Drawing lines: LASLI

       Name: LASLI                                                   [Show more]
       Type: Subroutine
   Category: Drawing lines
    Summary: Draw the laser lines for when we fire our lasers
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Main flight loop (Part 3 of 16) calls LASLI


 Draw the laser lines, aiming them to slightly different place each time so
 they appear to flicker and dance. Also heat up the laser temperature and drain
 some energy.


 Other entry points:

   LASLI-1              Contains an RTS


.LASLI

 JSR DORND              ; Set A and X to random numbers

 AND #7                 ; Restrict A to a random value in the range 0 to 7

 ADC halfScreenHeight   ; Set LASY to two pixels above the centre of the
 SBC #2                 ; screen (in halfScreenHeight), plus our random number,
 STA LASY               ; so the laser dances above and below the centre point

 JSR DORND              ; Set A and X to random numbers

 AND #7                 ; Restrict A to a random value in the range 0 to 7

 ADC #X-4               ; Set LASX to four pixels left of the centre of the
 STA LASX               ; screen (#X), plus our random number, so the laser
                        ; dances to the left and right of the centre point

 LDA GNTMP              ; Add 6 to the laser temperature in GNTMP
 ADC #6
 STA GNTMP

 JSR DENGY              ; Call DENGY to deplete our energy banks by 1

 LDA QQ11               ; If this is not a space view (i.e. QQ11 is non-zero)
 BNE LASLI-1            ; then jump to MA9 to return from the main flight loop
                        ; (as LASLI-1 is an RTS)

 LDA #32                ; Set A = 32 and Y = 224 for the first set of laser
 LDY #224               ; lines (the wider pair of lines)

 JSR las                ; Call las below to draw the first set of laser lines

 LDA #48                ; Fall through into las with A = 48 and Y = 208 to draw
 LDY #208               ; a second set of lines (the narrower pair)

                        ; The following routine draws two laser lines, one from
                        ; the centre point down to point A on the bottom row,
                        ; and the other from the centre point down to point Y
                        ; on the bottom row. We therefore get lines from the
                        ; centre point to points 32, 48, 208 and 224 along the
                        ; bottom row, giving us the triangular laser effect
                        ; we're after

.las

 STA X2                 ; Set X2 = A

 LDA LASX               ; Set (X1, Y1) to the random centre point we set above
 STA X1
 LDA LASY
 STA Y1

 LDA Yx2M1              ; Set Y2 to the height in pixels of the space view,
 STA Y2                 ; which is in the variable Yx2M1, so this sets Y2 to
                        ; the y-coordinate of the bottom pixel row of the space
                        ; view

 JSR LOIN               ; Draw a line from (X1, Y1) to (X2, Y2), so that's from
                        ; the centre point to (A, 191)

 LDA LASX               ; Set (X1, Y1) to the random centre point we set above
 STA X1
 LDA LASY
 STA Y1

 STY X2                 ; Set X2 = Y

 LDA Yx2M1              ; Set Y2 to the y-coordinate of the bottom pixel row
 STA Y2                 ; of the space view (as before)

 JMP LOIN               ; Draw a line from (X1, Y1) to (X2, Y2), so that's from
                        ; the centre point to (Y, 191), and return from
                        ; the subroutine using a tail call