OpenGL ES 2.0 Equivalent for ES 1.0 Circles Using GL_POINT_SMOOTH?

Question

OpenGL ES 2.0 doesn't have the GL_POINT_SMOOTH definition which ES 1.0 does. This means code I was using to draw circles no longer works:

glEnable(GL_POINT_SMOOTH);
glPointSize(radius*2);
glDrawArrays(GL_POINTS,0,nPoints);

Is there an equivalent in ES 2.0, perhaps something to go in the vertex shader, or must I use polygons for each circle?

Answer 1

You can use point sprites to emulate this. Just enable point sprites and you get a special variable gl_PointCoord that you can read in the fragment shader. This gives you the coordinates of the fragment in the square of the current point. You can just use these to read a texture that contains a circle (pixels not in circle have color of 0) and then discard every fragment, whose texture value is 0:

if(texture2d(circle, gl_PointCoord).r < 0.1)
   discard;

EDIT: Or you can do it without a texture, by trading texture access latency for computational complexity and just evaluating the circle equation:

if(length(gl_PointCoord-vec2(0.5)) > 0.5)
    discard;

This might be further optimized by dropping the square root (used in the length function) and comparing against the squared radius:

vec2 pt = gl_PointCoord - vec2(0.5);
if(pt.x*pt.x+pt.y*pt.y > 0.25)
    discard;

But maybe the builtin length function is even faster than this, being optimized for length computation and maybe implemented directly in hardware.