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#define FP highp
#extension GL_OES_standard_derivatives: enable
uniform FP sampler2D distanceFieldTexture;
uniform FP float minAlpha;
uniform FP float maxAlpha;
uniform FP float textureSize;
uniform FP vec4 color;
varying FP vec3 position;
varying FP vec2 texCoord;
void main()
{
// determine the scale of the glyph texture within pixel-space coordinates
// (that is, how many pixels are drawn for each texel)
FP vec2 texelDeltaX = abs(dFdx(texCoord));
FP vec2 texelDeltaY = abs(dFdy(texCoord));
FP float avgTexelDelta = textureSize * 0.5 * (texelDeltaX.x + texelDeltaX.y + texelDeltaY.x + texelDeltaY.y);
FP float texScale = 1.0 / avgTexelDelta;
// scaled to interval [0.0, 0.15]
FP float devScaleMin = 0.00;
FP float devScaleMax = 0.15;
FP float scaled = (clamp(texScale, devScaleMin, devScaleMax) - devScaleMin) / (devScaleMax - devScaleMin);
// thickness of glyphs should increase a lot for very small glyphs to make them readable
FP float base = 0.5;
FP float threshold = base * scaled;
FP float range = 0.06 / texScale;
FP float minAlpha = threshold - range;
FP float maxAlpha = threshold + range;
FP float distVal = texture2D(distanceFieldTexture, texCoord).r;
gl_FragColor = vec4(color.rgb, color.a * smoothstep(minAlpha, maxAlpha, distVal));
}
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