Lookup Tables

                             W E L C O M E         
                      To the VGA Trainer Program    
                          DENTHOR of ASPHYXIA        

                            --==[ PART 6 ]==--


 Hi there! I'm back, with the latest part in the series : Pregenerated
 arrays. This is a fairly simple concept that can treble the speed of
 your code, so have a look.

 I still suggest that if you haven't got a copy of TEXTER that you get it.
 This is shareware, written by me, that allows you to grab fonts and use
 them in your own programs.

 I downloaded the Friendly City BBS Demo, an intro for a PE BBS, written
 by a new group called DamnRite, with coder Brett Step. The music was
 excellent, written by Kon Wilms (If I'm not mistaken, he is an Amiga
 weenie ;-)). A very nice first production, and I can't wait to see more
 of their work. I will try con a local BBS to allow me to send Brett some

 If you would like to contact me, or the team, there are many ways you
 can do it : 1) Write a message to Grant Smith in private mail here on
                   the Mailbox BBS.
             2) Write a message here in the Programming conference here
                   on the Mailbox (Preferred if you have a general
                   programming query or problem others would benefit from)
             3) Write to ASPHYXIA on the ASPHYXIA BBS.
             4) Write to Denthor, Eze or Livewire on Connectix.
             5) Write to :  Grant Smith
                            P.O.Box 270 Kloof
             6) Call me (Grant Smith) at (031) 73 2129 (leave a message if you
                   call during varsity)

 NB : If you are a representative of a company or BBS, and want ASPHYXIA
        to do you a demo, leave mail to me; we can discuss it.
 NNB : If you have done/attempted a demo, SEND IT TO ME! We are feeling
         quite lonely and want to meet/help out/exchange code with other demo
         groups. What do you have to lose? Leave a message here and we can work
         out how to transfer it. We really want to hear from you!

   Why do I need a lookup table? What is it?

 A lookup table is an imaginary table in memory where you look up the
 answers to certain mathematical equations instead of recalculating them
 each time. This may speed things up considerably. Please note that a
 lookup table is sometimes referred to as a pregenerated array.

 One way of looking at a lookup table is as follows : Let us say that for
 some obscure reason you need to calculate a lot of multiplications (eg.
 5*5 , 7*4 , 9*2 etc.). Instead of actually doing a slow multiply each
 time, you can generate a kind of bonds table, as seen below :

 把田   1     2     3     4     5     6     7     8     9   
  1    1     2     3     4     5     6     7     8     9   
  2    2     4     6     8     10    12    14    16    18  
  3    3     6     9     12    15    18    21    24    27  
  4    4     8     12    16    20    24    28    32    36  
  5    5     10    15    20    25    30    35    40    45  
  6    6     12    18    24    30    36    42    48    54  
  7    7     14    21    28    35    42    49    56    63  
  8    8     16    24    32    40    48    56    64    72  
  9    9     18    27    36    45    54    63    72    81  

 This means that instead of calculating 9*4, you just find the 9 on the
 top and the 4 on the side, and the resulting number is the answer.  This
 type of table is very useful when the equations are very long to do.

 The example I am going to use for this part is that of circles. Cast
 your minds back to Part 3 on lines and circles. The circle section took
 quite a while to finish drawing, mainly because I had to calculate the
 SIN and COS for EVERY SINGLE POINT. Calculating SIN and COS is obviously
 very slow, and that was reflected in the speed of the section.

   How do I generate a lookup table?

 This is very simple. In my example, I am drawing a circle. A circle has
 360 degrees, but for greater accuracy, to draw my circle I will start
 with zero and increase my degrees by 0.4. This means that in each circle
 there need to be 8000 SINs and COSes (360/0.4=8000). Putting these into
 the base 64k that Pascal allocates for normal variables is obviously not
 a happening thing, so we define them as pointers in the following
         TYPE   table = Array [1..8000] of real;

         VAR    sintbl : ^table;
                costbl : ^table;

