//
// Fragment shader for environment mapping with an
// equirectangular 2D texture
//
// Authors: John Kessenich, Randi Rost
//
// Copyright (c) 2002-2004 3Dlabs Inc. Ltd. 
//
// See 3Dlabs-License.txt for license information
//

const vec3 Xunitvec = vec3 (1.0, 0.0, 0.0);
const vec3 Yunitvec = vec3 (0.0, 1.0, 0.0);

uniform vec3  BaseColor;
uniform float MixRatio;

uniform sampler2D EnvMap;

varying vec3  Normal;
varying vec3  EyeDir;
varying float LightIntensity;

void main (void)
{
    // Compute reflection vector
    
    vec3 reflectDir = reflect(EyeDir, Normal);

    // Compute altitude and azimuth angles

    vec2 index;

    index.y = dot(normalize(reflectDir), Yunitvec);
    reflectDir.y = 0.0;
    index.x = dot(normalize(reflectDir), Xunitvec) * 0.5;

    // Translate index values into proper range

    if (reflectDir.z >= 0.0)
        index = (index + 1.0) * 0.5;
    else
    {
        index.t = (index.t + 1.0) * 0.5;
        index.s = (-index.s) * 0.5 + 1.0;
    }
    
    // if reflectDir.z >= 0.0, s will go from 0.25 to 0.75
    // if reflectDir.z <  0.0, s will go from 0.75 to 1.25, and
    // that's OK, because we've set the texture to wrap.
  
    // Do a lookup into the environment map.

    vec3 envColor = vec3 (texture2D(EnvMap, index));

    // Add lighting to base color and mix

    vec3 base = LightIntensity * BaseColor;
    envColor = mix(envColor, base, MixRatio);

    gl_FragColor = vec4 (envColor, 1.0);
}