Re: 3d help for an idiot... =o)

["Josiah Avery" <josiahavery@hotmail.com>]

>Hi guys, village idiot here.
>I'm trying to learn 3d point plotting, and am trying to use this cam2scr() 
>function I got from a tutorial one you guys posted a link to, >to  make a 
>3d starfield thingie like the screen saver that comes with >windoze.  Right 
>now I'm having eye.* be fixed so I can get the program >down before I give 
>you the ability to pivot and move around. I tried >making the stars[].x, 
>stars[].y, and stars[].z, as well as screenx and >screeny, as fixed, 
>ints,and floats. Some gave me one of those memory >leaks screens where it 
>says "frame traceback bla bla bla", and in other >tries it set the gfx mode 
>and sat there with a blank screen 'til I >pressed a key.

*** not being very familiar with Allegro ( besides know that it was designed 
to be used with gnu compliers ), I can give it a shot. I didn't notice any 
intalization of a back buffer, so that kills my "you forgot to flip pages" 
theroy.....

>Here's the code. It's done using the allegro library, but even if you're 
>not familiar with it, you can hopefully still figure out the  >basic gist 
>of it and find what needs to be added/changed. Is this even >close to  
>working?
>thx guyz. ~Stephan~
>
>#include <stdio.h>
>#include <allegro.h>
>
>#define NUM_STARS 60
>
>struct POINT{
>  fixed x, y, z;
>//  fixed screenx, screeny;
>};
>
>struct POINT stars[NUM_STARS];
>struct POINT eye;
>fixed screenx, screeny;

*** note these variables are never initalized to anything.....

>int i;
>
>void cam2scr(int x, int y, int z, int *a, int *b){
>  *a = eye.x + x * eye.z / (z + eye.z);
>  *b = eye.y - y * eye.z / (z * eye.z);
>} // end cam2scr() func
>
>void init(){


>  allegro_init();

*** does this call create a back buffer? If so, then you have to flip the
offscreen page on to the screen before displaying the next screen....

>  install_keyboard();
>  set_gfx_mode(GFX_AUTODETECT, 640, 480, 0, 0);
>
>  for(i=0;i<NUM_STARS;i++){
>    stars[i].x = rand()%600; // <-- How do I make a rand()
>    stars[i].y = rand()%400; // <-- number that's a fixed
>    stars[i].z = rand()%400; // <-- or a float?
>    }

*** You can always get a random interger and divide it by another random 
interger to get a random float ( be sure implement error check; div by 0 )

>  eye.x = 0.0;
>  eye.y = 0.0;
>  eye.z = 300.0;
>} // end init() func
>
>int main(){
>  init();
>
>  while(!keypressed()){
>    vsync();
>    clear(screen);
>    for(i=0;i<NUM_STARS;i++){
>
>      if(--(stars[i].z) < 0){
>        stars[i].x = rand()%600;
>        stars[i].y = rand()%400;
>        stars[i].z = 400;
>        } //end if
>
>      cam2scr(stars[i].x, stars[i].y, stars[i].z, &screenx, &screeny);

*** Note: screenx and screeny have yet to be initalized.... I am assuming 
that they are in a header somewhere....

>     putpixel(screen, screenx, screeny, makecol(255, 255, 255));
>      } // end for loop thru the stars..
>    } // end while no keys pressed
>  return 0;
>} // end main


*** You can look through what I have written in your source for possible 
things you may have missed. I do however have a bit of a time finding the 
purpose of this function:

void cam2scr(int x, int y, int z, int *a, int *b)

I am assuming that this projects a vector so that it may be displayed on the 
screen. I think that it would be in better style if you changed it like so:

POINT cam2scr(POINT projectMe)
// note: I am unsure as to why you wish to point to 'a' and b', surely 
passing by value would be in better style; That and I am not sure what 
purpose they serve.

{
   POINT returnPoint;

   // project POINT

   return returnPoint;
}


*** I dunno... could you send me the link, perhaps that will clear things 
up.

   Josiah
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