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: [
    '#version 300 es',
    'in vec2 uv;',
    'in vec3 position;',
    'uniform mat4 projectionMatrix;',
    'uniform mat4 modelViewMatrix;',
    'out vec2 vUV;',
    'void main(void) {',
    '  gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);',
    '  vUV = uv;',
    '}'
  ].join('\n'),

  fragmentShader: [
    '#version 300 es',
    'precision highp float;',
    'uniform float alphaTest;',
    'uniform float opacity;',
    'uniform sampler2D map;',
    'uniform vec3 color;',
    'in vec2 vUV;',
    'out vec4 fragColor;',

    '#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 = texture(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(texture(map, box.xy).a, width)',
    '        + contour(texture(map, box.zw).a, width)',
    '        + contour(texture(map, box.xw).a, width)',
    '        + contour(texture(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 = texture(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',

    '  fragColor = vec4(color, opacity * alpha);',
    '}'
  ].join('\n')
});