From:	"23yrold3yrold" <cbarry AT pangea DOT ca>
Newsgroups:	comp.os.msdos.djgpp
References:	<39C9617E DOT A5EDE2E8 AT inwind DOT it>
Subject:	Re: problem with allegro
Date:	Sun, 24 Sep 2000 22:24:38 -0500
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

"Ivan Ferrara" <ifalpsfun AT inwind DOT it> wrote in message
news:39C9617E DOT A5EDE2E8 AT inwind DOT it...
> I finally solved my DPMI memory error (thanks to Martin!!!), but a new
> problem occurred.
> I'm trying to draw 3D polygons using allegro functions.

Jumping right into 3D, eh? ;)

> Here's a piece of code; It should draw a shaded triangle but my screen
> looks white! Why

Because your missing some steps. As I've learned myself lately, 3D is far
more complicated than that:

>
>
////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////
>
>
>
>
> #include <allegro.h>
> #include <stdio.h>
> #include <stdlib.h>
>
>
> BITMAP *texture;
> V3D v1 = {30 , 20 , 10 , 0 , 0 , 9 };
> V3D v2 = {30 , 40 , 10 , 0 , 0 , 9 };
> V3D v3 = {30 , 30 , 80 , 0 , 0 , 9 };

These coordinates will put your triangle in the far lower-right of the
screen, to make it more visible try

V3D v1 = {20, 0, 128, 0, 0, 9};
V3D v2 = {-25, -10, 140, 0, 0, 9};
V3D v3 = {-10, 20, 120, 0, 0, 9};

> int main()
> {
> allegro_init();
> install_keyboard();
> set_gfx_mode(GFX_AUTODETECT,800,600,0,0);
> set_palette(desktop_palette);
>
> triangle3d(screen,POLYTYPE_FLAT,texture,&v1,&v2,&v3);

Sorry, you can't just skip to that :). I'm not sure if it's mandatory, but
you should call

set_projection_viewport(0, 0, 800, 600);

Then - and this is the crucial part - you have to convert the 3D coordinates
to 2D ones (a monitor is 2D, not 3D, right?)

         persp_project(v1.x, v1.y, v1.z, &v1.x, &v1.y);
         persp_project(v2.x, v2.y, v2.z, &v2.x, &v2.y);
         persp_project(v3.x, v3.y, v3.z, &v3.x, &v3.y);

NOW you can call

triangle3d(screen, POLYTYPE_FLAT, NULL, &v1, &v2, &v3);

I haven't tested this, but it should work. Also keep in mind it is a VERY
sloppy way to do 3D; fine to make a triangle, totally unfeasible for any
real application. Spend much time studying Allegro's 3D samples, and a LOT
of time on 3D web tutorials to understand the basics (which you obviously
don't yet). Learning 3D takes a major comitment.......


Chris

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