shader_type canvas_item;

uniform vec2 u_offset;
uniform float u_scale = 0.02;
uniform float u_base_height = 0.0;
uniform float u_height_range = 100.0;
uniform int u_seed;
uniform int u_octaves = 5;
uniform float u_roughness = 0.5;
uniform float u_curve = 1.0;
uniform vec2 u_uv_offset;
uniform vec2 u_uv_scale = vec2(1.0, 1.0);

////////////////////////////////////////////////////////////////////////////////
// Perlin noise source:
// https://github.com/curly-brace/Godot-3.0-Noise-Shaders
//
// GLSL textureless classic 2D noise \"cnoise\",
// with an RSL-style periodic variant \"pnoise\".
// Author:  Stefan Gustavson (stefan.gustavson@liu.se)
// Version: 2011-08-22
//
// Many thanks to Ian McEwan of Ashima Arts for the
// ideas for permutation and gradient selection.
//
// Copyright (c) 2011 Stefan Gustavson. All rights reserved.
// Distributed under the MIT license. See LICENSE file.
// https://github.com/stegu/webgl-noise
//

vec4 mod289(vec4 x) {
    return x - floor(x * (1.0 / 289.0)) * 289.0;
}

vec4 permute(vec4 x) {
    return mod289(((x * 34.0) + 1.0) * x);
}

vec4 taylorInvSqrt(vec4 r) {
    return 1.79284291400159 - 0.85373472095314 * r;
}

vec2 fade(vec2 t) {
    return t * t * t * (t * (t * 6.0 - 15.0) + 10.0);
}

// Classic Perlin noise
float cnoise(vec2 P) {
    vec4 Pi = floor(vec4(P, P)) + vec4(0.0, 0.0, 1.0, 1.0);
    vec4 Pf = fract(vec4(P, P)) - vec4(0.0, 0.0, 1.0, 1.0);
    Pi = mod289(Pi); // To avoid truncation effects in permutation
    vec4 ix = Pi.xzxz;
    vec4 iy = Pi.yyww;
    vec4 fx = Pf.xzxz;
    vec4 fy = Pf.yyww;

    vec4 i = permute(permute(ix) + iy);

    vec4 gx = fract(i * (1.0 / 41.0)) * 2.0 - 1.0 ;
    vec4 gy = abs(gx) - 0.5 ;
    vec4 tx = floor(gx + 0.5);
    gx = gx - tx;

    vec2 g00 = vec2(gx.x,gy.x);
    vec2 g10 = vec2(gx.y,gy.y);
    vec2 g01 = vec2(gx.z,gy.z);
    vec2 g11 = vec2(gx.w,gy.w);
    
    vec4 norm = taylorInvSqrt(vec4(dot(g00, g00), dot(g01, g01), dot(g10, g10), dot(g11, g11)));
    g00 *= norm.x;
    g01 *= norm.y;
    g10 *= norm.z;
    g11 *= norm.w;
    
    float n00 = dot(g00, vec2(fx.x, fy.x));
    float n10 = dot(g10, vec2(fx.y, fy.y));
    float n01 = dot(g01, vec2(fx.z, fy.z));
    float n11 = dot(g11, vec2(fx.w, fy.w));
    
    vec2 fade_xy = fade(Pf.xy);
    vec2 n_x = mix(vec2(n00, n01), vec2(n10, n11), fade_xy.x);
    float n_xy = mix(n_x.x, n_x.y, fade_xy.y);
    return 2.3 * n_xy;
}
////////////////////////////////////////////////////////////////////////////////

float get_fractal_noise(vec2 uv) {
	float scale = 1.0;
	float sum = 0.0;
	float amp = 0.0;
	int octaves = u_octaves;
	float p = 1.0;
	uv.x += float(u_seed) * 61.0;
	
	for (int i = 0; i < octaves; ++i) {
		sum += p * cnoise(uv * scale);
		amp += p;
		scale *= 2.0;
		p *= u_roughness;
	}

	float gs = sum / amp;
	return gs;
}

float get_height(vec2 uv) {
	float h = 0.5 + 0.5 * get_fractal_noise(uv);
	h = pow(h, u_curve);
	h = u_base_height + h * u_height_range;
	return h;
}

void fragment() {
	vec2 uv = SCREEN_UV;

	// Handle screen padding: transform UV back into generation space
	uv = (uv + u_uv_offset) * u_uv_scale;

	// Offset and scale for the noise itself
	uv = (uv + u_offset) * u_scale;

	float h = get_height(uv);
	COLOR = vec4(h, h, h, 1.0);
}