Eevee: Ambient Occlusion: Enable Multibounce approximation and Change influence factor.

Making the influence a power for easy tuning. Works like a contrast knob.

source/blender/draw/engines/eevee/eevee_effects.c

@@ -391,7 +391,7 @@ void EEVEE_effects_init(EEVEE_Data *vedata)
 		/* Ambient Occlusion*/
 		stl->effects->ao_dist = BKE_collection_engine_property_value_get_float(props, "gtao_distance");
 		stl->effects->ao_samples = BKE_collection_engine_property_value_get_int(props, "gtao_samples");
-		stl->effects->ao_factor = 1.0f - BKE_collection_engine_property_value_get_float(props, "gtao_factor");
+		stl->effects->ao_factor = BKE_collection_engine_property_value_get_float(props, "gtao_factor");
 	}
 
 	/* MinMax Pyramid */

source/blender/draw/engines/eevee/shaders/ambient_occlusion_lib.glsl

@@ -124,7 +124,7 @@ void gtao(vec3 normal, vec3 position, vec2 noise, out float visibility
 #endif
 	}
 
-	visibility = min(1.0, visibility / aoSamples);
+	visibility = clamp(visibility / aoSamples, 1e-8, 1.0);
 
 #ifdef USE_BENT_NORMAL
 	/* The bent normal will show the facet look of the mesh. Try to minimize this. */
@@ -132,7 +132,7 @@ void gtao(vec3 normal, vec3 position, vec2 noise, out float visibility
 #endif
 
 	/* Scale by user factor */
-	visibility = max(0.0, mix(aoFactor, 1.0, visibility));
+	visibility = pow(visibility, aoFactor);
 }
 
 /* Multibounce approximation base on surface albedo.

source/blender/draw/engines/eevee/shaders/lit_surface_frag.glsl

@@ -494,7 +494,7 @@ vec3 eevee_surface_lit(vec3 world_normal, vec3 albedo, vec3 f0, float roughness,
 
 	vec3 indirect_radiance =
 	        spec_accum.rgb * F_ibl(f0, brdf_lut) * float(specToggle) * specular_occlusion(dot(sd.N, sd.V), final_ao, roughness) +
-	        diff_accum.rgb * albedo * final_ao;
+	        diff_accum.rgb * albedo * gtao_multibounce(final_ao, albedo);
 
 	return radiance + indirect_radiance;
 }