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base:waving_sprite_scroll [2020-11-14 00:00] (current)
mace created
Line 1: Line 1:
 +====== Waving sprite scroll ======
 +
 +The sprites 1 through 8 are positioned on the screen next to each other with a fixed spacing. The scrolling takes place by moving them all a bit to the left, 4 pixels at a time, until sprite 1 reaches X≤0. At that moment sprite 2 is 4 pixels to the right of where sprite 1 started.
 +The sprites are then all positioned back to their starting position and the content is shifted, so that the value of sprites 8 is in 7, 7 in 6, etc. A new letter is fetched from the scroll text and the corresponding pointer value is plotted in the pointer of sprite 8. So basically sprites 1 moves from X=43 to X=255, 4 pixels at a time, after which it resets to value 43 again.
 +
 +In addition to that all, the Y-coords of the sprites are changed based on sine data.
 +
 +This codes assumes a letter-per-sprite font, in the same order as the screencodes, located at $2000.
 +
 +The code is a stripped down and somewhat simplified version of what is used in "This Is Halloweed" (https://csdb.dk/release/?id=197114).
 +
 +<code>
 +// Made in Kick Assembler
 +
 + .const SPRITESPACING = 43 // minimal possible spacing
 + .const SINLEAP = 20 // choose anything here to change sine wave
 + .const SCROLLSPEED = 4 // lower value is slower
 + .const SPRITEPOINTER = $07f8
 + .label spriteMemory = $2000 // assumes sprite font from $2000 to $3000
 +
 +*=$0801
 + BasicUpstart($0810)
 +*=$0810
 + sei
 + jsr $e544 // KERNAL: clear screen
 + lda #$00 // reset sine wave pointers
 + sta sinreset+1
 + sta sinwave+1
 +
 + ldx #<scrolltext // reset scroll text pointer
 + ldy #>scrolltext
 + stx textpointer+1
 + sty textpointer+2
 +        
 + lda #$ff
 + sta $d015 // turn on all sprites
 +
 + ldx #$00
 +!: lda #$a0 // init with spaces in all sprite pointers
 + sta SPRITEPOINTER,x
 + lda #$0e // light blue
 + sta $d027,x // set sprite colours
 + inx
 + cpx #$08
 + bne !-
 +
 + lda #$01 // init IRQ
 + sta $d01a
 + lda #$7f
 + sta $dc0d
 + sta $dd0d
 + lda $dc0d
 + lda $dd0d
 +
 + lda #$32 // arbitrary value
 + sta $d012
 + lda #$1b
 + sta $d011
 +
 + ldx #<irq // set pointers to FLD IRQ routine
 + ldy #>irq
 + stx $0314
 + sty $0315
 +
 + cli
 +
 + jmp *
 +
 +irq:
 + inc $d019
 +
 + lda scrollpos+1 // X-position of 1st sprite
 + sec
 + sbc #SCROLLSPEED // decrease with SCROLLSPEED
 + bpl notext // skip text fetch if sprite 1 can still move left
 +
 + ldx #$00 // shift content of sprites pointers
 + scrollpointers:
 + lda SPRITEPOINTER+1,x
 + sta SPRITEPOINTER,x
 + inx
 + cpx #$07
 + bne scrollpointers
 + lda sinreset+1 // shift sine wave
 + clc
 + adc SINLEAP // this fixes sine offset
 + adc #$03 //  when resetting sprite position
 + sta sinreset+1
 + textpointer:
 + lda scrolltext // get next letter from scroll text
 + bne noreset // if not #$00 (end indicator)
 +
 + lda #<scrolltext // reset scroll text pointer when letter is $00
 + sta textpointer+1
 + lda #>scrolltext
 + sta textpointer+2
 + jmp textpointer // read new letter
 +
 + noreset:
 + clc
 + adc #(>spriteMemory<<2) // correct for location of sprite font
 + sta SPRITEPOINTER+7 // store new letter in sprite 8 pointer
 +
 + inc textpointer+1 // increase scroll text pointer
 + bne !+
 + inc textpointer+2
 + !:  
 + lda #SPRITESPACING // move sprite 1 to right most position
 +notext:
 + sta scrollpos+1
 +
 + ldx #$00 // position other sprites relative to sprite 1
 +scrollpos:
 + lda #$18 // set new X-coord for all sprites
 + sta $d000
 + clc
 + adc #SPRITESPACING
 + sta $d002
 + clc
 + adc #SPRITESPACING
 + sta $d004
 + clc
 + adc #SPRITESPACING
 + sta $d006
 + clc
 + adc #SPRITESPACING
 + sta $d008
 + clc
 + adc #SPRITESPACING
 + sta $d00a
 + bcc !+
 + ldx #%11100000 // take care of MSB
 + clc
 +!: adc #SPRITESPACING
 + sta $d00c
 + bcc !+
 + ldx #%11000000
 + clc
 +!: adc #SPRITESPACING
 + sta $d00e
 + bcc !+
 + ldx #%10000000
 +!: stx $d010 // set proper sprite MSB
 +
 +sinreset:
 + ldx #$00 // sine wave counter
 + stx sinwave+1 // store in sine wave pointer
 + inc sinreset+1
 + ldy #$00
 +sinwave:
 + lda sindata // read sine wave data
 + sta $d001,y // store in Y-coords sprites
 + lda sinwave+1
 + clc
 + adc #SINLEAP // to make wave more interesting
 + sta sinwave+1 // increase sine wave pointer by SINLEAP
 + iny
 + iny
 + cpy #$10 // next sprites
 + bne sinwave
 + jmp $ea31 // end of IRQ1
 +
 +.align $100
 +sindata:
 + .fill 256, 120 + 15.5*sin(toRadians(i*(3*360)/256))
 +
 +scrolltext:
 + .text "this is a sprite scroller for codebase64.org        "
 + .byte $00
 +</code>
  
base/waving_sprite_scroll.txt · Last modified: 2020-11-14 00:00 by mace