Here's the source code. I love Assembler, and this is right at home for me:
; P L a 256-byte intro by Rrrola - final version
; \ / \ rrrola@gmail.com
; U S http://rrrola.wz.cz
; greets to everyone who came to Riverwash 2009
;%define BLOWUP 86 ; "ambient occlusion" strength (default 86: -1 byte)
MAXSTEPSHIFT equ 6 ; shading strength (>=6 unless BLOWUP defined)
MAXITERS equ 26
BASECOLOR equ -34
%define BLOCKS ; render 4x1 blocks (for slower machines, +3 bytes)
org 100h ; assume ah=bx=0 cx=255 sp=di=-2 si=100h
mov al,13h ;<(byte)[100h]>>8 = 0.6875
push bx ; (word)[100h]>>16 = 0.0769
mov dx,3C8h ; (float)[100h] = -0.0008052
int 10h
;palette - 4 gradients of 32 shades
P mov al,bl ;<set al on 1st pass, then each red and green
Q test bl,cl ; parity: eooooeoeoeee eooooeoeoeee ...
jpe E ; index: #0gb1gb2gb3g b4gb5gb6gb7g ...
imul al
shr ax,7
E imul bl
mov al,ah
out dx,al
mov dl,0c9h
loop P
mov cl,3
dec bx
jnz Q
push 09FCEh ;<aligns with the screen ;-)
pop es
mov bh,56h ; xyz addressing trick from neon_station
; vecNN = (words){[5600h+N] [5656h+N] [56ACh+N]}
;frame loop - prepare constants
;ax=key bx=5600h bp=dx=0 cx=3 si=100h di=-2 sp=-4 word[5700h]=T
M fninit ; {} = fpu stack
add word[bx+si],byte 88 ; T++
fild word[bx+si] ;=14.00564 * 2pi
fsincos ; {s=sin(T*0.00564) c=cos(T*0.00564)}
fdiv st1,st0 ; {s t=cotg(T*0.00564)}
fild word[bx+si]
fmul dword[si]
fsin
fimul word[si] ; {r>>16=0.0769*sin(T*-0.0708) s t}
pop edx ;=9B6C0000... just get rid of it
push es
push bp ;dword[-4]=9FCE0000h
;pixel loop
;x = word[-3] ~ -0.5..0.5 (+= 0.003125 each column)
;y = word[-2] ~ -0.4..0.4 (+= 0.00390625 each row)
X
%ifdef BLOCKS
test bp,cx ; 4x1 blocks: keep last color?
jnz D
%endif
pusha ;[-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 ]
; di si bp sp bx dx cx ax yyyy
; FEFF0001adr FCFF005600000300col ..xxxx
;fisheye projection: z = C-x*x-y*y
mov [bx],bx ;=0.33594
Z mov ax,[di]
fild word[di]
imul ax
sub [bx],dx
dec di
jpo Z ;di=-4
fild word[bx] ; {z>>16=0.33594-x*x-y*y x y r s t}
;advance x and y for next pixel (low word)
%ifdef BLOCKS
add word[di],3334h
%else
add dword[di],0000CCCDh
%endif
;rotate direction vector
R fmul st4 ; {sz x y r s t}
fld st0
fmul st6 ; {cz sz x y r s t}
fxch st2
fmul st5
fsub st2,st0; {sx sz cz-sx y r s t}
fmul st6
faddp st1,st0; {cx+sz cz-sx y r s t}
fxch st2 ; {y cz-sx cx+sz r s t}
inc di
jpo R
dec di ;di=-2 ; {X Y Z r s t}
;advance x and y for next pixel (high word)
%ifdef BLOCKS
adc word[di],cx
%endif
;store ray origin and direction vector
imul dx,[bx+si],byte 10;=0.0134*T
S
fistp word[bx+di] ; d>>16 = v-2 = {X, Y, Z}
;fistp dword[bx+di] ;<slower, but fixes corners (+3 bytes)
;sar dword[bx+di],1
mov [bx],dx
add dh,[si] ; o>>16 = v0 = 0.0134*T + {0, 0.6875, 0.375}
