ScriptableRenderPipeline/Assets/ScriptableRenderLoop/common/SkyboxHelper.cs

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

public class SkyboxHelper
{
    public SkyboxHelper()
    {
    }

    const int k_NumFullSubdivisions = 3; // 3 subdivs == 2048 triangles
    const int k_NumHorizonSubdivisions = 2;

    public void CreateMesh()
    {
        var vertData = new Vector3[8 * 3];
        for (int i = 0; i < 8 * 3; i++)
        {
            vertData[i] = m_OctaVerts[i];
        }

        // Regular subdivisions
        for (int i = 0; i < k_NumFullSubdivisions; i++)
        {
            var srcData = vertData.Clone() as Vector3[];
            var verts = new List<Vector3>();

            for (int k = 0; k < srcData.Length; k += 3)
            {
                Subdivide(verts, srcData[k], srcData[k + 1], srcData[k + 2]);
            }
            vertData = verts.ToArray();
        }

        // Horizon subdivisions
        var horizonLimit = 1.0f;
        for (int i = 0; i < k_NumHorizonSubdivisions; i++)
        {
            var srcData = vertData.Clone() as Vector3[];
            var verts = new List<Vector3>();

            horizonLimit *= 0.5f; // First iteration limit to y < +-0.5, next one 0.25 etc.
            for (int k = 0; k < srcData.Length; k += 3)
            {
                var maxAbsY = Mathf.Max(Mathf.Abs(srcData[k].y), Mathf.Abs(srcData[k + 1].y), Mathf.Abs(srcData[k + 2].y));
                if (maxAbsY > horizonLimit)
                {
                    // Pass through existing triangle
                    verts.Add(srcData[k]);
                    verts.Add(srcData[k + 1]);
                    verts.Add(srcData[k + 2]);
                }
                else
                {
                    SubdivideYOnly(verts, srcData[k], srcData[k + 1], srcData[k + 2]);
                }
            }
            vertData = verts.ToArray();
        }

        // Write out the mesh
        var vertexCount = vertData.Length;
        var triangles = new int[vertexCount];
        for (int i = 0; i < vertexCount; i++)
        {
            triangles[i] = i;
        }

        m_Mesh = new Mesh
        {
            vertices = vertData,
            triangles = triangles
        };
    }

    public UnityEngine.Mesh mesh
    {
        get { return m_Mesh; }
    }

    public void Draw(RenderLoop loop, Camera camera)
    {
        if (camera.clearFlags != CameraClearFlags.Skybox)
        {
            return;
        }

        var mat = RenderSettings.skybox;

        if (mat == null)
        {
            return;
        }

        var cmd = new CommandBuffer { name = "Skybox" };

        var looksLikeSixSidedShader = true;
        looksLikeSixSidedShader &= (mat.passCount == 6); // should have six passes
        //looksLikeSixSidedShader &= !mat.GetShader()->GetShaderLabShader()->HasLightingPasses();

        if (looksLikeSixSidedShader)
        {
            Debug.LogWarning("Six sided skybox not yet supported.");
        }
        else
        {
            if (mesh == null)
            {
                CreateMesh();
            }

            var dist = camera.farClipPlane * 10.0f;

            var world = Matrix4x4.TRS(camera.transform.position, Quaternion.identity, new Vector3(dist, dist, dist));

            var skyboxProj = SkyboxHelper.GetProjectionMatrix(camera);
            cmd.SetProjectionAndViewMatrices(skyboxProj, camera.worldToCameraMatrix);
            cmd.DrawMesh(mesh, world, mat);

            cmd.SetProjectionAndViewMatrices(camera.projectionMatrix, camera.worldToCameraMatrix);
        }

        loop.ExecuteCommandBuffer(cmd);
        cmd.Dispose();
    }

    public static Matrix4x4 GetProjectionMatrix(Camera camera)
    {
        var skyboxProj = Matrix4x4.Perspective(camera.fieldOfView, camera.aspect, camera.nearClipPlane, camera.farClipPlane);

        var nearPlane = camera.nearClipPlane * 0.01f;
        skyboxProj = AdjustDepthRange(skyboxProj, camera.nearClipPlane, nearPlane, camera.farClipPlane);
        return MakeProjectionInfinite(skyboxProj, nearPlane);
    }

    static Matrix4x4 MakeProjectionInfinite(Matrix4x4 m, float nearPlane)
    {
        const float epsilon = 1e-6f;

        var r = m;
        r[2, 2] = -1.0f + epsilon;
        r[2, 3] = (-2.0f + epsilon) * nearPlane;
        r[3, 2] = -1.0f;
        return r;
    }

    static Matrix4x4 AdjustDepthRange(Matrix4x4 mat, float origNear, float newNear, float newFar)
    {
        var x = mat[0, 0];
        var y = mat[1, 1];
        var w = mat[0, 2];
        var z = mat[1, 2];

