.DrawLightning ; The rectangle is split into a top half and a bottom ; half, with a bolt in the top half and a bolt in the ; bottom half, and we draw each bolt in turn LDA K+1 ; Set XX2+1 = K+1 / 2 LSR A ; STA XX2+1 ; So XX2+1 contains a quarter of the height of the ; rectangle containing the lightning LDA K+3 ; Set K3 = K+3 - XX2+1 + 1 SEC ; SBC XX2+1 ; So K3 contains the y-coordinate of the centre of the CLC ; top lightning bolt (i.e. the invisible horizontal line ADC #1 ; through the centre of the top bolt) STA K3 JSR lite1 ; Call lite1 below to draw the top lightning bolt along ; a centre line at y-coordinate K+3 LDA K+3 ; Set K3 = K+3 + XX2+1 CLC ; ADC XX2+1 ; So K3 contains the y-coordinate of the centre of the STA K3 ; bottom lightning bolt (i.e. the invisible horizontal ; line through the centre of the bottom bolt) ; Fall through into lite1 to draw the second lightning ; bolt along a centre line at y-coordinate K+3 .lite1 ; We now draw a lightning bolt along an invisible centre ; line at y-coordinate K+3 LDA K ; Set STP = K / 4 LSR A ; LSR A ; As K is the half-width of the rectangle containing the STA STP ; lightning, this means STP is 1/8 of the width of the ; lightning rectangle ; ; We use this value to step along the rectangle from ; left to right, so we can draw the lightning bolt in ; eight equal-width segments LDA K+2 ; Set X1 = K+2 - K SEC ; SBC K ; So X1 contains the x-coordinate of the left edge of STA X1 ; the rectangle containing the lightning bolt LDA K3 ; Set Y1 = K3 STA Y1 ; ; So Y1 contains the y-coordinate of the centre of the ; lightning bolt, and (X1, Y1) therefore contains the ; pixel coordinate of the left end of the lightning bolt LDY #7 ; We now draw eight segments of lightning, zig-zagging ; above and below the invisible centre line at ; y-coordinate K3 .lite2 JSR DORND ; Set Q to a random number in the range 0 to 255 STA Q LDA K+1 ; Set A to K+1, which is half the height of the ; rectangle containing the lightning, which is the same ; as the full height of the rectangle containing the ; lightning bolt we are drawing JSR FMLTU ; Set A = A * Q / 256 ; = K+1 * rand / 256 ; ; So A is a random number in the range 0 to the maximum ; height of the lightning bolt we are drawing CLC ; Set Y2 = K3 + A - XX2+1 ADC K3 ; SEC ; In the above, K3 is the y-coordinate of the centre of SBC XX2+1 ; the lightning bolt, XX2+1 contains half the height of STA Y2 ; the lightning bolt, and A is a random number between 0 ; and the height of the lightning bolt, so this sets Y2 ; to a y-coordinate that is centred on the centre line ; of the lightning bolt, and is a random distance above ; or below the line, and which fits within the height of ; the lightning bolt ; ; We can therefore use this as the y-coordinate of the ; next point along the zig-zag of the lightning bolt LDA X1 ; Set X2 = X1 + STP CLC ; ADC STP ; So X2 is the x-coordinate of the next point along the STA X2 ; lightning bolt, and (X2, Y2) is therefore the next ; point along the lightning bolt JSR LOIN ; Draw a line from (X1, Y1) to (X2, Y2) to draw the next ; segment of the bolt LDA SWAP ; If SWAP is non-zero then we already swapped the line BNE lite3 ; coordinates around during the drawing process, so we ; can jump to lite3 to skip the following coordinate ; swap LDA X2 ; Set (X1, Y1) to (X2, Y2), so (X1, Y1) contains the new STA X1 ; end coordinates of the lightning bolt, now that we LDA Y2 ; just drawn another segment of the bolt STA Y1 .lite3 DEY ; Decrement the segment counter in Y BNE lite2 ; Loop back to draw the next segment until we have drawn ; seven of them ; We finish off by drawing the final segment, which we ; draw from the current end of the zig-zag to the right ; end of the invisible horizontal line through the ; centre of the bolt, so the bolt starts and ends at ; this height LDA K+2 ; Set X2 = K+2 + K CLC ; ADC K ; So X2 contains the x-coordinate of the right edge of STA X2 ; the rectangle containing the lightning LDA K3 ; Set Y2 = K3 STA Y2 ; ; So Y2 contains the y-coordinate of the centre of the ; lightning bolt, and (X2, Y2) therefore contains the ; pixel coordinate of the right end of the lightning ; bolt JSR LOIN ; Draw a line from (X1, Y1) to (X2, Y2) to draw the ; final segment of the bolt RTS ; Return from the subroutineName: DrawLightning [Show more] Type: Subroutine Category: Flight Summary: Draw a lightning effect for the launch tunnel and E.C.M. that consists of two random lightning bolts, one above the otherContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DrawLightning_b6 calls DrawLightning
Arguments: K Half the width of the rectangle containing the lightning K+1 Half the height of the rectangle containing the lightning K+2 The x-coordinate of the centre of the lightning K+3 The y-coordinate of the centre of the lightning
[X]
Subroutine DORND (category: Maths (Arithmetic))
Generate random numbers
[X]
Subroutine FMLTU (category: Maths (Arithmetic))
Calculate A = A * Q / 256
[X]
Subroutine LOIN (Part 1 of 7) (category: Drawing lines)
Draw a line: Calculate the line gradient in the form of deltas
[X]
Label lite1 is local to this routine
[X]
Label lite2 is local to this routine
[X]
Label lite3 is local to this routine