// Calm-water surface seen only through its shadow: the mesh is invisible
// (SHADOW_ONLY) and the alpha-scissor cut-outs let the fixed daytime sun carve
// light shafts into the volumetric fog below.
shader_type spatial;
render_mode cull_disabled, unshaded, depth_prepass_alpha;

uniform sampler2D ripple_noise : repeat_enable, filter_linear_mipmap;
uniform float ripple_scale = 0.025;
uniform float drift_speed = 0.012;
uniform float coverage : hint_range(0.0, 1.0) = 0.30;
// World-space pool where the surface lets sunlight through. Outside it the
// surface is fully opaque, so shafts only form in that patch of water.
uniform vec2 pool_center_xz = vec2(-14.0, -31.0);
uniform float pool_radius = 22.0;
uniform float pool_edge = 12.0;

varying vec2 world_xz;

void vertex() {
	world_xz = (MODEL_MATRIX * vec4(VERTEX, 1.0)).xz;
}

void fragment() {
	vec2 slow_drift = TIME * drift_speed * vec2(1.0, 0.35);
	vec2 cross_drift = TIME * drift_speed * vec2(-0.4, 0.9);
	float broad = texture(ripple_noise, world_xz * ripple_scale + slow_drift).r;
	float fine = texture(ripple_noise, world_xz * ripple_scale * 1.9 + cross_drift).r;
	float raw_ripple = broad * 0.65 + fine * 0.35;
	// Inside the pool the crests are sharpened so the gaps cut crisp shafts.
	float pool_ripple = smoothstep(0.35, 0.65, raw_ripple);
	// Outside the pool only the deepest troughs open: a dim dapple, not darkness.
	float dim_ripple = smoothstep(0.16, 0.30, raw_ripple);
	float pool = 1.0 - smoothstep(
		pool_radius - pool_edge, pool_radius, distance(world_xz, pool_center_xz)
	);
	ALBEDO = vec3(0.0);
	ALPHA = mix(dim_ripple, pool_ripple, pool);
	ALPHA_SCISSOR_THRESHOLD = coverage;
}