Tetraskelion by Řrřola [web]
; Tetraskelion
; a 256-byte intro by rrrola <rrrola@gmail.com>
; greets to everyone who likes glowsticks
; Render ten layers of broken shiny circles.
; Uses a low-res mode with 16-bit colors.
; To make it faster, compute only half of the screen.
org 100h ; assume cs<=0x2a18 [0xfd..ff]=0 bx=0 sp=-2 si=0x100
;Timer multipliers.
T1 equ 27*4 ; rotation
T2 equ 19*4 ; scale
T3 equ -32*4 ; circle radius
T4 equ 23*4 ; color cycling
;Prepare segments:
; ss constants|stack|scratch
; ds cosine table
; fs color_mul/cosine table
; gs mirror buffer
; es screen
%define w(xx) word[byte bp+si-0x100+xx]
%define d(xx) dword[byte bp+si-0x100+xx]
push 0xa000 ;=68 00 a0 scratch (word|dword), scale (qword)
; pop later to make sp=-4 in the loop below
lds bp,[si-3] ; bp=0, ds=0x6800 table: cos
mov ax,0x4f02
mov fs,ax ; fs=0x4f02 table: color_mul / cos()
fninit
;Precompute tables with 16384 entries: cosine, color_mul/cosine
COS_TAB:
fild word[bp+di] ; bx/4=[bp+di]=[-2]=angle (0 on init)
fmul d(TABLE_STEP)
fcos ;; cos(angle/65536*2pi): adjust period to 2pi
fst dword[bx]
fldpi ; color_mul = 3.14
fdivrp st1,st0 ; color_mul / cos(...)
; fidivr [bp+si+?] ; -1 byte, but I like color_mul = pi
fstp dword[fs:bx]
sub bx,sp ; bx+=4
inc word[bp+di] ; next angle
jnz COS_TAB ; bx=0 [-2]=0
mov bx,0x10e
int 10h ; set mode 320x200 with 65536 colors; assume it's ok (ax=0x004f)
pop es ; es=0xa000 screen
add ax,0x39c9 ; constant: 07 05 c9 39, should be "db 0f c9 39"
TABLE_STEP equ $-4 ;= 0.000383495197 = 2pi / 16384, also ~ 1 / (256 * pi^2)
mov gs,ax ; gs=0x3a18 mirror buffer
; Frame loop: cx = time
M:
; Precompute the per-iteration scale.
imul bx,cx,T2
fld d(ZOOM)
fsub dword[bx] ;; zoom-cos(t2)
fdiv d(ZOOM) ;; (zoom-cos(t2))/zoom
fstp qword[bp+si] ;; scale = 1 - cos(t2)/zoom
inc cx ; time++
ZOOM equ $-4 ; =9.679
;Pixel loop: di = pixel index = pixel address / 2
X mov ax,0xcccd ; convert width 320 -> 65536
mul di
xchg ax,bx ; full 16-bit precision of X
mov ax,0x4f05
add bx,ax
adc dx,0x9b80 ; put center at [100, 159.5]: should add 0x9b804d46
pusha ; [-18-16-14-12-10 -8 -6 -4] on the stack
; di si bp sp bx dx cx ax
; yy tt
; x x
;Is it time to set the VESA bank?
add di,di ; di=pixel address
jnz D
cwd
adc dx,dx ; dx=page (0 or 1)
xor bx,bx ; bh=0 (set bank), bl=0 (window id)
int 10h ; assume 64kB granularity and window at 0xA000
;Compute a new color or load it from the mirror buffer.
D mov ax,[gs:di] ; load from mirror buffer
jc COPY_MIRROR ; compute only the top half of the screen
push di
call IT ; compute a new color
pop di
;Set pixel color
sub bp,di ; mirrored store to mirror buffer
mov [gs:bp+320*201*2-65536 - 2],ax
COPY_MIRROR:
stosw
; stosw ; 2x faster, also adjust mirroring to -4
popa
; inc di ; 2x faster
inc di
jnz X ; di=0
; call SCREENSHOT
;ESC check.
in al,60h
cmp al,1
jne M ; fallthrough, exit later
IT:
;Color accumulators = 0, load coordinates and radius.
