In the previous post, a drawing program was successfully run on 6502 Emulator, now I want to create a kaleidoscope drawing program.
My plan is to allow the user to only draw on the second quadrant which is the top left quadrant in the 6502 display and reflecting whatever the user draw on it to the other three quadrant to create an kaleidoscope effect.
A couple more things needs to be done in order to achieve this.
First it needs to restrict the cursor's movement to only allow user draw on the second quadrant, in drawing cursor by setting the range of row and column to [$00 : $0F] will make this happen.
drawing_cursor:
lda ROW ; ensure ROW is in range 0:0F
and #$0f
sta ROW
lda COL ; ensure COL is in range 0:0F
and #$0f
sta COL
ldy ROW ; load POINTER with start-of-row
lda table_low,y
sta POINTER
lda table_high,y
sta POINTER_H
ldy COL ; store CURSOR at POINTER plus COL
lda #CURSOR
sta (POINTER),y
rts
Then whatever pixels that are in the second quadrant should be reflected to other three quadrant. This is done by storing a copy of the current position and adding the x or y offset to the cursor every time user moves the cursor.
drawing_other_quads:
LDA POINTER ;; save the pointer to the
PHA ;; original location in top_left_quad
LDA POINTER_H
PHA
; top right quadrant
LDA #$10
CLC
SBC COL
CLC
ADC #$10
TAY
LDA DOT
STA (POINTER),y
TYA
PHA ; save the y offset
; bottom left quadrant
lda #$10 ; load POINTER with start-of-row
CLC
SBC ROW
CLC
ADC #$10
TAY
lda table_low,y
sta POINTER
lda table_high,y
sta POINTER_H
ldy COL ; store CURSOR at POINTER plus COL
lda DOT
sta (POINTER),y
PLA
TAY
; bottom right quadrant
lda DOT
sta (POINTER),y
PLA
STA POINTER_H
PLA
STA POINTER
RTS
Complete source code:
; zero-page variable locations
define ROW $20 ; current row
define COL $21 ; current column
define POINTER $10 ; ptr: start of row
define POINTER_H $11
; constants
define DOT $01 ; dot colour location
define CURSOR $04 ; purple colour
setup: lda #$0f ; set initial ROW,COL
sta ROW
sta COL
LDA #$01
STA DOT
draw: jsr drawing_cursor
getkey:
ldx #$00 ; clear out the key buffer
lda $ff ; get a keystroke
stx $ff
cmp #$30
bmi getkey
cmp #$40
bpl continue
SEC
sbc #$30
tay
lda color_pallete, y
sta DOT
jmp done
continue:
cmp #$43 ; handle C or c
beq clear
cmp #$63
beq clear
cmp #$80 ; if not a cursor key, ignore
bmi getkey
cmp #$84
bpl getkey
pha ; save A
lda DOT ; set current position to DOT
sta (POINTER),y
jsr drawing_other_quads
pla ; restore A
cmp #$80 ; check key == up
bne check1
dec ROW ; ... if yes, decrement ROW
jmp done
check1:
cmp #$81 ; check key == right
bne check2
inc COL ; ... if yes, increment COL
jmp done
check2:
cmp #$82 ; check if key == down
bne check3
inc ROW ; ... if yes, increment ROW
jmp done
check3:
cmp #$83 ; check if key == left
bne done
dec COL ; ... if yes, decrement COL
clc
bcc done
clear:
lda table_low ; clear the screen
sta POINTER
lda table_high
sta POINTER_H
ldy #$00
tya
c_loop:
sta (POINTER),y
iny
bne c_loop
inc POINTER_H
ldx POINTER_H
cpx #$06
bne c_loop
done:
clc ; repeat
bcc draw
drawing_cursor:
lda ROW ; ensure ROW is in range 0:0F
and #$0f
sta ROW
lda COL ; ensure COL is in range 0:0F
and #$0f
sta COL
ldy ROW ; load POINTER with start-of-row
lda table_low,y
sta POINTER
lda table_high,y
sta POINTER_H
ldy COL ; store CURSOR at POINTER plus COL
lda #CURSOR
sta (POINTER),y
rts
drawing_other_quads:
LDA POINTER ;; save the pointer to the
PHA ;; original location in top_left_quad
LDA POINTER_H
PHA
; top right quadrant
LDA #$10
CLC
SBC COL
CLC
ADC #$10
TAY
LDA DOT
STA (POINTER),y
TYA
PHA ; save the y offset
; bottom left quadrant
lda #$10 ; load POINTER with start-of-row
CLC
SBC ROW
CLC
ADC #$10
TAY
lda table_low,y
sta POINTER
lda table_high,y
sta POINTER_H
ldy COL ; store CURSOR at POINTER plus COL
lda DOT
sta (POINTER),y
PLA
TAY
; bottom right quadrant
lda DOT
sta (POINTER),y
PLA
STA POINTER_H
PLA
STA POINTER
RTS
; these two tables contain the high and low bytes
; of the addresses of the start of each row
table_high:
dcb $02,$02,$02,$02,$02,$02,$02,$02
dcb $03,$03,$03,$03,$03,$03,$03,$03
dcb $04,$04,$04,$04,$04,$04,$04,$04
dcb $05,$05,$05,$05,$05,$05,$05,$05,
table_low:
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
dcb $00,$20,$40,$60,$80,$a0,$c0,$e0
color_pallete:
dcb $01,$02,$03,$04,$05,$06,$07,$08,$09,$0a
Results:
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