add bl,bh
jnc S ;bl=2 ; {r s t}
;intersect ray with scene, count number of bounding volume misses
cwd ; dx = hit(-1|0): grainy rendering
mov ah,-MAXITERS ; ah = iters(-MAXITERS..0), al = hue(0..3)
;cwd ;<for smooth rendering with bands
%ifdef BLOWUP
mov cx,BLOWUP*256+MAXSTEPSHIFT; cl = stepshift(0..MAXSTEPSHIFT)
%else
adc cx,bx ;=86*256 + 6 ;-)
%endif
call I
sub ah,cl
aad 4 ;ah=0
add al,MAXITERS*4+BASECOLOR
mov [di-4],ax ; pushed ax = color = (iters-stepshift)*4 + hue
popa
;draw pixel
D
%ifdef BLOCKS
mov [es:bp+si],al
inc bp
%else
inc bp
mov [es:bp+si],al
%endif
jnz X ; 65536 pixels
;next frame: fall through on esc - ah=word[sp]=0
in al,60h
dec ax
jnz M ;<assume no one presses ESC after 1 frame ;)
;raycasting using unbounded binary search
;start with the smallest step size
;v-2 = d = {dx, dy, dz}
;v0 = o = current {x, y, z} mod 1
;last probe was inside: halve step and go back
; outside: double step and go forward
I mov bl,0 ;bl=0
A mov bp,[bx+di] ; hit ? (o -= d>>stepshift) : (o += d>>stepshift)
sar bp,cl
xor bp,dx
add [bx],bp
add bl,bh
jnc A ;bl=2
salc ;al=FFh
fist word[bx+si] ; word[5702h] = r
;bounding volumes: "blow up" the scene by the current step size
push cx
shr cx,cl
add ch,37 ; cx = hitlimit = 0.1445 + (BLOWUP >> stepshift+8)
;inside test
;hue=[0,1]
;green octahedra: (|x|+|y|+|z|)/2 - 0.1445 + r < blowup
;orange octahedra: (|x+0.5|+|y+0.5|+|z+0.5|)/2 - 0.1445 - r < blowup
O mov dx,[bx+si] ; dx = [r,-r]
neg word[bx+si]
C imul bp,ax,32768 ; bp = [0.5, 0] - v0.xyz
sub bp,[bx+di]
jns T
neg bp
T shr bp,1
add dx,bp ; v2 = |signed(bp)|/2
mov [bx],bp
add bl,bh
jnc C ;bl=4 ; dx = sum(v2) + [r,-r]
cmp dx,cx ; if (dx < hitlimit) hit!
inc ax ;al=0,ah++
jc H
mov bl,2 ;bl=2
jpe O ;al=1 ; repeat if al==0
;hue=[2,3]
;bars: (||x|-|z|| + ||y|-|x|| + ||z|-|y||)/2 - 0.0676 < blowup
;bolts: (||x|-|z|| + ||y|-|x|| + ||z|-|y||)/2 - 0.1445 < blowup
sub dx,[bx+si]
inc ax ;al=2
;sub dx,bx ;<simple bolt movement: sum(v2)-2*r-0.3359
sub dx,[bx+si] ;<precise bolt movement: sum(v2)-3*r-0.375
sub dh,60h ; (+3 bytes)
imul dx,byte 13 ; if (|sum(v2)-3*r-0.375| < 0.03846)
mov dx,[si] ; dx = extra_width = 0.0769, hue = 2
jo B ; else
inc ax ;al=3 ; dx = extra_width = 0, hue = 3
cwd
B sub bp,[bx] ; bp = v2.zxy - v2.xyz
jns L
neg bp
L add dx,bp
mov bp,[bx]
add bl,bh
jnc B ;bl=4 ; dx = sum(|signed(bp)|) + extra_width
cmp dx,cx ; if (dx < hitlimit) hit!
;adjust step size according to the hit result
H pop cx ;cf=hit
sbb dx,dx ; dx = hit?-1:0
cmc
sbb cl,dl ; if (hit) stepshift++
adc cl,dl ; else if (stepshift > 0) stepshift--
;more probes?
cmp cl,MAXSTEPSHIFT
jae F ; if (stepshift >= maxstepshift) break
add ah,dl ; iters++, if (hit) iters--
jne I ; if (iters >= maxiters) break
F ret