 Then in the program we get the memory for these two pointers. Asphyxia
 was originally thinking of calling itself Creative Reboot Inc., mainly
 because we always forgot to get the necessary memory for our pointers.
 (Though a bit of creative assembly coding also contributed to this. We
 wound up rating our reboots on a scale of 1 to 10 ;-)). The next obvious
 step is to place our necessary answers into our lookup tables.  This can
 take a bit of time, so in a demo, you would do it in the very beginning
 (people just think it's slow disk access or something), or after you
 have shown a picture (while the viewer is admiring it, you are
 calculating pi to its 37th degree in the background ;-)) Another way of
 doing it is, after calculating it once, you save it to a file which you
 then load into the variable at the beginning of the program. Anyway,
 this is how we will calculate the table for our circle :

     Procedure Setup;
     VAR deg:real;
       for loop1:=1 to 8000 do BEGIN
         costbl^[loop1]:=cos (rad(deg));
         sintbl^[loop1]:=sin (rad(deg));

 This will calculate the needed 16000 reals and place them into our two
 variables. The amount of time this takes is dependant on your computer.

   How do I use a lookup table?

 This is very easy. In your program, wherever you put
                cos (rad(deg)),
 you just replace it with :

 Easy, no? Note that the new "deg" variable is now an integer, always
 between 1 and 8000.

  Where else do I use lookup tables?

 Lookup tables may be used in many different ways. For example, when
 working out 3-dimensional objects, sin and cos are needed often, and are
 best put in a lookup table. In a game, you may pregen the course an
 enemy may take when attacking. Even saving a picture (for example, a
 plasma screen) after generating it, then loading it up later is a form
 of pregeneration.

 When you feel that your program is going much too slow, your problems
 may be totally sorted out by using a table. Or, maybe not. ;-)

   In closing

 As you have seen above, lookup tables aren't all that exciting, but they
 are useful and you need to know how to use them. The attached sample
 program will demonstrate just how big a difference they can make.

 Keep on coding, and if you finish anything, let me know about it! I
 never get any mail, so all mail is greatly appreciated ;-)

 Sorry, no quote today, it's hot and I'm tired. Maybe next time ;-)

   - Denthor
 USES crt;

 CONST VGA = $a000;

 TYPE tbl = Array [1..8000] of real;
              { This will be the shape of the 'table' where we look up
                values, which is faster then calculating them }

 VAR loop1:integer;
     Pall : Array [1..20,1..3] of byte;
       { This is our temporary pallette. We ony use colors 1 to 20, so we
         only have variables for those ones. }

 Procedure SetMCGA;  { This procedure gets you into 320x200x256 mode. }
      mov        ax,0013h
      int        10h

 Procedure SetText;  { This procedure returns you to text mode.  }
      mov        ax,0003h
      int        10h

 Procedure Cls (Col : Byte);
    { This clears the screen to the specified color }
   Fillchar (Mem [VGA:0],64000,col);

 Procedure Putpixel (X,Y : Integer; Col : Byte);
   { This puts a pixel on the screen by writing directly to memory. }
   Mem [VGA:X+(Y*320)]:=Col;

 procedure WaitRetrace; assembler;
   {  This waits for a vertical retrace to reduce snow on the screen }
   l1, l2;
     mov dx,3DAh
     in al,dx
     and al,08h
     jnz l1
     in al,dx
     and al,08h
     jz  l2

 Procedure Pal(ColorNo : Byte; R,G,B : Byte);
   { This sets the Red, Green and Blue values of a certain color }
    Port[$3c8] := ColorNo;
    Port[$3c9] := R;
    Port[$3c9] := G;
    Port[$3c9] := B;

 Function rad (theta : real) : real;
   {  This calculates the degrees of an angle }
   rad := theta * pi / 180

 Procedure NormCirc;
   { This generates a spireal without using a lookup table }
 VAR deg,radius:real;

   gotoxy (1,1);
   Writeln ('Without pregenerated arrays.');
   for loop1:=60 downto 43 do BEGIN
       X:=round(radius*COS (rad (deg)));
       Y:=round(radius*sin (rad (deg)));
       putpixel (x+160,y+100,61-loop1);
       deg:=deg+0.4;           { Increase the degree so the circle is round }
       radius:=radius-0.02;    { Decrease the radius for a spiral effect }
     until radius<0; {  Continue till at the centre (the radius is zero) }

 Procedure LookupCirc;
   {  This draws a spiral using a lookup table }
 VAR radius:real;
     costbl : ^tbl;
     sintbl : ^tbl;

