; Star update routine (C) 1993 M. Mackey ; MASM 5.0 compatible assembly code ; ; Turbo Pascal near-callable as ; Procedure Update(var star:StarType;i:integer); ; ; where StarType=record ; x,y,z:integer; ; xz,yz:integer; ; end; ; ; This procedure erases the star, recalculates its position and redraws ; it. Z values are constrained within 256...16012, and X, Y values are ; constrained within -8192..8191 with wraparound. (Letting X, Y go ; out to -32768..32767 works, but 80% of the stars are not visible at ; any given time, which is a waste of processor power. The view angle ; can be expanded to compensate for this, but that results in a 'fish- ; eye' effect which looks stupid.) ; This code is released freeware. You may use and modify it as you see ; fit, but you may not claim this code as your own work. Please give ; me a copy of anything you do with this, especially any improved ; versions. ; .MODEL small,pascal .386 ; note: uses 386 opcodes and registers! parms struc dw 2 dup (?) ; pushed bp and return address i dw ? ; star number (for calling newstar proc) starp dd ? ; far pointer to star variable parms ends .DATA XWidth equ 319 YWidth equ 199 XCenter equ 160 YCenter equ 100 EXTRN xltsin:word,xltcos:word ;external variables & procedures EXTRN yltsin:word,yltcos:word EXTRN zltsin:word,zltcos:word EXTRN speed:word EXTRN rotx:byte,roty:byte,rotz:byte EXTRN setpix:proc EXTRN newstar:proc STAR struc x dw ? y dw ? z dw ? xz dw ? ; screen coord yz dw ? ; screen coord STAR ends .CODE PUBLIC Update Update PROC near push bp ; Entry sequence - save old BP mov bp,sp ; point to local stack frame les si,[bp+starp] ; ES:SI points to star record ; check screen bounds cmp word ptr es:[si+xz],0 jl DontErase cmp word ptr es:[si+yz],0 jl DontErase cmp word ptr es:[si+xz],xwidth jg DontErase cmp word ptr es:[si+yz],ywidth jg DontErase push es ; es and si may be modified by setpix() push si push es:[si+xz] ; push xz push es:[si+yz] ; push yz push 0F000h ; push color (lo byte only counts) ; (I know this is weird but push 00000h ; pushed a byte only. This is kludgy but ; at least it works!) call setpix ; erase old star position pop si pop es DontErase: mov ax,ds:[speed] sub word ptr es:[si+z],ax ; move all stars towards viewer ; do rotations: 16-bit arithmetic Yrotate: cmp [roty],0 ; rotate about y axis? jz XRotate movsx ecx,word ptr es:[si+z] ; load and sign-extend z movsx edi,ds:[yltcos] ; load and sign-extend cos value imul ecx,edi ; ecx holds signed 32-bit cos*z movsx ebx,word ptr es:[si+x] ; load and sign-extend x movsx edx,ds:[yltsin] ; load and sign-extend sin value imul ebx,edx ; ebx holds signed 32-bit sin*x sub ecx,ebx ; ecx= yltcos * z -yltsin * x sar ecx,15 ; div 32768 ; cx now contains signed word z value movsx eax,word ptr es:[si+x] ; load and sign-extend x imul eax,edi ; eax holds cos*x movsx ebx,word ptr es:[si+z] ; load and sign-extend z imul ebx,edx ; ebx holds sin*z add eax,ebx ; eax= yltcos * x + yltsin * z sar eax,15 ; div 32768 ; ax now contains signed word x value mov es:[si+x],ax mov es:[si+z],cx ; finished rotation XRotate: cmp [rotx],0 ; rotate about x axis? jz ZRotate movsx ecx,word ptr es:[si+z] movsx edi,ds:[xltcos] imul ecx,edi ; ecx holds signed 32-bit cos*z movsx ebx,word ptr es:[si+y] movsx edx,ds:[xltsin] imul ebx,edx ; ebx holds signed 32-bit sin*x sub ecx,ebx ; ecx= xltcos * z - xltsin * y sar ecx,15 ; div 32768 ; cx now contains signed word z value movsx eax,word ptr es:[si+y] imul eax,edi ; eax holds cos*y movsx ebx,word ptr es:[si+z] imul ebx,edx ; ebx holds sin*z add eax,ebx ; eax= xltcos * y + xltsin * z sar eax,15 ; div 32768 ; ax now contains signed word y value mov es:[si+y],ax mov es:[si+z],cx ; finished rotation ZRotate: cmp [rotz],0 ; rotate about z axis? jz Endrotations movsx ecx,word ptr es:[si+y] movsx edi,ds:[zltcos] imul ecx,edi ; ecx holds signed 32-bit cos*z movsx ebx,word ptr es:[si+x] movsx edx,ds:[zltsin] imul ebx,edx ; ebx holds signed 32-bit sin*x sub ecx,ebx ; ecx= zltcos * y - zltsin * x sar ecx,15 ; div 32768 ; cx now contains signed word y value movsx eax,word ptr es:[si+x] imul eax,edi ; eax holds cos*x movsx ebx,word ptr es:[si+y] imul ebx,edx ; ebx hold sin*z add eax,ebx ;eax= zltcos * x + zltsin * y sar eax,15 ; div 32768 ; ax now contains signed word x value mov es:[si+x],ax mov es:[si+y],cx ; finished rotation Endrotations: ;check bounds cmp es:[si+z],256 ; z<256 gets a divide ; by zero exception later on jl GetNewstar cmp es:[si+z],15756 jg GetNewstar jmp Z_OK GetNewstar: push es ; es, di may be modified by newstar() push si push [bp+i] call newstar ; problem: stars are different when you reverse direction ; However, if new (random) stars are not added the ; existing stars clump together because of inaccuracy ; in the 16-bit arithmetic. Change to 32-bit asap. pop si pop es Z_OK: ; z is between 256 and 16012 shl es:[si+x],2 sar es:[si+x],2 ; get x modulo 8192 (it works! try it) shl es:[si+y],2 sar es:[si+y],2 ; get y modulo 8192 movsx edx,es:[si+x] mov ax,dx ; sign-extend x to dx:ax shr edx,16 mov cx,es:[si+z] shr cx,8 ; get hi(z) in cx idiv cx ; ax = x div hi(z) (dx:ax div cx) add ax,xcenter ; xz = (x div hi(z)) + xcenter ; (perspective projection) mov es:[si+xz],ax movsx edx,es:[si+y] mov ax,dx ; sign-extend y to dx:ax shr edx,16 idiv cx add ax,ycenter mov es:[si+yz],ax ; yz = (y div hi(z)) + ycenter ; (perspective projection) ; check screen bounds cmp word ptr es:[si+xz],0 jl Finished cmp word ptr es:[si+yz],0 jl Finished cmp word ptr es:[si+xz],xwidth jg Finished cmp word ptr es:[si+yz],ywidth jg Finished ; draw star in grayscale based on z distance push es:[si+xz] push es:[si+yz] mov ax,31 mov cx,es:[si+z] shr cx,10 ; cx = z shr 10 ( ranges from 0 to 15) sub ax,cx ; ax = 31- (z shr 10) ; i.e colour between 16 and 31 ; (VGA colours 16-31 are a grayscale) push ax call setpix Finished: pop bp retn 6 ; return 6 bytes of arguments Update endp end