====== Sprite FPP ======

The basic theory behind a Sprite FPP is quite simple: Change sprite-pointer value every rasterline. Though there are a few things to prepare before it works in reality.

First we want to use a bigger graphics area than one sprite. With a normal [[base:stretching_sprites|$d017-stretcher]] we can make the sprites arbitrary high, and at the same time we only use one line of the sprite for graphics. If we then place our 8 sprites beside eachother, increasing the y-position by 1 for each sprite, then our FPP will display different graphics for each sprite. We have a 192 pixels wide sprite-layer with any desired height.

Note that all sprite-pointers have to be set each line for the FPP to work as we wish. Since they all use different lines of the sprite, the same value can be set to all sprite pointers. Using all 8 sprites, there will be 44 cycles availible each line for you to do this, and the $d017-stretching. Unrolled code is about the only option. There is no time for badlines here, so mostly you will need to use the $d011 idle-mode, which requires opening the upper/lower borders.

In the following code example i have kept it simple by using a loop for the FPP. Thus not all 8 sprites are being updated, only 6, so i save some cycles there to be able to loop the code. "FppCalc" is just a st0pid routine that generates some values to the FPP-table. Here you can do the real beauty of the effect by putting some more effort to it :).

<code>
	*= $0900

	sei
	lda #$ff ; Enable sprites
	sta $d015

	ldx #14 ; Set some sprite x-positions
	sec
	lda #$f0
	sta $d000,x
	sbc #$18
	dex
	dex
	bpl *-7

	ldx #14 ; Set some sprite y-positions
	clc
	lda #$40
	sta $d001,x
	adc #1
	dex
	dex
	bpl *-7

	lda #$bd ; Set idle-pattern
	sta $3fff
loop1
	lda #$f9 ; Remove upper/lower border
	cmp $d012
	bne *-3
	lda #0
	sta $d011
	bit $d011
	bpl *-3
	lda #8 ; Set $d011 to idle mode -> no badlines
	sta $d011

	jsr FppCalc ; Make beautiful FPP.

	lda #$48 ; Wait for sprite y-position
	cmp $d012
	bne *-3

	ldx #5 ; Wait a few cycles to make the d017-stretch work..
	dex    ; ..and all sprite pointers update on the same line.
	bne *-1
	nop

	ldx #0
loop2
	dec $d017 ; Do $d017-stretching as we have learned
	lda FppTab,x ; Get value from table..
	sta $07f8    ; ..and store to all sprite pointers
	sta $07f9
	sta $07fa
	sta $07fb
	sta $07fc
	sta $07fd

	; Note: here we should store to $07fe and $07ff also,
	; but there are not enough cycles in this loop (44 cycles).
	; To make FPP on all sprites, unroll the loop.

	inc $d017 ; Set back $d017
	inx
	bpl loop2

	jmp loop1

FppCalc
	lda #0 ; Increase adding value
	inc *-1
	sta AddVal

	clc
	lda #0 ; Increase starting value
	adc #3
	sta *-3
	sta Value

	ldy #0
FC_1
	clc ; Calculate some obscure values to the FppTable
	lda AddVal
	adc #3
	sta AddVal
	bpl *+4
	eor #$ff
	lsr
	lsr
	lsr
	lsr
	lsr
	clc
	adc Value
	sta Value
	sta FppTab,y
	iny
	bpl FC_1
	rts

AddVal	.byte 0
Value	.byte 0

	.align $100 ; Align the table to a new page, this way lda FppTab,x always takes 4 cycles.
FppTab
	.dsb 256 ; Reserve 256 bytes for the table
</code>