        var r = ((2.0f * origNear) / x) * ((w + 1) * 0.5f);
        var t = ((2.0f * origNear) / y) * ((z + 1) * 0.5f);
        var l = ((2.0f * origNear) / x) * (((w + 1) * 0.5f) - 1);
        var b = ((2.0f * origNear) / y) * (((z + 1) * 0.5f) - 1);

        var ratio = (newNear / origNear);

        r *= ratio;
        t *= ratio;
        l *= ratio;
        b *= ratio;

        var ret = new Matrix4x4();

        ret[0, 0] = (2.0f * newNear) / (r - l); ret[0, 1] = 0; ret[0, 2] = (r + l) / (r - l); ret[0, 3] = 0;
        ret[1, 0] = 0; ret[1, 1] = (2.0f * newNear) / (t - b); ret[1, 2] = (t + b) / (t - b); ret[1, 3] = 0;
        ret[2, 0] = 0; ret[2, 1] = 0; ret[2, 2] = -(newFar + newNear) / (newFar - newNear); ret[2, 3] = -(2.0f * newFar * newNear) / (newFar - newNear);
        ret[3, 0] = 0; ret[3, 1] = 0; ret[3, 2] = -1.0f; ret[3, 3] = 0;

        return ret;
    }

    // Octahedron vertices
    readonly Vector3[] m_OctaVerts =
    {
        new Vector3(0.0f, 1.0f, 0.0f),      new Vector3(0.0f, 0.0f, -1.0f),     new Vector3(1.0f, 0.0f, 0.0f),
        new Vector3(0.0f, 1.0f, 0.0f),      new Vector3(1.0f, 0.0f, 0.0f),      new Vector3(0.0f, 0.0f, 1.0f),
        new Vector3(0.0f, 1.0f, 0.0f),      new Vector3(0.0f, 0.0f, 1.0f),      new Vector3(-1.0f, 0.0f, 0.0f),
        new Vector3(0.0f, 1.0f, 0.0f),      new Vector3(-1.0f, 0.0f, 0.0f),     new Vector3(0.0f, 0.0f, -1.0f),
        new Vector3(0.0f, -1.0f, 0.0f),     new Vector3(1.0f, 0.0f, 0.0f),      new Vector3(0.0f, 0.0f, -1.0f),
        new Vector3(0.0f, -1.0f, 0.0f),     new Vector3(0.0f, 0.0f, 1.0f),      new Vector3(1.0f, 0.0f, 0.0f),
        new Vector3(0.0f, -1.0f, 0.0f),     new Vector3(-1.0f, 0.0f, 0.0f),     new Vector3(0.0f, 0.0f, 1.0f),
        new Vector3(0.0f, -1.0f, 0.0f),     new Vector3(0.0f, 0.0f, -1.0f),     new Vector3(-1.0f, 0.0f, 0.0f),
    };

    Vector3 SubDivVert(Vector3 v1, Vector3 v2)
    {
        return Vector3.Normalize(v1 + v2);
    }

    void Subdivide(ICollection<Vector3> dest, Vector3 v1, Vector3 v2, Vector3 v3)
    {
        var v12 = SubDivVert(v1, v2);
        var v23 = SubDivVert(v2, v3);
        var v13 = SubDivVert(v1, v3);

        dest.Add(v1);
        dest.Add(v12);
        dest.Add(v13);
        dest.Add(v12);
        dest.Add(v2);
        dest.Add(v23);
        dest.Add(v23);
        dest.Add(v13);
        dest.Add(v12);
        dest.Add(v3);
        dest.Add(v13);
        dest.Add(v23);
    }

    void SubdivideYOnly(ICollection<Vector3> dest, Vector3 v1, Vector3 v2, Vector3 v3)
    {
        // Find out which vertex is furthest out from the others on the y axis

        var d12 = Mathf.Abs(v2.y - v1.y);
        var d23 = Mathf.Abs(v2.y - v3.y);
        var d31 = Mathf.Abs(v3.y - v1.y);

        Vector3 top, va, vb;

        if (d12 < d23 && d12 < d31)
        {
            top = v3;
            va = v1;
            vb = v2;
        }
        else if (d23 < d12 && d23 < d31)
        {
            top = v1;
            va = v2;
            vb = v3;
        }
        else
        {
            top = v2;
            va = v3;
            vb = v1;
        }

        var v12 = SubDivVert(top, va);
        var v13 = SubDivVert(top, vb);

        dest.Add(top);
        dest.Add(v12);
        dest.Add(v13);

        // A bit of extra logic to prevent triangle slivers: choose the shorter of (13->va), (12->vb) as triangle base
        if ((v13 - va).sqrMagnitude > (v12 - vb).sqrMagnitude)
        {
            dest.Add(v12);
            dest.Add(va);
            dest.Add(vb);
            dest.Add(v13);
            dest.Add(v12);
            dest.Add(vb);
        }
        else
        {
            dest.Add(v13);
            dest.Add(v12);
            dest.Add(va);
            dest.Add(v13);
            dest.Add(va);
            dest.Add(vb);
        }
    }

    Mesh m_Mesh;
}