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precision mediump float;

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in vec2 pos;

out vec4 colour;

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uniform sampler2D image;

uniform vec2 res;

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vec4 packHeight(float value) {

    int rgba = floatBitsToInt(value);

    float a = float((rgba & 0xff000000) >> 24) / 255.0;

    float b = float((rgba & 0x00ff0000) >> 16) / 255.0;

    float g = float((rgba & 0x0000ff00) >> 8) / 255.0;

    float r = float(rgba & 0x000000ff) / 255.0;

    a = max(1./255., a);

    // return vec4(r, g, b, a);

  return vec4(vec3(value), 1.);

}

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float unpackHeight(vec4 color) {

    // int rgba = (int(color.a * 255.0) << 24) | (int(color.b * 255.0) << 16) | (int(color.g * 255.0) << 8) | int(color.r * 255.0);

    // return intBitsToFloat(rgba);

  return colour.r;

}

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void main() {

  vec2 bottomLeft = (floor(pos * res) - vec2(0.5))/res;

  vec2 topRight = (floor(pos * res) + vec2(0.5))/res;

  float t = topRight.y;

  float b = bottomLeft.y;

  float l = bottomLeft.x;

  float r = topRight.x;

  float tl = unpackHeight(texture(image, vec2(l, t)));

  float tr = unpackHeight(texture(image, vec2(r, t)));

  float bl = unpackHeight(texture(image, vec2(l, b)));

  float br = unpackHeight(texture(image, vec2(r, b)));

  float maxHeight = max(max(tl, tr), max(bl, br));

  colour = packHeight(maxHeight);

}

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