distance_field.frag

#extension GL_OES_standard_derivatives : enable
precision highp float;
varying vec2 v_texCoord;
uniform sampler2D texture0;
const float distancethreshold = 0.5;

float aastep(float threshold, float dist)
{
	float afwidth = 0.5 * length(vec2(dFdx(dist), dFdy(dist)));
	return smoothstep(threshold - afwidth, threshold + afwidth, dist);
}

void main()
{
	float dist = texture2D(texture0, v_texCoord).a;
	float alpha = aastep(distancethreshold, dist);
	gl_FragColor = vec4(vec3(1.0), alpha);
}