/*
    Copyright (C) 2021  hiimgoodpack

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/
#version 330 core

in vec2 vFragCoord;
in vec2 vTexCoord;
uniform sampler2D aTexture;
uniform float aHorizontalOffset;

// Set to 1 or -1 to flip Y
uniform float aTexCoordYScale;

// Adds the Z value to a vec2, calculating it assuming
// it's a sphere with a radius of 2
vec3 getZInSphere(vec2 v) {
	const float radius = 2;
	return vec3(v, sqrt(radius-dot(v,v)));
}

void main() {
	float distanceFromCenterSquared = dot(vFragCoord, vFragCoord);

	// alpha = 0 if fragment is outside circle
	// alpha = 1 if fragment is inside circle
	float alpha = distanceFromCenterSquared > 1 ? 0 : 1;

	vec3 normal;
	{
		// Form a triangle to approximate the normal or something like that
		const float offset = 0.001;
		vec3 p0 = getZInSphere(vFragCoord + vec2(0, offset));
		vec3 p1 = getZInSphere(vFragCoord + vec2(offset, -offset));
		vec3 p2 = getZInSphere(vFragCoord + vec2(-offset, -offset));

		// Now calculate the normal of the triangle
		vec3 a = p0 - p1;
		vec3 b = p2 - p1;
		normal = normalize(cross(a, b));
	}

	const float pi = radians(180);
	// Based off of https://stackoverflow.com/questions/41957890/how-to-properly-map-a-2d-image-texture-to-an-icosphere
	vec2 texCoord = vec2(
		atan(normal.x, normal.z) * (1.0f/pi),
		asin(-normal.y) * (2.0f/pi) * aTexCoordYScale
	) + vec2(0.5f+aHorizontalOffset, 0.5f);

	// Some equation which seems to work good enough
	float specular = (dot(normal, normalize(vec3(-1, 2, 0.5f))) - 0.25f) * 1.3f + 0.1f;
	specular = specular*specular * sign(specular);

	gl_FragColor = vec4(texture(aTexture, texCoord).rgb + (vec3(1,1,1) * specular), alpha);
}