     Procedure Setupvars;
       {  This is a nested procedure (a procedure in a procedure), and may
          therefore only be used from within the main part of this procedure.
          This section gets the memory for the table, then generates the
          table. }
     VAR deg:real;
       getmem (costbl,sizeof(costbl^));
       getmem (sintbl,sizeof(sintbl^));
       for loop1:=1 to 8000 do BEGIN         { There are 360 degrees in a    }
         deg:=deg+0.4;                       { circle. If you increase the   }
         costbl^[loop1]:=cos (rad(deg));     { degrees by 0.4, the number of }
         sintbl^[loop1]:=sin (rad(deg));     { needed parts of the table is  }
       END;                                  { 360/0.4=8000                  }
     { NB : For greater accuracy I increase the degrees by 0.4, because if I
            increase them by one, holes are left in the final product as a
            result of the rounding error margin. This means the pregen array
            is bigger, takes up more memory and is slower to calculate, but
            the finished product looks better.}

   cls (0);
   gotoxy (1,1);
   Writeln ('Generating variables....');
   gotoxy (1,1);
   Writeln ('With pregenerated arrays.');
   for loop1:=60 downto 43 do BEGIN
       X:=round (radius*costbl^[pos]);   { Note how I am not recalculating sin}
       Y:=round (radius*sintbl^[pos]);   { and cos for each point.            }
       putpixel (x+160,y+100,61-loop1);
       inc (pos);
       if pos>8000 then pos:=1;    { I only made a table from 1 to 8000, so it}
                                   { must never exceed that, or the program   }
                                   { will probably crash.                     }
     until radius<0;
   freemem (costbl,sizeof(costbl^));   { Freeing the memory taken up by the   }
   freemem (sintbl,sizeof(sintbl^));   { tables. This is very important.      }

 Procedure PalPlay;
   { This procedure mucks about with our "virtual pallette", then shoves it
     to screen. }
 Var Tmp : Array[1..3] of Byte;
   { This is used as a "temporary color" in our pallette }
     loop1 : Integer;
      { This copies color 1 from our virtual pallette to the Tmp variable }
      { This moves the entire virtual pallette down one color }
      { This copies the Tmp variable to no. 18 of the virtual pallette }
    For loop1:=1 to 18 do
      pal (loop1,pall[loop1,1],pall[loop1,2],pall[loop1,3]);

   writeln ('Hi there! This program will demonstrate the usefullness of ');
   writeln ('pregenerated arrays, also known as lookup tables. The program');
   writeln ('will first draw a spiral without using a lookup table, rotate');
   writeln ('the pallette until a key is pressed, the calculate the lookup');
   writeln ('table, then draw the same spiral using the lookup table.');
   writeln ('This is merely one example for the wide range of uses of a ');
   writeln ('lookup table.');
   Write ('  Hit any key to contine ...');
   directvideo:=FALSE;  { This handy trick allows you to use GOTOXY and }
                        { Writeln in GFX mode. Hit CTRL-F1 on it for more }
                        { info/help }
   For Loop1 := 1 to 18 do BEGIN
     Pall[Loop1,1] := (Loop1*3)+9;
     Pall[Loop1,2] := 0;
     Pall[Loop1,3] := 0;
        { This sets colors 1 to 18 to values between 12 to 63. }

    For loop1:=1 to 18 do
      pal (loop1,pall[loop1,1],pall[loop1,2],pall[loop1,3]);
         { This sets the true pallette to variable Pall }

   normcirc;         { This draws a spiral without lookups }
   Until keypressed;
   lookupcirc;       { This draws a spiral with lookups }
   Until keypressed;

   Writeln ('All done. This concludes the sixth sample program in the ASPHYXIA');
   Writeln ('Training series. You may reach DENTHOR under the name of GRANT');
   Writeln ('SMITH on the MailBox BBS, or leave a message to ASPHYXIA on the');
   Writeln ('ASPHYXIA BBS. I am also an avid Connectix BBS user.');
   Writeln ('Get the numbers from Roblist, or write to :');
   Writeln ('             Grant Smith');
   Writeln ('             P.O. Box 270');
   Writeln ('             Kloof');
   Writeln ('             3640');
   Writeln ('I hope to hear from you soon!');
   Writeln; Writeln;
   Write   ('Hit any key to exit ...');

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Date this article was posted to GameDev.net: 7/16/1999
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Denthor's Asphyxia Tutorials

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