Set a global exposure shader variable based on light probes and raycasts to lights

Exposure.cs

using UnityEngine;
using System.Collections;
using UnityEngine.Rendering;

[ExecuteInEditMode]
public class Exposure : MonoBehaviour {

    const float DEFAULT_EXPOSURE = 3f;
    const string EXPOSURE_PROPERTY = "_PA_Exposure";

    // Linear color space luminence from Unity built-in shaders
    static readonly Vector3 UNITY_COLORSPACE_LUMINANCE = new Vector3(0.0396819152f, 0.458021790f, 0.00609653955f);

    // is the exposure for this camera fixed or dynamic
    public bool fixedExposure = false;

    // if the light probes dont contain direct light then it must be added seperately
    public bool addDirectLight = true;

    // Overall multiplier for exposure, if fixed exposure is set this is the exposure used
    public float multiplier = DEFAULT_EXPOSURE;

    // The speed at which the exposure adapts to changes
    public float lerpSpeed = 2f;

    // The layers that direct light will be cast against
    public LayerMask layerMask;

    private float lastExposure;
    RaycastHit[] hit;
    Light[] lights;

    private void Start() {
        lastExposure = DEFAULT_EXPOSURE;

        if (addDirectLight) {
            hit = new RaycastHit[1];
            lights = FindObjectsOfType<Light>();
        }
    }

    private void OnPreRender() {
        UpdateExposure();
    }

    private void OnPostRender() {
        ResetExposure();
    }

    void UpdateExposure () {
        //if fixed exposure just set the global exposure directly
        if (fixedExposure) {
            Shader.SetGlobalFloat(EXPOSURE_PROPERTY, multiplier);
            return;
        }

        SphericalHarmonicsL2 probe;
        // fill the interpolated probe, the camera has an empty renderer for a performance boost
        LightProbes.GetInterpolatedProbe(transform.position, GetComponent<Renderer>(), out probe);

        //sampler the color from the probe in the cameras direction
        Vector3 color = ShadeSH9(probe, transform.forward);

        //calculate luminocity
        float luma = Vector3.Dot(color, UNITY_COLORSPACE_LUMINANCE);

        if (addDirectLight) {
            //Raycast to each light to determine if its shadowed or not
            for (int i = 0; i < lights.Length; i++) {

                Light light = lights[i];
                Vector3 directionToLight = light.type == LightType.Directional ? -light.transform.forward : (light.transform.position - transform.position).normalized;

                //raycast to the light (note the 100 unit hardcoded distance)
                if (Physics.RaycastNonAlloc(new Ray(transform.position, directionToLight), hit, 100, layerMask.value) == 0) {
                    //if its not shadowed then add the lights intensity multiplied by dot product of the cameras forward direction and the direction to the light
                    float lightLuma = light.intensity * Vector3.Dot(new Vector3(light.color.r, light.color.g, light.color.b), UNITY_COLORSPACE_LUMINANCE);
                    lightLuma *= Mathf.Clamp01(Vector3.Dot(transform.forward, directionToLight));
                    luma += lightLuma;
                }
            }
        }

        float exposure = 1 / luma * multiplier;

        //in play mode lerp the exposure for a gradual change, in the editor apply it instantly
        lastExposure = Application.isPlaying ? Mathf.Lerp(lastExposure, exposure, Mathf.Clamp01(Time.deltaTime * lerpSpeed)) : exposure;

        if (float.IsNaN(lastExposure)) { lastExposure = DEFAULT_EXPOSURE; }

        //set the exposure
        Shader.SetGlobalFloat(EXPOSURE_PROPERTY, lastExposure);
	}

    void ResetExposure() {
        Shader.SetGlobalFloat(EXPOSURE_PROPERTY, DEFAULT_EXPOSURE);
    }

    // Get the color from a lightprobe
    // https://forum.unity3d.com/threads/getinterpolatedlightprobe-interpreting-the-coefficients.209223/#post-2124031
    Vector3 ShadeSH9(SphericalHarmonicsL2 l, Vector3 n) {
        var bx = n.x * n.y;
        var by = n.y * n.z;
        var bz = n.z * n.z * 3;
        var bw = n.z * n.x;

        var c = n.x * n.x - n.y * n.y;

        return new Vector3 {
            x = l[0, 3] * n.x + l[0, 1] * n.y + l[0, 2] * n.z + l[0, 0] - l[0, 6] + l[0, 4] * bx + l[0, 5] * by + l[0, 6] * bz + l[0, 7] * bw + c * l[0, 8],
            y = l[1, 3] * n.x + l[1, 1] * n.y + l[1, 2] * n.z + l[1, 0] - l[1, 6] + l[1, 4] * bx + l[1, 5] * by + l[1, 6] * bz + l[1, 7] * bw + c * l[1, 8],
            z = l[2, 3] * n.x + l[2, 1] * n.y + l[2, 2] * n.z + l[2, 0] - l[2, 6] + l[2, 4] * bx + l[2, 5] * by + l[2, 6] * bz + l[2, 7] * bw + c * l[2, 8]
        };
    }
}