vec3 TRIANGLE(vec3 color, vec3 background, vec2 vertices[3], vec2 uv);

vec2 rotate_point(vec2 point, vec2 pivot_point, float angle);

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 uv = fragCoord/iResolution.xy;


    vec3 color = vec3(0.0);
    float len = .5f;
    vec2 middle = vec2(iResolution.z/2.0, iResolution.z/2.0);
    vec2 a = vec2(iResolution.z/2.0, iResolution.z/2.0  - len);
    vec2 b = vec2(iResolution.z/2.0 - len, iResolution.z/2.0);

    float angle = 105.0f  - atan(iMouse.y - middle.y, iMouse.x - middle.x);

    vec2 VERTS[3] = vec2[]
    (
        middle,
        rotate_point(a, middle, angle),
        rotate_point(b, middle, -angle)
    );

    color = TRIANGLE(vec3(1.0), color, VERTS, uv);

    fragColor = vec4(color,1.0);
}

float TRIANGLE_sign(vec2 p1, vec2 p2, vec2 p3)
{
    return (p1.x - p3.x) * (p2.y - p3.y) - (p2.x - p3.x) * (p1.y - p3.y);
}

vec3 TRIANGLE(vec3 color, vec3 background, vec2 vertices[3], vec2 uv)
{
    bool b1 = TRIANGLE_sign(uv, vertices[0], vertices[1]) < 0.0f;
    bool b2 = TRIANGLE_sign(uv, vertices[1], vertices[2]) < 0.0f;
    bool b3 = TRIANGLE_sign(uv, vertices[2], vertices[0]) < 0.0f;

    if((b1 == b2) && (b2 == b3))return color;
    return background;
}


vec2 rotate_point(vec2 point, vec2 pivot_point, float angle)
{
    vec2 new_p;
    new_p.x =  - cos(angle) * (point.x - pivot_point.x) + sin(angle) * (point.y - pivot_point.y) + pivot_point.x;

    new_p.y = - sin(angle) * (point.x - pivot_point.x) + cos(angle) * (point.y - pivot_point.y) + pivot_point.y;
    return new_p;
}