Alpha blending a triangle with a textured triangle in openGL

no-reply@allegro.cc (altalena) — Thu, 09 May 2013 08:32:52 +0000

in this program,
https://www.allegro.cc/forums/thread/612527/981976#target
I've got pairs of 3d triangles making up a terrain. I also subdivide them into powers of 2, depending on how close the camera is to the middle of the 2 triangles' shared edge.

I want to draw the triangles with the 3 corner colours interpolated smoothly blended with a repeating procedurally generated detail texture.

I'm wondering if there's a way to draw a normal triangle, like this

glBegin(GL_TRIANGLE_STRIP);
glColor3d(0.75,0.25,0.25);
glVertex3d( 0,1,0 );
glColor3d(0.25,0.75,0.25);
glVertex3d( 1,0,0 );
glColor3d(0.25,0.25,0.75);
glVertex3d( 1,1,0 );
glEnd();

with a repeating texture modifying the colour of each pixel in the triangle. The detail textures in Deus Ex are my inspiration for wanting to do this.

Sorry if there's an obvious answer I should have found.

no-reply@allegro.cc (Trent Gamblin) — Thu, 09 May 2013 09:38:18 +0000

Assuming you're using Allegro and you have loaded the texture into a bitmap, then

GLint tex = al_get_opengl_texture(allegro_bitmap);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, tex);

glBegin(GL_TRIANGLES);
glTexCoord3d(tu, tv)
/* Define the rest of this vertex */
... more vertices ...

tu and tv are texture coordinates. In OpenGL by default those are from 0 (left and bottom) to 1 (right and top).

no-reply@allegro.cc (altalena) — Fri, 10 May 2013 00:07:47 +0000

I tried what you suggested, but the program crashes now. Is there a short example program that just loads a .png and draws a textured triangle?

edit:

I attached the program I'm working on
here's some of the code I'm using(example):

#SelectExpand
  1#include 
  2ALLEGRO_BITMAP *image = NULL;
  3image = al_load_bitmap("noise.png");
  4GLuint texture = al_get_opengl_texture(image);
  5if (texture != 0)
  6    glBindTexture(GL_TEXTURE_2D, texture);
  7glEnable(GL_DEPTH_TEST);
  8glDepthMask(GL_TRUE);
  9glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 10glMatrixMode(GL_PROJECTION);
 11glLoadIdentity();
 12gluPerspective(85,aspect_ratio,0.001,1000000);
 13gluLookAt(eye[0],eye[1],eye[2],                            // camera is here
 14          looking_at[0],looking_at[1],looking_at[2],       // camera is looking towards this point
 15          0.0f,0.0f,1.0f);                                 // up vector
 16
 17glMatrixMode(GL_MODELVIEW);
 18glLoadIdentity();
 19glClear(GL_DEPTH_BUFFER_BIT);
 20glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
 21glClear(GL_DEPTH_BUFFER_BIT);
 22
 23glBegin(GL_TRIANGLES);
 24glColor3d(0.5,0.5,0.5);
 25glTexCoord2d(1, 1);
 26glVertex3d( 1,1,0 );
 27glTexCoord2d(1, 0);
 28glVertex3d( 1,0,0 );
 29glTexCoord2d(0, 1);
 30glVertex3d( 0,1,0 );
 31glEnd();

no-reply@allegro.cc (Trent Gamblin) — Fri, 10 May 2013 01:28:24 +0000

The code I posted will not crash as long as allegro_bitmap is valid.

no-reply@allegro.cc (altalena) — Fri, 10 May 2013 02:06:38 +0000

Thanks. I'm gonna try http://wiki.allegro.cc/index.php?title=2D_using_OpenGL, and I'll edit if I get it to work.

edit:

I'm embarrassed to say I couldn't get it to work. Is there an example program anywhere that draws textured triangles?