Question

I am trying to make a few effects in a C+GL game. So far I draw all my sprites as a quad, and it works.

However, I am trying to make a large ring appear at times, with a texture following that ring, as it takes less memory than a quad with the ring texture inside.
The type of ring I want to make is not a round-shaped GL mesh ring (the “tube” type) but a “paper” 2D ring. That way I can modify the “width” of the ring, getting more of the effect than a simple quad+ring texture.
So far all my attempts have been…kind of ridiculous, as I don’t understand GL’s coordinates too well (and I can’t really understand the available documentation…I am just a designer with no coder help or background. A n00b, basically).

glBegin(GL_POLYGON);
for(i = 0;i < 360; i += 10){
  glTexCoord2f(0, 0);
  glVertex2f(Cos(i)*(H-10),Sin(i)*H);
  glTexCoord2f(0, HP);
  glVertex2f(Sin(i)*(H-10),Cos(i)*(H-10));
  glTexCoord2f(WP, HP);
  glVertex2f(Cos(i)*H,Sin(i)*(H-10));
  glTexCoord2f(WP, 0);
  glVertex2f(Sin(i)*H,Cos(i)*H);
} 
glEnd();

This is my last attempt, and it seems to generate a “sunburst” from the right edge of the circle instead of a ring. It’s an amusing effect but definitely not what I want. Other results included the circle looking exactly the same as the quad textured (aka drawing a sprite literally) or something that looked like a pop-art filter, by working on this train of thought.

Seems like my logic here is entirely flawed, so, what would be the easiest way to obtain such a ring? No need to reply in code, just some guidance for a non-math-skilled user…

Edit: A different way to word what I want, would be a sequence of rotated rectangles connected to each other, forming a low-resolution ring.

Answer 1

So you want an annulus? That is, the area between two circles with the same center but different radii? I’d try a quad strip like this:

glBegin(GL_QUAD_STRIP);
for(i = 0; i <= 360; i += 10){
  glTexCoord2f(WP*i/360, 0);
  glVertex2f(Cos(i)*(H-10),Sin(i)*(H-10));
  glTexCoord2f(WP*i/360, HP);
  glVertex2f(Cos(i)*H,Sin(i)*H);
} 
glEnd();

Each quad is a 10-degree sector of the ring. Note that if you want to draw N quads in a strip, it takes 2*(N+1) points, so we draw a total of 2*(36+1) = 74 points.