// fog.shader 构造 fog 效果
// noise 为配置好的噪音取样纹理
// texture 为蒙版 mask，用于控制雾气浓度
shader_type canvas_item;
uniform sampler2D noise: repeat_enable;
// 数字越小（越接近 0）越黑，范围在 0-1 之间
uniform float grey_level = 0.6;
uniform float speed = 1.;

void fragment() {
	float delta = TIME * .004 * speed;
	float fog_alpha_1 = texture(noise, vec2(UV.x + delta * 7., UV.y + delta * 2.)).r;
	float fog_alpha_2 = texture(noise, vec2(UV.x + 0.5 + delta * 4., UV.y + 0.5 - delta)).r;
	float fog_alpha_3 = texture(noise, vec2(UV.x + 0.25 + delta, UV.y - 0.25 + delta * 3.)).r;
	float fog_alpha = mix(fog_alpha_3, mix(fog_alpha_1, fog_alpha_2, 0.5), 0.66);
	// factor 越靠近 1， fog_alpha 也靠近 1； factor 越靠近 0.，fog_alpha 越靠近自己
	COLOR.a = mix(fog_alpha, 1., COLOR.a * COLOR.a) * smoothstep(.0, .2, COLOR.a);
	COLOR.rgb *= mix(grey_level, 1., fog_alpha);
}