precision mediump float;
varying vec2 v_texcoord;
uniform sampler2D tex;

const float temperature = 3500.0;
const float temperatureStrength = 1.0;

#define WithQuickAndDirtyLuminancePreservation
const float LuminancePreservationFactor = 1.0;

// function from https://www.shadertoy.com/view/4sc3D7
// valid from 1000 to 40000 K (and additionally 0 for pure full white)
vec3 colorTemperatureToRGB(const in float temperature){
    // values from: http://blenderartists.org/forum/showthread.php?270332-OSL-Goodness&p=2268693&viewfull=1#post2268693
    mat3 m = (temperature <= 6500.0) ? mat3(vec3(0.0, -2902.1955373783176, -8257.7997278925690),
                                            vec3(0.0, 1669.5803561666639, 2575.2827530017594),
                                            vec3(1.0, 1.3302673723350029, 1.8993753891711275)) :
                                       mat3(vec3(1745.0425298314172, 1216.6168361476490, -8257.7997278925690),
                                            vec3(-2666.3474220535695, -2173.1012343082230, 2575.2827530017594),
                                            vec3(0.55995389139931482, 0.70381203140554553, 1.8993753891711275));
    return mix(
        clamp(vec3(m[0] / (vec3(clamp(temperature, 1000.0, 40000.0)) + m[1]) + m[2]), vec3(0.0), vec3(1.0)),
        vec3(1.0),
        smoothstep(1000.0, 0.0, temperature)
    );
}

void main() {
    vec4 pixColor = texture2D(tex, v_texcoord);

    // RGB
    vec3 color = vec3(pixColor[0], pixColor[1], pixColor[2]);

#ifdef WithQuickAndDirtyLuminancePreservation
    color *= mix(1.0,
                 dot(color, vec3(0.2126, 0.7152, 0.0722)) / max(dot(color, vec3(0.2126, 0.7152, 0.0722)), 1e-5),
                 LuminancePreservationFactor);
#endif

    color = mix(color, color * colorTemperatureToRGB(temperature), temperatureStrength);

    vec4 outCol = vec4(color, pixColor[3]);

    gl_FragColor = outCol;
}