text: var registerShader = require('../core/shader').registerShader;
  
  /**
   * Signed distance field.
   * Used by text component.
   */
  module.exports.Shader = registerShader('sdf', {
    schema: {
      alphaTest: {type: 'number', is: 'uniform', default: 0.5},
      color: {type: 'color', is: 'uniform', default: 'white'},
      map: {type: 'map', is: 'uniform'},
      opacity: {type: 'number', is: 'uniform', default: 1.0}
    },
  
    raw: true,
  
    vertexShader: [
      'attribute vec2 uv;',
      'attribute vec3 position;',
      'uniform mat4 projectionMatrix;',
      'uniform mat4 modelViewMatrix;',
      'varying vec2 vUV;',
      'void main(void) {',
      '  gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);',
      '  vUV = uv;',
      '}'
    ].join('\n'),
  
    fragmentShader: [
      '#ifdef GL_OES_standard_derivatives',
      '#extension GL_OES_standard_derivatives: enable',
      '#endif',
  
      'precision highp float;',
      'uniform float alphaTest;',
      'uniform float opacity;',
      'uniform sampler2D map;',
      'uniform vec3 color;',
      'varying vec2 vUV;',
  
      '#ifdef GL_OES_standard_derivatives',
      '  float contour(float width, float value) {',
      '    return smoothstep(0.5 - value, 0.5 + value, width);',
      '  }',
      '#else',
      '  float aastep(float value, float afwidth) {',
      '    return smoothstep(0.5 - afwidth, 0.5 + afwidth, value);',
      '  }',
      '#endif',
  
      // FIXME: Experimentally determined constants.
      '#define BIG_ENOUGH 0.001',
      '#define MODIFIED_ALPHATEST (0.02 * isBigEnough / BIG_ENOUGH)',
      '#define ALL_SMOOTH 0.4',
      '#define ALL_ROUGH 0.02',
      '#define DISCARD_ALPHA (alphaTest / (2.2 - 1.2 * ratio))',
  
      'void main() {',
         // When we have derivatives and can get texel size for supersampling.
      '  #ifdef GL_OES_standard_derivatives',
      '    vec2 uv = vUV;',
      '    vec4 texColor = texture2D(map, uv);',
      '    float dist = texColor.a;',
      '    float width = fwidth(dist);',
      '    float alpha = contour(dist, width);',
      '    float dscale = 0.353505;',
  
      '    vec2 duv = dscale * (dFdx(uv) + dFdy(uv));',
      '    float isBigEnough = max(abs(duv.x), abs(duv.y));',
  
           // When texel is too small, blend raw alpha value rather than supersampling.
           // FIXME: experimentally determined constant
      '    if (isBigEnough > BIG_ENOUGH) {',
      '      float ratio = BIG_ENOUGH / isBigEnough;',
      '      alpha = ratio * alpha + (1.0 - ratio) * dist;',
      '    }',
  
           // Otherwise do weighted supersampling.
           // FIXME: why this weighting?
      '    if (isBigEnough <= BIG_ENOUGH) {',
      '      vec4 box = vec4 (uv - duv, uv + duv);',
      '      alpha = (alpha + 0.5 * (',
      '        contour(texture2D(map, box.xy).a, width)',
      '        + contour(texture2D(map, box.zw).a, width)',
      '        + contour(texture2D(map, box.xw).a, width)',
      '        + contour(texture2D(map, box.zy).a, width)',
      '      )) / 3.0;',
      '    }',
  
           // Do modified alpha test.
      '    if (alpha < alphaTest * MODIFIED_ALPHATEST) { discard; return; }',
  
      '  #else',
           // When we don't have derivatives, use approximations.
      '    vec4 texColor = texture2D(map, vUV);',
      '    float value = texColor.a;',
           // FIXME: if we understood font pixel dimensions, this could probably be improved
      '    float afwidth = (1.0 / 32.0) * (1.4142135623730951 / (2.0 * gl_FragCoord.w));',
      '    float alpha = aastep(value, afwidth);',
  
           // Use gl_FragCoord.w to guess when we should blend.
           // FIXME: If we understood font pixel dimensions, this could probably be improved.
      '    float ratio = (gl_FragCoord.w >= ALL_SMOOTH) ? 1.0 : (gl_FragCoord.w < ALL_ROUGH) ? 0.0 : (gl_FragCoord.w - ALL_ROUGH) / (ALL_SMOOTH - ALL_ROUGH);',
      '    if (alpha < alphaTest) { if (ratio >= 1.0) { discard; return; } alpha = 0.0; }',
      '    alpha = alpha * ratio + (1.0 - ratio) * value;',
      '    if (ratio < 1.0 && alpha <= DISCARD_ALPHA) { discard; return; }',
      '  #endif',
  
      '  gl_FragColor = vec4(color, opacity * alpha);',
      '}'
    ].join('\n')
  });
  