OpenGL ES iPhone - drawing anti aliased lines

Question

Normally, you'd use something like:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
glEnable(GL_LINE_SMOOTH);

glLineWidth(2.0f);

glVertexPointer(2, GL_FLOAT, 0, points);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_LINE_STRIP, 0, num_points);

glDisableClientState(GL_VERTEX_ARRAY);

It looks good in the iPhone simulator, but on the iPhone the lines get extremely thin and w/o any anti aliasing.

How do you get AA on iPhone?

Answer 1

One can achieve the effect of anti aliasing very cheaply using vertices with opacity 0. Here's an image example to explain:

Comparison with AA:

You can read a paper about this here:

http://research.microsoft.com/en-us/um/people/hoppe/overdraw.pdf

You could do something along this way:

// Colors is a pointer to unsigned bytes (4 per color).
// Should alternate in opacity.
glColorPointer(4, GL_UNSIGNED_BYTE, 0, colors);
glEnableClientState(GL_COLOR_ARRAY);

// points is a pointer to floats (2 per vertex)
glVertexPointer(2, GL_FLOAT, 0, points);
glEnableClientState(GL_VERTEX_ARRAY);

glDrawArrays(GL_TRIANGLE_STRIP, 0, points_count);

glDisableClientState(GL_VERTEX_ARRAY);
glDisableClientState(GL_COLOR_ARRAY);