Drawing ships: SHPPT

       Name: SHPPT                                                   [Show more]
       Type: Subroutine
   Category: Drawing ships
    Summary: Draw a distant ship as a point rather than a full wireframe
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LL9 (Part 2 of 12) calls SHPPT


.SHPPT

 JSR PROJ               ; Project the ship onto the screen, returning:
                        ;
                        ;   * K3(1 0) = the screen x-coordinate
                        ;   * K4(1 0) = the screen y-coordinate
                        ;   * A = K4+1

 ORA K3+1               ; If either of the high bytes of the screen coordinates
 BNE nono               ; are non-zero, jump to nono as the ship is off-screen

 LDY K4                 ; Set Y = the y-coordinate of the dot

 CPY #Y*2-2             ; If the y-coordinate is bigger than the y-coordinate of
 BCS nono               ; the bottom of the screen, jump to nono as the ship's
                        ; dot is off the bottom of the space view

                        ; The C flag is clear at this point as we just passed
                        ; through a BCS, so we call Shpt with the C flag clear

 JSR Shpt               ; Call Shpt to draw a horizontal 4-pixel dash for the
                        ; first row of the dot (i.e. a four-pixel dash)

 INY                    ; Increment Y to the next row (so this is the second row
                        ; of the two-pixel-high dot)

 CLC                    ; Cleat the C flag to pass to Shpt

 JSR Shpt               ; Call Shpt to draw a horizontal 4-pixel dash for the
                        ; second row of the dot (i.e. a four-pixel dash)

 BIT XX1+31             ; If bit 6 of the ship's byte #31 is clear, then there
 BVC nono               ; are no lasers firing, so jump to nono to record that
                        ; we didn't draw anything and return from the subroutine

 LDA XX1+31             ; Clear 6 in the ship's byte #31 to denote that there
 AND #%10111111         ; are no lasers firing (bit 6), as we are about to draw
 STA XX1+31             ; the laser line and this will ensure it flickers off in
                        ; the next iteration

                        ; We now draw the laser line, from the ship dot at
                        ; (X1, Y1), as set in the call to Shpt, to a point on
                        ; the edge of the screen

 LDX #1                 ; Set X = 1 to use as the x-coordinate for the end of
                        ; the laser line for when z_lo < 128 (so the ship fires
                        ; to our left)

 LDA XX1+6              ; Set A = z_lo

 BPL shpt1              ; If z_lo < 128, jump to shpt1 to leave X = 1

 LDX #255               ; Set X = 255 to use as the x-coordinate for the end of
                        ; the laser line for when z_lo >= 128 (so the ship fires
                        ; to our left)
                        ;
                        ; This makes the ship fire to our left and right as it
                        ; gets closer to us, as z_lo reduces from 255 to 0 for
                        ; each reduction in z_hi

.shpt1

 STX X2                 ; Set X2 to the x-coordinate of the end of the laser
                        ; line

 AND #63                ; Set Y2 = z_lo, reduced to the range 0 to 63, plus 32
 ADC #32                ;
 STA Y2                 ; So the end of the laser line moves up and down the
                        ; edge of the screen (between y-coordinate 32 and 95) as
                        ; the ship gets closer to us, as z_lo reduces from 255
                        ; to 0 for each reduction in z_hi

 JSR LOIN               ; Draw the laser line from (X1, Y1) to (X2, Y2)

.nono

 LDA #%11110111         ; Clear bit 3 of the ship's byte #31 to record that
 AND XX1+31             ; nothing is being drawn on-screen for this ship
 STA XX1+31

 RTS                    ; Return from the subroutine

.Shpt

                        ; This routine draws a horizontal 4-pixel dash, for
                        ; either the top or the bottom of the ship's dot
                        ;
                        ; We always call this routine with the C flag clear

 LDA K3                 ; Set A = screen x-coordinate of the ship dot

 STA X1                 ; Set X1 to the screen x-coordinate of the ship dot

 ADC #3                 ; Set A = screen x-coordinate of the ship dot + 3
                        ; (this works because we know the C flag is clear)

 BCS shpt2              ; If the addition overflowed, jump to shpt2 to return
                        ; from the subroutine without drawing the dash

 STA X2                 ; Store the x-coordinate of the ship dot in X1, as this
                        ; is where the dash starts

 STY Y1                 ; Store Y in both y-coordinates, as this is a horizontal
 STY Y2                 ; dash at y-coordinate Y

 JMP LOIN               ; Draw the dash from (X1, Y1) to (X2, Y2), returning
                        ; from the subroutine using a tail call

.shpt2

 PLA                    ; Pull the return address from the stack, so the RTS
 PLA                    ; below actually returns from the subroutine that called
                        ; LL9 (as we called SHPPT from LL9 with a JMP)

 JMP nono               ; Jump to nono to record that we didn't draw anything
                        ; and return from the subroutine