anchor.diff:  
   #pragma tangram: global
   
  -float contour(in float d, in float w, float t) {
  -    return smoothstep(t - w, t + w, d);
  -}
  -
  -float sample(in vec2 uv, float w, float t) {
  -    return contour(texture2D(u_tex, uv).a, w, t);
  -}
  -
  -float sampleAlpha(in vec2 uv, float distance, float threshold) {
  -    const float smoothing = 0.0625 * emSize; // 0.0625 = 1.0/1em ratio
  -    float alpha = contour(distance, smoothing, threshold);
  -
  -#ifdef TANGRAM_SDF_MULTISAMPLING
  -    const float aaSmooth = smoothing / 2.0;
  -    float dscale = 0.354; // 1 / sqrt(2)
  -    vec2 duv = dscale * (dFdx(uv) + dFdy(uv));
  -    vec4 box = vec4(uv - duv, uv + duv);
  -
  -    float asum = sample(box.xy, aaSmooth, threshold)
  -               + sample(box.zw, aaSmooth, threshold)
  -               + sample(box.xw, aaSmooth, threshold)
  -               + sample(box.zy, aaSmooth, threshold);
  -
  -    alpha = mix(alpha, asum, 0.25);
  -#endif
  -
  -    return alpha;
  -}
  -
   void main(void) {
   
       vec4 color = v_color;
   
  -    float dist = texture2D(u_tex, v_texcoords).a;
  +    float signed_distance = texture2D(u_tex, v_texcoords).a;
   
       // - At the glyph outline alpha is 0.5
       //
  
positive.code: 
  #pragma tangram: defines
  
  const float emSize = 15.0 / 16.0;
  
  uniform sampler2D u_tex;
  uniform vec3 u_map_position;
  uniform vec3 u_tile_origin;
  
negative.code: #pragma tangram: extensions
  
  #ifdef GL_ES
      precision mediump float;
  