Z fldz
inc bp
jpo Z ; loop 3x, bp=3 ;; R=0 G=0 B=0
dec bp
L dec bp ; load y=[-9], x=[-8]
fild word[bp-9]
fadd st0 ;; x[-65536..65536] y R G B
jpo L ; loop 2x, bp=0 again, zero flag = 1
jmp LEN ; call LEN subroutine
LEN_RET:
imul di,[bp+si],4 ; di = d = 65536/2pi * length(x,y)/2
imul dx,cx,T4
sub dx,di ; dx = t4-d
imul cx,T3
add di,cx ; di = d-t3
mov ax,0x8000 + 10 ; al = number of iterations, ah=0x80 = fold offset
;Rotate.
; [x] = [C -S] * [x]
; [y] [S C] [y]
I mov cl,0x25 ; RGB phase shift, later shift length
;cl: horrible parity hack for looping
; 25 00100101 o <- start
; 26 00100110 o <- after Q
; 27 00100111 e <- after R
; 28 00101000 e
; 29 00101001 o <- after F
imul bx,[bp-6],T1 ; bx = t1 (cx is used for looping)
R fld st1 ;; y x y R G B | x Sy x Cy R G B
fmul dword[bx] ;; Cy x y R G B | Cx Sy x Cy R G B
fxch st2 ;; y x Cy R G B | x Sy Cx Cy R G B
fmul dword[bx-0x4000]
inc cx ;; Sy x Cy R G B | Sx Sy Cx Cy R G B
jpo R ; loop 2x: cl=0x27
faddp st3,st0 ;; Sy Cx Sx+Cy R G B
fsubp st1,st0 ;; x=Cx-Sy y=Sx+Cy R G B
;Scale, square fold.
F fmul qword[bp+si] ; scale
fistp dword[bp+si] ; wrap: keep only bottom 16 bits
add [bp+si],ax ;~0x8000, can also be xor (cf=0)
fild word[bp+si] ;; x = x-round(x) | y = y-round(y)
fxch st1
inc cx
jpe F ; loop 2x: cl=0x29, zero flag = 0
;Subroutine: compute length of 2D vector, scale to access cos table
LEN: ;; x y -> [bp+si] = sqrt(x*x+y*y)/65536/2 * 16384/2pi ~ sqrt(C * (x*x+y*y))
fld st1
fmul st0
fld st1 ; -1 byte: cmc, jc LEN (but it's slow in DOSBox)
fmul st0
faddp
fmul d(TABLE_STEP) ; exact: (16384/2pi/65536/2)^2 = 0.000395785+
fsqrt
fistp word[bp+si]
jz LEN_RET ; return up if zero flag = 1
;Add colorful circles.
; k = color_mul / cos(5*length(x,y) + d - t3)
; [R G B] += k * ( 1 + 2 * cos(3*(i/40 + t4-d) + 0.9*[2 1 0]) );
dec dh
imul bx,dx,3 ; bx = q = 65536/2pi * 3*(i/40 + t4-d)
Q:add bh,0x25 ; q += ~ 0.9 * 65536/2pi
;add bh,cl ; q += ~ 1.0 * 65536/2pi ; -1 byte, I like the 0x25 colors better
fld1
fsub dword[bx]
fsub dword[bx] ;; 1+2cos(q) x y R G B
dec cx ; cl=0x26
jpe Q ; loop 3x ;; [dR dG dB]=1+2cos(q+0.9*[2 1 0])) x y R G B ; bp=3
imul bx,[bp+si],10*4 ; 65536/2pi * 5*length(x,y)
K fmul dword[fs:bx+di] ; k = color_mul / cos(5*length(x,y) + d - t3)
faddp st5,st0 ;; dG gB x y R+=k*dR G B
add al,0x55 ; +55 +aa +ff(=-1),carry
jnc K ; loop 3x ;; x y R+=k*dR G+=k*dG B+=k*dB
jnz I ; al=0, cl=0x26
fcompp ;; R G B
; Assemble R,G,B into 16-bit high color (5+6+5 bits). Clamp to 0..0xffff.
inc ax ; ax=0x8001
A fmul st0 ;; R^2 G^2 B^2
fistp word[bp+si] ; if it's > 0x7fff, clamp to 0x8000
imul bx,[bp+si],2 ; double, set carry if it was > 0x3fff
sbb bx,bp ; overflow -> 0xffff
xor cl,5^6 ; cl & 0x1f = shift length: flip 5<->6
shld ax,bx,cl ; rrrrrggggggbbbbb
jnc A ; loop 3x until you shift the 1 bit out of ax
ret
;%include "screenshot320_16.inc"
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