fits nicely
 SEC                    ; into the Long-range Chart
 SBC T1
 LSR A
 CLC
 ADC #8
 STA QQ19+1

 LDA #7                 ; Set QQ19+2 = 7, the size of the crosshairs on the
 STA QQ19+2             ; Long-range Chart

 JSR TT15               ; Draw the set of crosshairs defined in QQ19, which will
                        ; be drawn 24 pixels to the right of QQ19+1

 LDA QQ19+1             ; Add 24 to the y-coordinate of the crosshairs in QQ19+1
 CLC                    ; so that the centre of the circle matches the centre
 ADC #24                ; of the crosshairs
 STA QQ19+1

                        ; Fall through into TT128 to draw a circle with the
                        ; centre at the same coordinates as the crosshairs,
                        ; (QQ19, QQ19+1), and radius K that reflects the
                        ; current fuel levels