#version 330 core
out vec4 FragColor;

in vec3 ourColor;
in vec2 TexCoord;

uniform sampler2D ourTexture;
uniform sampler2D ourSecondTexture;
uniform float visibility1;

vec4 temp1, temp2, temp3;
float temp_red, temp_green, temp_blue, temp_alpha;

vec2 reverse(vec2 someCoord)
{
    return vec2(1 - someCoord.x, someCoord.y);
}

vec4 calc(vec4 underColor, vec4 aboveColor)
{
    // let's consider vec4 beforeColor and vec4 afterColor and resultColor
    // x = beforeColor.r
    // y = underColor.r
    // z = aboveColor.r
    // a = underColor.a
    // b = aboveColor.a
    // h = afterColor.r
    // magic = resultColor.r
    // h = (x * (1-a) + y * a) * (1-b) + z * b
    // h = x * (1-a) * (1-b) + y * a * (1-b) + z * b
    // h = x * (1-a-b+ab) + magic * (a+b-ab)
    temp_alpha = underColor.a + aboveColor.a - underColor.a * aboveColor.a;
    // y * a * (1-b) + z * b = magic * (a+b-ab)
    // magic = (y * a * (1-b) + z * b) / (a+b-ab)
    temp_red = (underColor.r * underColor.a * (1 - aboveColor.a) + aboveColor.r * aboveColor.a) / temp_alpha;
    temp_green = (underColor.g * underColor.a * (1 - aboveColor.a) + aboveColor.g * aboveColor.a) / temp_alpha;
    temp_blue = (underColor.b * underColor.a * (1 - aboveColor.a) + aboveColor.b * aboveColor.a) / temp_alpha;
    return vec4(temp_red, temp_green, temp_blue, temp_alpha);
}

vec4 changeVisibility(vec4 initialColor, float visibility)
{
    return vec4(initialColor.rgb, initialColor.a * visibility);
}

void main()
{
    temp1 = texture(ourTexture, TexCoord);
    temp2 = texture(ourSecondTexture, reverse(TexCoord));
    temp3 = calc(temp1, changeVisibility(temp2, visibility1));
    FragColor = temp3;
}