; 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