Glossary/Demos
Changing one single colour while the raster is drawing into the screen occurs in a split of the scanline, which looks like horizontally divide a scanline in two (or more) colors, from side to side of the screen.
It has not to be confused with a similar result obtained by changing different colors into specific chars, or similar tricks.
The Raster Scroll is a form of split raster where the position of the first raster is offset each frame. This way, the entire split raster moves like a big horizontal scrolltext.
On TED based machines, this effect can be easily distinguished from the fake one, because of the White Pixel's Bug.
Far from technicalities, it's possible to imagine that the TED swaps between two color in the same scanline in a slowly way, producing a white pixel into the transition zone of both colors, which results in a "snow" view where several of those are performed. This bug can't be circumvented, albeit it's still possible to cover it by characters'masking.
This quickly written code displays a simple split and its original bug, also performing a FLD in order to keep it clean from potential badlines:
SEI
STA $FF3F
up LDA #$B2 ; set red background color
STA $FF15 ; ---
LDA #$42 ; wait for a
loop CMP $FF1D ; fixed scanline
BNE loop ; ---
NOP
bigloop LDA $FF1D ; do FLD
ADC #$04 ;
AND #$07 ;
ORA #$18 ;
STA $FF06 ; ---
LDA #$46 ; set blue scanline
STA $FF15 ; ---
LDX #$80 ; set FLD ending
NOP
NOP
NOP
NOP
NOP
NOP
LDA $0000
LDA #$55 ; set green scanline
STA $FF15 ; ---
NOP
NOP
NOP
NOP
NOP
NOP
NOP
CPX $FF1D
BNE bigloop
LDA #$1B
STA $FF06
JMP up
Examples:
Raster splits pre-90ies from Muffbusters in MB Demo's 2nd part.
Digital Dream has a FLD bouncing raster scroll almost anywhere.
Snow, snow, snow like xmas! Slowflow is a good specimen about multiple white pixel bug.
To Be Ashamed shows some great examples about real and fake raster splits. Pit/A-System put some real ones in the bottom, whereas all the others are differently coloured characters with $FF14 changed every scanline.
raster, split, bug, scroll, white, pixel, badline
FLD, White Pixel Bug |
