#version 110

#define PI 3.141592653589793238462643383279

#define IMAGE_FRONT 0
#define IMAGE_BACK 1
#define IMAGE_LEFT 2
#define IMAGE_RIGHT 3
#define IMAGE_BOTTOM 4
#define IMAGE_TOP 5

//the six input textures
uniform sampler2D frontTex;
uniform sampler2D backTex;
uniform sampler2D leftTex;
uniform sampler2D rightTex;
uniform sampler2D bottomTex;
uniform sampler2D topTex;

//whether or not to flip the image
uniform int flip;

//the amount of yaw and pitch correction to apply (in radians)
uniform float yawCorrection;
uniform float pitchCorrection;

/**
 * Rotates a cartesian vector around the X and Y axes.
 * @param rotX the amount to rotate around the X axis (resulting in pitch change)
 * @param rotY the amount to rotate around the Y axis (resulting in yaw change)
 */
vec3 rotateVector(vec3 vector, float rotX, float rotY) {
    //using a rotation matrix to calculate the new vector

    //rotate around the X-axis
    vector.x = vector.x;
    vector.y = vector.y * cos(rotX) - vector.z * sin(rotX);
    vector.z = vector.y * sin(rotX) + vector.z * cos(rotX);

    //rotate around the Y-axis
    vector.x = vector.x * cos(rotY) + vector.z * sin(rotY);
    vector.y = vector.y;
    vector.z = -vector.x * sin(rotY) + vector.z * cos(rotY);

    return vector;
}

void main() {
    //normalized texture coordinates (from 0 to 1)
    float u = gl_TexCoord[0].x;
    float v = gl_TexCoord[0].y;

    //vertically flip the image if needed
    if(flip == 1) {
        v = 1.0 - gl_TexCoord[0].y;
    }

    //convert the texture coordinates to polar coordinates
    //which still resolve to points on the 2d face
    float phi = u * 2.0 * PI;
    float theta = v * PI;

    //calculate an unit vector in an cartesian coordinate system
    //that points from the center of a hypothetical sphere to the point on the sphere
    //that is mapped to the current pixel's location on the 2d face.
    float x = sin(phi) * sin(theta) * -1.0;
    float y = cos(theta);
    float z = cos(phi) * sin(theta) * -1.0;

    //rotate the vector to respect pitch and yaw correction
    vec3 rotated = rotateVector(vec3(x, y, z), -pitchCorrection, -yawCorrection);
    x = rotated.x;
    y = rotated.y;
    z = rotated.z;

    //divide the vector directions by the longest direction
    //to get the general direction of the vector,
    //thus finding out which of the cube's faces should be mapped to the point on the sphere.
    float a = max(abs(x), max(abs(y), abs(z)));

    float xa = x/a;
    float ya = y/a;
    float za = z/a;

    //normalized pixel position on target face
    float texX, texY;

    //determine target face
    //int target;
    if(xa == 1.0) {
        //target = IMAGE_RIGHT;
        texX = abs(((za + 1.0) / 2.0) - 1.0);
        texY = abs(((ya + 1.0) / 2.0));
        gl_FragColor = texture2D(rightTex, vec2(texX, texY));
        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
    } else if(xa == -1.0) {
        //target = IMAGE_LEFT;
        texX = abs(((za + 1.0) / 2.0));
        texY = abs(((ya + 1.0) / 2.0));
        gl_FragColor = texture2D(leftTex, vec2(texX, texY));
        gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
    } else if(ya == 1.0) {
        //target = IMAGE_TOP;
        texX = abs(((xa + 1.0) / 2.0));
        texY = abs(((za + 1.0) / 2.0) - 1.0);
        gl_FragColor = texture2D(topTex, vec2(texX, texY));
        gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);
    } else if(ya == -1.0) {
        //target == IMAGE_BOTTOM;
        texX = abs(((xa + 1.0) / 2.0));
        texY = abs(((za + 1.0) / 2.0));
        gl_FragColor = texture2D(bottomTex, vec2(texX, texY));
        gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
    } else if(za == 1.0) {
        //target = IMAGE_FRONT;
        texX = abs(((xa + 1.0) / 2.0));
        texY = abs(((ya + 1.0) / 2.0));
        gl_FragColor = texture2D(frontTex, vec2(texX, texY));
        gl_FragColor = vec4(1.0, 0.0, 1.0, 1.0);
    } else if(za == -1.0) {
        //target = IMAGE_BACK;
        texX = abs(((xa + 1.0) / 2.0) - 1.0);
        texY = abs(((ya + 1.0) / 2.0));
        gl_FragColor = texture2D(backTex, vec2(texX, texY));
        gl_FragColor = vec4(0.0, 1.0, 1.0, 1.0);
    } else {
        gl_FragColor = vec4(xa, ya, za, 1.0);
    }

}