Question

There are three main ways I know of to draw a simple circle in OpenGL ES, as provided by the iPhone. They are all based on a simple algorithm (the VBO version is below).

void circleBufferData(GLenum target, float radius, GLsizei count, GLenum usage) {
  const int segments = count - 2;

  const float coefficient = 2.0f * (float) M_PI / segments;

  float *vertices = new float[2 * (segments + 2)];

  vertices[0] = 0;
  vertices[1] = 0;
  for (int i = 0; i <= segments; ++i) {
    float radians = i * coefficient;
    float j = radius * cosf(radians);
    float k = radius * sinf(radians);
    vertices[(i + 1) * 2] = j;
    vertices[(i + 1) * 2 + 1] = k;
  }

  glBufferData(target, sizeof(float) * 2 * (segments + 2), vertices, usage);
  glVertexPointer(2, GL_FLOAT, 0, 0);

  delete[] vertices;
}

The three ways that I know of to draw a simple circle are by using glDrawArray from an array of vertices held by the application; using glDrawArray from a vertex buffer; and by drawing to a texture on initialization and drawing the texture when rendering is requested. The first two methods I know fairly well (though I have not been able to get anti-aliasing to work). What code is involved for the last option (I am very new to OpenGL as a whole, so a detailed explanation would be very helpful)? Which is most efficient?

Answer 1

Antialiasing in the iOS OpenGL ES impelmentation is severely limited. You won’t be able to draw antialiased circles using traditional methods.

However, if the circles you’re drawing aren’t that large, and are filled, you could take a look at using GL_POINT_SMOOTH. It’s what I used for my game, Pizarro, which involves a lot of circles. Here’s a detailed writeup of my experience with drawing antialiased circles on the iOS:

http://sveinbjorn.org/drawing_antialiased_circles_opengl_iphone