====== Merge char bullets with background chars ======
by Achim

Char bullets look quite ugly when flying around. A way to achieve a more aesthetic look is to generate new chars on the fly everytime the bullet's moving one char ahead.

First thing to do is to reserve as many empty chars in your charset as you need bullets in your game.

For example 8 chars:
<code>
charset:	$2000-$2800
8 empty chars:	$27c0, $27c8, $27d0, $27d8, $27e0, $27e8, $27f0, $27f8
char values:	$f8, $f9, $fa, $fb, $fc, $fd, $fe, $ff
</code>

Now you need a positive and a negative image/char of your char bullet.\\
\\
Negative image\\
{{:base:negativechar.gif?200|}}\\
Positive image\\
{{:base:positivechar.gif?200|}}


The negative has to be done with $d021 and $d800/colorRAM, the positive should be done with $d022 and/or $d023.
$d800 should only be used for the positive when all chars got the same color RAM. Otherwise the bullets
change their color accordingly everytime they move ahead which might look stupid.

Everytime the main program places a new bullet on screen respectively moving the char bullet on screen,
it first has to read and save the char underneath the bullet.

<code>
ldy screencolumn	//column of char bullet
lda (screenposition),y	//screenposition=zp address holding the screen row		
sta charstorage,x	//save char at specific position, x pointing at currently used char bullet
jsr chargenerator	//now create new char and print it on screen
</code>

Now the new char has to be generated:

<code>
.const charsethibyte	= $20		/charset at $2000 
.const negativechar 	= $2008		//2. char in charset
.const positivechar 	= $2010		//3. char in charset

//acc still holding value of saved char
//x-reg still pointing at currently used char bullet
//y-reg still holding current screen column

chargenerator:	asl		        //figure out chardata of saved char	
		asl                     
		asl                     
		sta tmp0		//get low byte of chardata in charset -> tmp0=zp address
		lda (screenposition),y
		lsr 
		lsr
		lsr
		lsr
		lsr
		clc
		adc #charsethibyte	//charset located at $2000
		sta tmp1		//get hi byte of chardata in charset
				
		lda charsetposition,x	//fetch low byte of new chardata to be written
		sta writechar+1		//selfmod
		
		ldy #$07
!:		lda (tmp0),y		//read chardata from charset
genand:		and negativechar,y	//use negative char to punch a hole into chardata
genora:		ora positivechar,y	//and fill it with positive chardata
writechar:	sta $2700,y		//write new chardata (low byte: selfmodifying code, mind the hi byte)					
         	dey
		bpl !-			
				
		lda newcharvalue,x	//get char value of new char bullet
		ldy screencolumn	//column of char bullet
		sta (screenposition),y	//print new char bullet on screen
		rts

charsetposition:	.byte		$c0, $c8, $d0, $d8, $e0, $e8, $f0, $f8		//low bytes
newcharvalue:		.byte		$f8, $f9, $fa, $fb, $fc, $fd, $fe, $ff	
</code>

Check out these games to see how it looks:

http://csdb.dk/release/?id=16770
http://csdb.dk/release/?id=105361