Automatically fade to Z far in environment depth fog

doc/classes/Camera3D.xml

@@ -167,6 +167,7 @@
 		</member>
 		<member name="far" type="float" setter="set_far" getter="get_far" default="4000.0">
 			The distance to the far culling boundary for this camera relative to its local Z axis.
+			[b]Note:[/b] When [member Environment.fog_enabled] is [code]true[/code], this is used as a base for open world fog fading (with fog starting at 50% of the Z far distance).
 		</member>
 		<member name="fov" type="float" setter="set_fov" getter="get_fov" default="75.0">
 			The camera's field of view angle (in degrees). Only applicable in perspective mode. Since [member keep_aspect] locks one axis, [code]fov[/code] sets the other axis' field of view angle.

doc/classes/Environment.xml

@@ -99,6 +99,7 @@
 		</member>
 		<member name="fog_enabled" type="bool" setter="set_fog_enabled" getter="is_fog_enabled" default="false">
 			If [code]true[/code], fog effects are enabled.
+			[b]Note:[/b] Enabling fog automatically fades starting from 50% of the [member Camera3D.far] distance, regardless of [member fog_depth_density]. This is used for open world fog fading. To disable this, increase the camera's [member Camera3D.far] property.
 		</member>
 		<member name="fog_height" type="float" setter="set_fog_height" getter="get_fog_height" default="0.0">
 			The height at which the height fog effect begins.

servers/rendering/renderer_rd/shaders/scene_forward_clustered.glsl

@@ -530,7 +530,16 @@ vec4 fog_process(vec3 vertex) {
 		}
 	}
 
-	float fog_amount = 1.0 - exp(min(0.0, -length(vertex) * scene_data.fog_density));
+	// Quadratic fade off to the Z far distance (for open world fog fading).
+	// Not physically accurate, but prevents visible sudden cutoffs near the Z far clip plane.
+	// This starts at 50% of the Z far distance, which is a good compromise between visibility
+	// and smoothness.
+	float fog_amount_quad = pow(smoothstep(scene_data.z_far * 0.5, scene_data.z_far, length(vertex)), 2.0);
+
+	// Exponential fog (physically accurate).
+	float fog_amount_exp = 1.0 - exp(min(0.0, -length(vertex) * scene_data.fog_density));
+
+	float fog_amount = max(fog_amount_quad, fog_amount_exp);
 
 	if (abs(scene_data.fog_height_density) >= 0.0001) {
 		float y = (scene_data.camera_matrix * vec4(vertex, 1.0)).y;