base:stretching_sprites
Sprite Stretching
Sprite stretching uses the technique of setting the bits of $d017 to 1, and then back to 0 on the next rasterline. This will fool the VIC not to increase the internal sprite-gfx-pointer and display the same line of sprite-graphics again. By repeating this trick every rasterline, you can decide how many times each line of sprite-graphics will be shown.
Code example follows. Note that $d017 is set from the table on the first rasterline. On the second rasterline it is set back to 0 and then set again to a new value from the table. Repeating on the third line and on.
*= $0900 sei bit $d011 ; Wait for new frame bpl *-3 bit $d011 bmi *-3 lda #$ff ; Enable sprites sta $d015 ldx #14 ; Set some x-positions clc lda #$f0 sta $d000,x sbc #$18 dex dex bpl *-7 ldx #14 ; Set some y-positions lda #$40 sta $d001,x dex dex bpl *-5 lda #$24 ; Set sprite pointers to display this code :). ldx #7 sta $07f8,x dex bpl *-4 lda #$bd ; Set idle-pattern sta $3fff loop1 jsr StretchCalc ; Make beautiful stretching. lda #$40 ; Wait for sprite y-position cmp $d012 bne *-3 ldx #4 ; Wait a few cycles to make the d017-stretch work dex bne *-1 ldx #0 loop2 lda StretchTab,x ; $ff will stretch, 0 will step one line of graphics in the sprite sta $d017 sec lda $d011 sbc #7 ora #$18 sta $d011 ; Step d011 each line to avoid badlines bit $ea ; Make the whole loop 44 cycles = one raster line when using 8 sprites nop nop nop lda #0 ; Set back for the next line sta $d017 inx cpx #100 bne loop2 ; Loop 100 times lda #$1b ; Set back char-screen mode sta $d011 jmp loop1 StretchCalc ; Setup the stretch table ldy #0 sty YPos lda #$ff ; First clear the table sta StretchTab,y iny bne *-4 lda #0 ; Increase the starting value inc *-1 asl sta AddVal ldy #0 ; This loop will insert 16 0:s into the table.. ; At those positions the sprites will not stretch SFT_1 lda AddVal clc adc #10 sta AddVal bpl *+4 eor #$ff lsr lsr lsr lsr sec adc YPos sta YPos tax lda #0 sta StretchTab,x iny cpy #20 bcc SFT_1 rts YPos .byte 0 AddVal .byte 0 .align $100 ; Align the table to a new page, this way lda StretchTab,x always takes 4 cycles. StretchTab .dsb 256 ; Reserve 256 bytes for the table
base/stretching_sprites.txt · Last modified: 2015-04-17 04:34 by 127.0.0.1