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DenTut16-Pas

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                   ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͸
                   ³         W E L C O M E         ³
                   ³  To the VGA Trainer Program   ³ ³
                   ³              By               ³ ³
                   ³      DENTHOR of ASPHYXIA      ³ ³ ³
                   ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ; ³ ³
                     ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ³
                       ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
                           --==[ PART 16 ]==--

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• Introduction

Hi there. This trainer is on the scaling of an arbitrary sized bitmap to screen in two dimensions. The sample program seems to work quite quickly, and the code is document. The scaling procedure is however totally in assembler ... hopefully this won't cause to many problems.

EzE has released a trainer! It is on the speeding up of 3D for normal 3D and for virtual worlds. Check it out, it is quite good (even though I get a bit of ribbing in his quote ) It will be in PCGPE ][, to be released shortly.

I have set up a mailserver (that doesn't seem to work all the time, but the ones that miss I post manually). It works like this :

Send mail to denthor@beastie.cs.und.ac.za with the subject line : request-list ... it will automatically mail you back with a list of subject lines with which you can grab certain files. You will then mail me with the subject line of a specific file and it will send you a uuencoded version of that file automatically. Cool, huh?

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• What is scaling?

I think that most of you know this one already, but here goes. Let us say you have a picture (10x10 pixels) and you want to draw it to a different size (say 5x7 pixel), the process of altering the picture to fit into the new size is called scaling. Scaling only works on rectangular areas.

With scaling to can easily strech and shrink your bitmaps.

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• Okay, so how do we code it?

Right. The way I am going to do scaling is as follows :

For the horizontal area, I am going to calculate a certain step value. I will then trace along the bitmap, adding this step to my position, and placing the nearest pixel on to the screen. Let me explain this simpler ...

Let us say I have a 10 pixel wide bitmap. I want to squish it into 5 pixels. Along the bitmap, I would draw every second pixel to screen. In ascii :

  1234567890   13579
  +--------+   +---+
  |        |   |   |
  | bitmap |   |   |dest
  |        |   |   |
  +--------+   +---+

As you can see, by stepping through every second pixel, I have shrunk the bitmap to a width of 5 pixels.

The equation is as follows :

            step = origionalwidth / wantedwidth;

Let us say we have a 100 pixel wide bitmap, which we want to get to 20 pixels.

            step = 100 / 20
            step = 5

If we draw every fifth pixel from the origional bitmap, we have scaled it down correctly! This also works for all values, if step is of type real.

We also find the step for the height in the same way.

Our horizontal loop is as follows :

       For loop1:=1 to wantedwidth do BEGIN
         putpixel (loop1,height,bitmap[round (curpos)],vga);
         curpos:=curpos+xstep;
       END;

And the vertical loop is much the same. Easy huh? So east in fact, I wrote the procedure in pure assembler for you ... don't worry, it's commented.

In the sample program, instead of using reals I have used fixed point math. Refer to tut 14 if you have any hassles with fixed point, it is fairly straightforward.

I also use psuedo 3-d perspective transforms to get the positions smooth... after Tut8, this should be a breeze.

There are no new commands in the assembler for you, so you should get by with what you learned in tut7/8 ... whew! A lot of back referencing there ;) We really are building on our knowledge :)

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• In closing

Well, that is about it. As you can see the concept is easy, and in fact fairly obvious, but that didn't stop me from having to sit down with a pencil and a piece of paper a few months ago and puzzle it out

I have a lot of work ahead of me for a while, so this may be the last trainer for a few months ... unless I can find some free time available. So please be patient!

       [ "Sir! My computer has just gone haywire!"
         "What?" shouts the CO. "That is a multimilliondollar machine!
           find out what's wrong! This is a critical time lieutenant!"
         "Yes sir"
         The young lieutenant furiously types away at the keyboard, but
           the machine totally ignores her.
         "What is going on, soldier?"
         "I don't know sir! It is just doing totally arbitrary things
           after it's assigned tasks are completed. In the computer world
           this is known as Denthorisms."
         The computer starts to make random beeps, and prints out a payroll
           program.
         "Get it working NOW soldier"
         The lieutenant ignores him, and contines typing. She gets partial
           control of the system, she can run programs, but the computer is
           continually running arb tasks in the background. One of the
           techhies who have gathered behing her suddenly cries "Hey! It's
           accessing the missile codes! It wants to launch them!"
         The typing gathers speed, but to no avail. Another techhie says
           "I could disconnect the computer from the link, but that would take
           hours! And this thing will have the codes in under five minutes
           at the speed it's going!"
         A smile forms on the lieutanants face, and she leans back in her chair.
         "What the hell are you doing?" yells the CO. "Why have you stopped?"
         Again ignoring him, the lieutenant instead turns to the techhie. "Go
           disconnect the machine, I know how to get you the time you need."
         "How on earth will you do that? The machines going at top speed!"
         She smiles again, leans forward, types in three letters and hits the
           carriage return. The computer grinds to a halt.
         The smile breaks into a grin. "Maybe it _does_ have it's uses after
           all."
                                                                        ]
                                                         - Grant Smith
                                                             15:30
                                                               23-9-94

Byeeeee.....
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