An idea

Just to give you an idea of the complexity of it heres how to code [one] square on a screen.


þ How to draw a polygon

Sounds easy enough, right? WRONG! There are many, many different ways to go about this, and today I’ll only be showing you one. Please don’t take what is written here as anything approaching the best method, it is just here to get you on your way…

The procedure I will be using here is based on something most of us learned in standard eight … I think. I seem to recall doing something like this in Mrs. Reids maths class all those years ago 😉 Take two points, x1,y1 and x2,y2. Draw them :

+ (x1,y1)
\  <– Point a somewhere along the line
+ (x2,y2)

Right, so what we have to do is this : if we know the y-coord of a, what is it’s x-coord? To prove the method we will give the points random values.

+ (2,10)
\  <– a.y = 12
+  (15,30)

Right. Simple enough problem. This is how we do it :

(a.y-y1) = (12 – 10)  {to get a.y as though y1 was zero}
*(x2-x1) = *(15 – 2)  {the total x-length of the line}
/(y2-y1) = /(30 – 10) {the total y-length of the line}
+x1 = +2         { to get the equation back to real coords}
So our equation is :  (a.y-y1)*(x2-x1)/(y2-y1)+x4    or
which gives you :
2*13/20+2 = 26/20+2
= 3.3

That means that along the line with y=12, x is equal to 3.3. Since we are not concerned with the decimal place, we replace the / with a div, which in Pascal gives us an integer result, and is faster too. All well and good, I hear you cry, but what does this have to do with life and how it relates to polygons in general. The answer is simple. For each of the four sides of the polygon we do the above test for each y line. We store the smallest and the largest x values into separate variables for each line, and draw a horizontal line between them. Ta-Dah! We have a cool polygon!

For example : Two lines going down :

+             +
/ <-x1     x2->|   <–For this y line
/                |
+                  +

Find x1 and x2 for that y, then draw a line between them. Repeat for all y values.

Of course, it’s not as simple as that. We have to make sure we only check those y lines that contain the polygon (a simple min y, max y test for all the points). We also have to check that the line we are calculating actually extends as far as where our current y is (check that the point is between both y’s). We have to compare each x to see weather it is smaller then the minimum x value so far, or bigger then the maximum (the original x min is set as a high number, and the x max is set as a small number). We must also check that we only draw to the place that we can see ( 0-319 on the x ; 0-199 on the y (the size of the MCGA screen))

To see how this looks in practice, have a look at the sample code provided. (Mrs. Reid would probably kill me for the above explanation, so when you learn it in school, split it up into thousands of smaller equations to get the same answer ;))

Okay, that’s it! What’s that? How do you draw a vertical line? Thats simple …


þ Drawing a vertical line

Right, this is a lot easier than drawing a normal line (Tut 5 .. I think), because you stay on the same y value. So, what you do is you set ES to the screen you want to write to, and get DI to the start of the y-line (see earlier trainers for a description of how SEGMENT:OFFSET works.

IN   : x1 , x2, y, color, where
mov    ax,where
mov    es,ax
mov    di,y
mov    ax,y
shl    di,8   { di:=di*256 }
shl    ax,6   { ax:=ax*64 }
add    di,ax  { di := (y*256)+(y*64) := y*320 Faster then a
straight multiplication }

Right, now you add the first x value to get your startoff.

add    di,x1

Move the color to store into ah and al

mov    al,color
mov    ah,al       { ah:=al:=color }

then get CX equal to how many pixels across you want to go

mov    cx,x2
sub    cx,x1   { cx:=x2-x1 }

Okay, as we all know, moving a word is a lot faster then moving a byte, so we halve CX

shr    cx,1    { cx:=cx/2 }

but what happens if CX was an odd number. After a shift, the value of the last number is placed in the carry flag, so what we do is jump over a single byte move if the carry flag is zero, or execute it if it is one.

jnc     @Start  { If there is no carry, jump to label Start }
stosb           { ES:[DI]:=al ; increment DI }
@Start :            { Label Start }
rep     stosw   { ES:[DI]:=ax ; DI:=DI+2; repeat CX times }

Right, the finished product looks like this :

Procedure Hline (x1,x2,y:word;col:byte;where:word); assembler;
{ This draws a horizontal line from x1 to x2 on line y in color col }
mov   ax,where
mov   es,ax
mov   ax,y
mov   di,ax
shl   ax,8
shl   di,6
add   di,ax
add   di,x1
mov   al,col
mov   ah,al
mov   cx,x2
sub   cx,x1
shr   cx,1
jnc   @start
@Start :
rep   stosw
void Hline (word X1, word X2, word Y, byte Col, word Where) {
asm {
mov     ax, [Where]  // move segment of Where to AX
mov     es, ax       // set ES to segment of Where
mov     ax, [Y]      // set AX to Y
mov     di, ax       // set DI to Y
shl     ax, 8        // shift AX left 8 places (multiply Y by 256)
shl     di, 6        // shift DI left 6 places (multiply Y by 64)
add     di, ax       // add AX to DI (Y*64 + Y*256 = Y*320)
add     di, [X1]     // add the X1 offset to DI
mov     al, [Col]    // move Col to AL
mov     ah, al       // move Col to AH (we want 2 copies for word moving)
mov     cx, [X2]     // move X2 to CX
sub     cx, [X1]     // move the change in X to CX
shr     cx, 1        // divide change in X by 2 (for word moving)
jnc     Start        // if we have an even number of moves, go to Start
stosb                // otherwise, move one byte more
Start: asm {
rep     stosw        // do it!



þ In closing

This 3D system is still not perfect. It needs to be faster, and now I have also dumped the problem of face-sorting on you! Nyahahahaha!

[ My sister and I were driving along the other day when she asked me, what would I like for my computer. I thought long and hard about it, and came up with the following hypothesis. When a girl gets a Barbie doll, she then wants the extra ballgown for the doll, then the hairbrush, and the car, and the house, and the friends etc. When a guy gets a computer, he wants the extra memory, the bigger hard drive, the maths co-pro, the better motherboard, the latest software, and the bigger monitor etc. I told my sister all of this, and finished up with : “So as you can see, computers are Barbie dolls for MEN!” She called me a chauvinist. And hit me. Hard. ]

– Grant Smith

See you next time!
– Denthor

These fine BBS’s carry the ASPHYXIA DEMO TRAINER SERIES : (alphabetical)

ºBBS Name                  ºTelephone No.   ºOpen ºMsgºFileºPastº
ºASPHYXIA BBS #1           º(031) 765-5312  ºALL  º * º *  º *  º
ºASPHYXIA BBS #2           º(031) 765-6293  ºALL  º * º *  º *  º
ºConnectix BBS             º(031) 266-9992  ºALL  º   º *  º *  º
Open = Open at all times or only A/H
Msg  = Available in message base
File = Available in file base
Past = Previous Parts available
Does no other BBS’s ANYWHERE carry the trainer? Am I writing this for
three people who get it from one of these BBS’s each week? Should I go
on? (Hehehehe … I was pleased to note that Tut 8 was THE most
downloaded file from ASPHYXIA BBS last month … )


~ by infamouskiller on June 24, 2008.

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