// world.gdshader
// shader for earth globe with heightmap and color texture
shader_type spatial;

uniform sampler2D height_map : hint_default_white;
uniform sampler2D earth_texture : hint_default_white;
uniform float height_scale : hint_range(0.0, 0.2) = 0.02;
uniform bool apply_height = true;
uniform bool show_height_debug = false;

// convert point on unit sphere to lat/lon uv coordinates
vec2 point_to_uv(vec3 point) {
    float latitude = asin(point.y);
	float longitude = atan(point.x, -point.z);
    // optional : move seam by rotating longitude (0 = current)
    float lon_offset = 0.0;
    longitude += lon_offset;
	float u = (longitude + PI) / (2.0 * PI);
	float v = (latitude + PI / 2.0) / PI;

	return vec2(u, v);
}

void vertex() {
	// get point on unit sphere (normalized vertex position)
	vec3 sphere_point = normalize(VERTEX);
	// convert to uv using lat/lon projection
	vec2 tex_coord = point_to_uv(sphere_point);
	// sample heightmap
	// apply height displacement along normal
	if (apply_height) {
    	float height = texture(height_map, tex_coord).r;
		VERTEX = sphere_point * (1.0 + height * height_scale);
		NORMAL = normalize(VERTEX);
	}
	// pass uv to fragment shader
	UV = tex_coord;
}

void fragment() {
	// sample earth texture using lat/lon uv
	vec4 earth_color = texture(earth_texture, UV);	
	if (show_height_debug) {
		// debug: show heightmap as grayscale
		float height = texture(height_map, UV).r;
		ALBEDO = vec3(height);
	} else {
		// normal rendering: show earth texture
		ALBEDO = earth_color.rgb;
	}
	// basic lighting
	ROUGHNESS = 0.8;
	METALLIC = 0.0;
}