**Overlapping Raster Bars**

You've seen them all in demos, intros and possibly games. Overlapping raster (or rasters that scroll or bounce over existing raster bars) may look and sound difficult, but apparently after making a complete raster bar. They are actually quite easy to make and learn. These also make pretty nice effects.

//An example of an overlapping raster bar in WOD intro designer 2 //

{{:base:overlappingrasters2.png?400|}}

//An example of an overlapping raster I made//

{{:base:overlappingrasters.png?400|}}

So then how does this trick occur? We have three tables. One of which stores the actual colour table to the raster at the start. Then there is a second table which creates the bars that should scroll over the existing raster. Finally, there is a third table, which should backup the colours of the first table, before the second raster is stored.

What about performing the trick? First of all, an IRQ with a raster bar should be created, like as usual. Then a subroutine which calls the raster control code. Here's the code for the actual raster overlap routine. This version will only scroll the overlapping raster (it doesn't do the cool bounce tricks):

<code>overlapping_raster:
  lda rspeed         ;Control speed for scrolling/overlap
  cmp #2             ;before calling routine.
  beq waitend
  inc rspeed
  rts
waitend
  lda #0               
  sta rspeed
  jsr movesilver     ;Subroutine to move silver bars
  jsr backup         ;Subroutine to backup still raster colours
  rts
  
;This subroutine performs the main movement of the black + grey raster bars
  
movesilver
   lda ras2end-1          ;Grab last byte of the silver bar raster table
   sta rtemp1             ;put to a temp byte.
   ldx #ras2end-ras2start 
loop2   
   lda ras2start,x        ;Shift the raster forward to the next
   sta ras2start+1,x      ;byte
   dex
   bpl loop2
   lda rtemp1
   sta ras2start
   rts
   
   ;This routine does a backup of the main static raster bar
   
backup
   ldx #ras1backupend-ras1backup 
loop3   
   lda ras1backup,x 
   sta ras1start,x 
   dex
   bpl loop3
   
   ;This routine reads the silver raster bars. If the zero byte (colour BLACK)
   ;is detected on the table, it skips that byte and will read 
   ;the next byte. Any colour other than the zero byte will be stored
   ;to the main raster's table.
   
copysilver
   ldx #ras2end-ras2start 
loop4   
   lda ras2start,x
   cmp #0
   beq next 
   sta ras1start,x
next
   dex
   bpl loop4
   rts
   
; ... Some pointers and tables:

rtemp1 !byte 0
rtemp2 !byte 0
rspeed !byte 0


ras1start
  !byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $00,$06,$00,$06,$06,$00,$06,$06,$0e,$06,$0e,$0e,$06,$0e,$0e,$03
  !byte $0e,$03,$03,$0e,$03,$03,$01,$03,$01,$0e,$01,$03,$0e,$03,$03,$0e
  !byte $03,$0e,$0e,$06,$0e,$0e,$06,$0e,$06,$06,$00,$06,$06,$00,$06,$00
  !byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
ras1end
  !byte 0
  
ras2start
  !byte $0b,$0c,$0f,$01,$0f,$0c,$0b,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $0b,$0c,$0d,$01,$0f,$0c,$0b,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $0b,$0c,$0d,$01,$0f,$0c,$0b,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $0b,$0c,$0d,$01,$0f,$0c,$0b,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $0b,$0c,$0d,$01,$0f,$0c,$0b,$00,$00,$00,$00,$00,$00,$00,$00,$00
ras2end 
  !byte 0
  
ras1backup
  !byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  !byte $00,$06,$00,$06,$06,$00,$06,$06,$0e,$06,$0e,$0e,$06,$0e,$0e,$03
  !byte $0e,$03,$03,$0e,$03,$03,$01,$01,$01,$01,$01,$03,$0e,$03,$03,$0e
  !byte $03,$0e,$0e,$06,$0e,$0e,$06,$0e,$06,$06,$00,$06,$06,$00,$06,$00
  !byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
ras1backupend
  !byte 0 
  
 </code>