Simulating Cloud Shadows Analytically

ShaderError June 1, 2023June 2, 2023

The shadow value is sampling from noise. More detailed explanation you can find here.

To use full power of this shader, some minor adjustments are required in the Godot Shader Pipeline. Specifically, you’ll need to add a custom built-in function to sample the shadows. A tutorial with the necessary steps you can find here

See more about this shader

Shader code

shader_type spatial;

uniform float noise_strength = 1.0;
uniform vec4 albedo : source_color;
uniform float shadow_strenght = 0.95;
uniform sampler2D noise_texture : source_color, filter_linear_mipmap, repeat_enable;
uniform vec2 uv_scale = vec2(0.1);
uniform vec2 speed = vec2(0.03);

varying mat4 world_mat;

void vertex() {
	world_mat = INV_VIEW_MATRIX;
}

varying vec3 _vertex;

void fragment(){
	
	_vertex = VERTEX;

	ALBEDO = albedo.rgb;
}

void light() {
	
  if(LIGHT_IS_DIRECTIONAL){
	
	//ray
	// p = ray_start + ray_dir * t
	vec3 ray_dir = -LIGHT; // in view space
	ray_dir = mat3(world_mat) * ray_dir; // in world space
	vec3 ray_start = (INV_VIEW_MATRIX * vec4(_vertex, 1.0f)).xyz;
	
	//plane
	// (p - p0) . n = 0
	vec3 n = vec3(0.0, 1.0, 0.0); 
	vec3 PO = vec3(0.0, 1.0, 0.0); // can make offset to the top
	vec3 v = PO - ray_start;

	// solve for parameter t
	// t = ((p0 - ray_start) . n) / (ray_dir . n)
	float t = dot(normalize(v), n) * length(v) / dot(ray_dir, n);
	vec3 P = ray_start + t * ray_dir;  // in world space
	
	vec2 uv = P.xz; // convert it to texture UV
	vec2 uv_offset = vec2(TIME, TIME) * speed;
	
	vec4 rg = texture( noise_texture, uv * uv_scale + uv_offset) * noise_strength;
	
	float clouds = smoothstep(0.2, 1.0, 1.0 - rg.r);
	
	//make it less dark, can skip it
	clouds *= 0.9;
	clouds += 0.1;
	
	//To sample shadow with different blur param, check the tutorial linked in the description
	//float blur = 1.;
	//if(clouds < 1.0) blur += 10. * smoothstep(0.2, 1.0, rg.r) ;	
	//float shadow = get_directional_shadow(LIGHT_INDEX, _vertex, blur);
	float shadow = ATTENUATION;
	
	if( clouds < 1.0 && shadow < 1.0) {
		shadow *= 1.0 - 0.2 * smoothstep(0.1, 1.0, rg.r) ;
		shadow += 0.2 * smoothstep(0.1, 1.0, rg.r) ;
		clouds = min(clouds, shadow);
	}

	DIFFUSE_LIGHT += clamp(dot(NORMAL, LIGHT), 0.0, 1.0) * clouds * shadow * ALBEDO * LIGHT_COLOR;

  }	
}