RaymarchPostProcess.cs

using System;
using UnityEngine;
using UnityEngine.Rendering;
using UnityEngine.Rendering.PostProcessing;

[Serializable]
public sealed class ShaderParameter : ParameterOverride<Shader> { }

[Serializable]
[PostProcess(typeof(RaymarchPostProcessRenderer), PostProcessEvent.AfterStack, "Raymarch/RaymarchHDRP")]
public sealed class RaymarchPostProcess : PostProcessEffectSettings
{
    public ShaderParameter _shader = new ShaderParameter { value = null };
}

public sealed class RaymarchPostProcessRenderer : PostProcessEffectRenderer<RaymarchPostProcess>
{
    [SerializeField]
    private Shader _shader;

    public Material _raymarchMaterial
    {
        get
        {
            if (!_raymarchMat && _shader)
            {
                _raymarchMat = new Material(_shader);
                _raymarchMat.hideFlags = HideFlags.HideAndDontSave;
            }
            return _raymarchMat;
        }
    }

    private Material _raymarchMat;

    public Camera _camera
    {
        get
        {
            if (!_cam)
            {
                _cam = Camera.main;
            }

            return _cam;
        }
    }

    private Camera _cam;

    public override void Render(PostProcessRenderContext context)
    {
        if (!_shader)
            _shader = settings._shader.value;

        if (!_cam)
            _cam = Camera.main;

        PropertySheet sheet = context.propertySheets.Get(Shader.Find("Raymarch/Raymarch"));

        sheet.properties.SetMatrix("_CamFrustum", CamFrustum(_camera));
        sheet.properties.SetMatrix("_CamToWorld", _camera.cameraToWorldMatrix);
        sheet.properties.SetVector("_CamWorldSpace", _camera.transform.position);

        context.command.BlitFullscreenTriangle(context.source, context.destination, sheet, 0);
    }

    private Matrix4x4 CamFrustum(Camera cam)
    {
        Matrix4x4 frustum = Matrix4x4.identity;
        float fov = Mathf.Tan((cam.fieldOfView * 0.5f) * Mathf.Deg2Rad);

        Vector3 goUp = Vector3.up * fov;
        Vector3 goRight = Vector3.right * fov * cam.aspect;

        Vector3 TL = (-Vector3.forward - goRight + goUp);
        Vector3 TR = (-Vector3.forward + goRight + goUp);
        Vector3 BR = (-Vector3.forward + goRight - goUp);
        Vector3 BL = (-Vector3.forward - goRight - goUp);

        frustum.SetRow(0, TL);
        frustum.SetRow(1, TR);
        frustum.SetRow(2, BR);
        frustum.SetRow(3, BL);

        return frustum;
    }
}