GlobalFog.cs

using System;
using UnityEngine;

namespace UnityStandardAssets.ImageEffects
{
    [ExecuteInEditMode]
    [RequireComponent (typeof(Camera))]
    [AddComponentMenu ("Image Effects/Rendering/Global Fog")]
    class GlobalFog : PostEffectsBase
    {
        [Tooltip("Apply distance-based fog?")]
        public bool  distanceFog = true;
        [Tooltip("Exclude far plane pixels from distance-based fog? (Skybox or clear color)")]
        public bool  excludeFarPixels = true;
        [Tooltip("Distance fog is based on radial distance from camera when checked")]
        public bool  useRadialDistance = false;
        [Tooltip("Apply height-based fog?")]
        public bool  heightFog = true;
        [Tooltip("Fog top Y coordinate")]
        public float height = 1.0f;
        [Range(0.001f,10.0f)]
        public float heightDensity = 2.0f;
        [Tooltip("Push fog away from the camera by this amount")]
        public float startDistance = 0.0f;

        public Shader fogShader = null;
        private Material fogMaterial = null;


        public override bool CheckResources ()
        {
            CheckSupport (true);

            fogMaterial = CheckShaderAndCreateMaterial (fogShader, fogMaterial);

            if (!isSupported)
                ReportAutoDisable ();
            return isSupported;
        }

        [ImageEffectOpaque]
        void OnRenderImage (RenderTexture source, RenderTexture destination)
        {
            if (CheckResources()==false || (!distanceFog && !heightFog))
            {
                Graphics.Blit (source, destination);
                return;
            }

            Camera cam = GetComponent<Camera>();
            Transform camtr = cam.transform;
            float camNear = cam.nearClipPlane;
            float camFar = cam.farClipPlane;
            float camFov = cam.fieldOfView;
            float camAspect = cam.aspect;

            Matrix4x4 frustumCorners = Matrix4x4.identity;

            float fovWHalf = camFov * 0.5f;

            Vector3 toRight = camtr.right * camNear * Mathf.Tan (fovWHalf * Mathf.Deg2Rad) * camAspect;
            Vector3 toTop = camtr.up * camNear * Mathf.Tan (fovWHalf * Mathf.Deg2Rad);

            Vector3 topLeft = (camtr.forward * camNear - toRight + toTop);
            float camScale = topLeft.magnitude * camFar/camNear;

            topLeft.Normalize();
            topLeft *= camScale;

            Vector3 topRight = (camtr.forward * camNear + toRight + toTop);
            topRight.Normalize();
            topRight *= camScale;

            Vector3 bottomRight = (camtr.forward * camNear + toRight - toTop);
            bottomRight.Normalize();
            bottomRight *= camScale;

            Vector3 bottomLeft = (camtr.forward * camNear - toRight - toTop);
            bottomLeft.Normalize();
            bottomLeft *= camScale;

            frustumCorners.SetRow (0, topLeft);
            frustumCorners.SetRow (1, topRight);
            frustumCorners.SetRow (2, bottomRight);
            frustumCorners.SetRow (3, bottomLeft);

            var camPos= camtr.position;
            float FdotC = camPos.y-height;
            float paramK = (FdotC <= 0.0f ? 1.0f : 0.0f);
            float excludeDepth = (excludeFarPixels ? 1.0f : 2.0f);
            fogMaterial.SetMatrix ("_FrustumCornersWS", frustumCorners);
            fogMaterial.SetVector ("_CameraWS", camPos);
            fogMaterial.SetVector ("_HeightParams", new Vector4 (height, FdotC, paramK, heightDensity*0.5f));
            fogMaterial.SetVector ("_DistanceParams", new Vector4 (-Mathf.Max(startDistance,0.0f), excludeDepth, 0, 0));

            var sceneMode= RenderSettings.fogMode;
            var sceneDensity= RenderSettings.fogDensity;
            var sceneStart= RenderSettings.fogStartDistance;
            var sceneEnd= RenderSettings.fogEndDistance;
            Vector4 sceneParams;
            bool  linear = (sceneMode == FogMode.Linear);
            float diff = linear ? sceneEnd - sceneStart : 0.0f;
            float invDiff = Mathf.Abs(diff) > 0.0001f ? 1.0f / diff : 0.0f;
            sceneParams.x = sceneDensity * 1.2011224087f; // density / sqrt(ln(2)), used by Exp2 fog mode
            sceneParams.y = sceneDensity * 1.4426950408f; // density / ln(2), used by Exp fog mode
            sceneParams.z = linear ? -invDiff : 0.0f;
            sceneParams.w = linear ? sceneEnd * invDiff : 0.0f;
            fogMaterial.SetVector ("_SceneFogParams", sceneParams);
            fogMaterial.SetVector ("_SceneFogMode", new Vector4((int)sceneMode, useRadialDistance ? 1 : 0, 0, 0));

            int pass = 0;
            if (distanceFog && heightFog)
                pass = 0; // distance + height
            else if (distanceFog)
                pass = 1; // distance only
            else
                pass = 2; // height only
            CustomGraphicsBlit (source, destination, fogMaterial, pass);
        }

        static void CustomGraphicsBlit (RenderTexture source, RenderTexture dest, Material fxMaterial, int passNr)
        {
            RenderTexture.active = dest;

            fxMaterial.SetTexture ("_MainTex", source);

            GL.PushMatrix ();
            GL.LoadOrtho ();

            fxMaterial.SetPass (passNr);

            GL.Begin (GL.QUADS);

            GL.MultiTexCoord2 (0, 0.0f, 0.0f);
            GL.Vertex3 (0.0f, 0.0f, 3.0f); // BL

            GL.MultiTexCoord2 (0, 1.0f, 0.0f);
            GL.Vertex3 (1.0f, 0.0f, 2.0f); // BR

            GL.MultiTexCoord2 (0, 1.0f, 1.0f);
            GL.Vertex3 (1.0f, 1.0f, 1.0f); // TR

            GL.MultiTexCoord2 (0, 0.0f, 1.0f);
            GL.Vertex3 (0.0f, 1.0f, 0.0f); // TL

            GL.End ();
            GL.PopMatrix ();
        }
    }
}