[A5] Drawing a textured trapezoid (2D)

no-reply@allegro.cc (CrizerPL) — Thu, 28 Jul 2016 17:48:10 +0000

Hello,

I've been recently trying to draw a textured trapezoid using al_draw_prim() so I can make these pseudo-3D buildings like in GTA 1 possible.
And I ran into an obvious issue:
{"name":"t.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/0\/7\/0710027359231792b6461c324445e409.png","w":303,"h":251,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/0\/7\/0710027359231792b6461c324445e409"}
Is there any way to make it look properly? I'm using Allegro 5.0.4.

Here's the code, just in case:

#SelectExpand
  int bmp_w = al_get_bitmap_width(textures.GetBitmapResource("Brickwall"))-1;
  int bmp_h = al_get_bitmap_height(textures.GetBitmapResource("Brickwall"))-1;
  ALLEGRO_VERTEX v[] = 
  {
      {200, 200, 0, 0, 0, al_map_rgb(255,255,255)},
      {350, 400, 0, bmp_w, bmp_h, al_map_rgb(255,255,255)},
      {250, 400, 0, 0, bmp_h, al_map_rgb(255,255,255)},
      
      {200, 200, 0, 0, 0, al_map_rgb(255,255,255)},
      {400, 200, 0, bmp_w, 0, al_map_rgb(255,255,255)},
      {350, 400, 0, bmp_w, bmp_h, al_map_rgb(255,255,255)}
  };
  al_draw_prim(v, NULL, textures.GetBitmapResource("Brickwall"), 0, 6, ALLEGRO_PRIM_TRIANGLE_LIST);

no-reply@allegro.cc (jmasterx) — Thu, 28 Jul 2016 18:27:07 +0000

First, define your definition of 'looks right' (has screenshot)?

The main issue is that shingles do not naturally warp inwards like that.

You need to play with your UV coordinates such that either:

Your bottom UV coordinates form at straight line with the top ones:

eg:

*              *

*              *

Or:
You make the UV parellel eg:

*             *

     *             *

Right now you have the UV like a trapezoid because you have a 1 to 1 mapping with your texture, so the shingles will warp of course.

If we imagine your cords are currently:

0,0                        1,0

0,1                        1,1

Then you want to solve for the UV coord that forms a right triangle with your trapezoid.

no-reply@allegro.cc (SiegeLord) — Thu, 28 Jul 2016 20:19:47 +0000

Basically, as jmasterx is explaining, this transformation is non-linear, so you can't do it without introducing some non-linearity. The typical way of accomplishing this is to use a projection transform (i.e. literally draw your rectangle in 3D and use perspective to accomplish what you want). Unfortunately, the feature to do that is only in Allegro 5.2. Here's some sample code (I forget how I came up with the constants, I'll think about it some more later):

#SelectExpand
  1#include 
  2#include 
  3
  4int main()
  5{
al_init();
al_init_primitives_addon();
ALLEGRO_DISPLAY* d = al_create_display(800, 600);

ALLEGRO_VERTEX vtx[4];
vtx[0].x = -400;
vtx[0].y = -200;
vtx[0].z = -400;
vtx[0].color = al_map_rgb_f(0, 0, 1);
 15
vtx[1].x = -400;
vtx[1].y = 200;
vtx[1].z = -800;
vtx[1].color = al_map_rgb_f(0, 1, 0);
 20
vtx[2].x = 400;
vtx[2].y = 200;
vtx[2].z = -800;
vtx[2].color = al_map_rgb_f(1, 0, 1);
 25
vtx[3].x = 400;
vtx[3].y = -200;
vtx[3].z = -400;
vtx[3].color = al_map_rgb_f(1, 1, 1);

ALLEGRO_TRANSFORM t;
al_identity_transform(&t);
al_perspective_transform(&t, -400, -300, 400, 400, 300, 10000);
al_use_projection_transform(&t);

al_clear_to_color(al_map_rgb_f(0, 0, 0));
al_draw_prim(vtx, NULL, NULL, 0, 4, ALLEGRO_PRIM_TRIANGLE_FAN);
al_flip_display();
al_rest(1.0);
 40}

no-reply@allegro.cc (CrizerPL) — Thu, 28 Jul 2016 23:44:12 +0000

This is what I'm looking for, if it wasn't clear.
{"name":"bw.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/a\/3aecb940a35bebc8b17fbb3fa434d6f5.png","w":256,"h":256,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/a\/3aecb940a35bebc8b17fbb3fa434d6f5"}
Thanks though, I guess my main priority now is to get Allegro 5.2 working

no-reply@allegro.cc (Edgar Reynaldo) — Fri, 29 Jul 2016 02:04:53 +0000

CrizerPL said:

Thanks though, I guess my main priority now is to get Allegro 5.2 working

I offer binaries for MinGW 4.8.1 and Allegro 5.2 if you're interested. The A5.2 package includes all dependencies.

https://sourceforge.net/projects/unofficialmingw/files/mingw_4_8_1-4.tar.7z/download

https://sourceforge.net/projects/unofficialallegro5distribution/files/A52GITdistro2.tar.7z/download

no-reply@allegro.cc (Arthur Kalliokoski) — Fri, 29 Jul 2016 02:42:48 +0000

https://www.allegro.cc/forums/thread/614834

no-reply@allegro.cc (CrizerPL) — Sat, 30 Jul 2016 16:40:59 +0000

It works! Thanks to all of you

{"name":"finally.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/1\/c1f0941ed0cc3251c2df118f1cd16f73.png","w":1219,"h":680,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/1\/c1f0941ed0cc3251c2df118f1cd16f73"}