float aa = 0.75 * length(vec2(dFdx(d), dFdy(d)));  // antialias

// GLSL 1.2 to correspond with OpenGL 2.0
#version 120

// predefined variables
uniform sampler2D tex0;

float contour(in float d, in float w) {
    // smoothstep(lower edge0, upper edge1, x)
    return smoothstep(0.5 - w, 0.5 + w, d);
}

float samp(in vec2 uv, float w) {
    return contour(texture2D(tex0, uv).a, w);
}

void main(void) {

    // retrieve distance from texture
    vec2 uv = gl_TexCoord[0].xy;
    float dist = texture2D(tex0, uv).a;

    // fwidth helps keep outlines a constant width irrespective of scaling
    // GLSL's fwidth = abs(dFdx(uv)) + abs(dFdy(uv))
    float width = fwidth(dist);
    // Stefan Gustavson's fwidth
    //float width = 0.7 * length(vec2(dFdx(dist), dFdy(dist)));

// basic version
    //float alpha = smoothstep(0.5 - width, 0.5 + width, dist);

// supersampled version

    float alpha = contour( dist, width );
    //float alpha = aastep( 0.5, dist );

    // ------- (comment this block out to get your original behavior)
    // Supersample, 4 extra points
    float dscale = 0.354; // half of 1/sqrt2; you can play with this
    vec2 duv = dscale * (dFdx(uv) + dFdy(uv));
    vec4 box = vec4(uv-duv, uv+duv);

    float asum = samp( box.xy, width )
               + samp( box.zw, width )
               + samp( box.xw, width )
               + samp( box.zy, width );

    // weighted average, with 4 extra points having 0.5 weight each,
    // so 1 + 0.5*4 = 3 is the divisor
    alpha = (alpha + 0.5 * asum) / 3.0;

    // -------

    gl_FragColor = vec4(gl_Color.rgb, alpha);
}