using System;
using System.Numerics;
using AcDream.App.Rendering;
using Xunit;

namespace AcDream.App.Tests.Rendering;

public class PortalProjectionTests
{
    // A simple GL-style perspective looking down -Z, camera at origin.
    private static Matrix4x4 ViewProj()
    {
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(1.0f, 1.0f, 0.1f, 1000f);
        return view * proj;
    }

    [Fact]
    public void Project_QuadInFront_ProducesNdcInsideViewport()
    {
        // A 2x2 quad at z=-5 (in front), cell-local == world (identity transform).
        var poly = new[]
        {
            new Vector3(-1, -1, -5), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, -5),
        };
        var r = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(r.Length >= 3);
        foreach (var v in r)
        {
            Assert.InRange(v.X, -1.001f, 1.001f);
            Assert.InRange(v.Y, -1.001f, 1.001f);
        }
    }

    [Fact]
    public void Project_QuadFullyBehind_ReturnsEmpty()
    {
        var poly = new[]
        {
            new Vector3(-1, -1, 5), new Vector3(1, -1, 5), new Vector3(1, 1, 5), new Vector3(-1, 1, 5),
        };
        var r = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(r.Length < 3);
    }

    [Fact]
    public void Project_QuadStraddlingCamera_ClipsWithoutInversion()
    {
        // Spans from behind (z=+2) to in front (z=-5). Must clip to the in-front part,
        // never produce a wildly out-of-range inverted vertex.
        var poly = new[]
        {
            new Vector3(-1, -1, 2), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, 2),
        };
        var r = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(r.Length >= 3);
        foreach (var v in r)
        {
            // A corner a few cm in front of the eye and ~1 m to the side genuinely
            // projects large (~±37 NDC) but finite. ±50 still catches the ±7852
            // perspective-inversion blow-up the old w-clip produced.
            Assert.InRange(v.X, -50f, 50f); // bounded — no inversion blow-up
            Assert.InRange(v.Y, -50f, 50f);
        }
    }

    [Fact]
    public void Project_QuadStraddlingCamera_DownstreamIntersectionIsValidOnScreen()
    {
        var poly = new[]
        {
            new Vector3(-1, -1, 2), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, 2),
        };
        var projected = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(projected.Length >= 3);

        // The viewport region (NDC [-1,1]^2), same as CellView.FullScreen()'s single polygon.
        var viewport = CellView.FullScreen().Polygons[0].Vertices;
        var onScreen = ScreenPolygonClip.Intersect(projected, viewport);

        Assert.True(onScreen.Length >= 3); // a non-empty visible region survives
        foreach (var v in onScreen)
        {
            Assert.InRange(v.X, -1.001f, 1.001f);
            Assert.InRange(v.Y, -1.001f, 1.001f);
        }
    }

    [Fact]
    public void Project_QuadStraddlingCamera_NdcStaysWithinScreen()
    {
        // R1 void-flap fix (2026-06-05): the eye-plane-only clip (w>MinW) let small-w verts
        // explode under the perspective divide (~±37 NDC). Those off-screen NDC then corrupted
        // the downstream 2D ScreenPolygonClip, which at glancing/close angles collapsed to EMPTY
        // -> OutsideView empty -> terrain Skip -> the bluish "void" at the cottage doorway.
        // Clipping the 4 frustum SIDE planes in clip space (homogeneous, before the divide)
        // bounds every projected vertex to the screen [-1,1] by construction. RED before the fix.
        var poly = new[]
        {
            new Vector3(-1, -1, 2), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, 2),
        };
        var r = PortalProjection.ProjectToNdc(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(r.Length >= 3);
        foreach (var v in r)
        {
            Assert.InRange(v.X, -1.001f, 1.001f); // bounded to the screen — no off-screen explosion
            Assert.InRange(v.Y, -1.001f, 1.001f);
        }
    }

    [Fact]
    public void Project_CloseDoorway_NdcStaysWithinScreen_AndCoversScreen()
    {
        // The probe-confirmed void frame (2026-06-05): the chase eye is ~0.28 m from the front-door
        // EXIT portal — well inside RetailChaseCamera's 1.0 m near plane — and looking through it.
        // The door subtends the whole screen, but the old clip produced NDC like (10.2,-67.4) and
        // ScreenPolygonClip reduced it to clip=0 (the void). After the homogeneous side-plane clip
        // the NDC stays on-screen AND the door still covers the viewport (non-empty), not the void.
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1.0f, 5000f); // RetailChaseCamera
        var viewProj = view * proj;

        // A 2 m x 2 m doorway 0.28 m in front of the eye, facing it.
        var doorway = new[]
        {
            new Vector3(-1f, -1f, -0.28f), new Vector3(1f, -1f, -0.28f),
            new Vector3(1f,  1f, -0.28f),  new Vector3(-1f, 1f, -0.28f),
        };
        var projected = PortalProjection.ProjectToNdc(doorway, Matrix4x4.Identity, viewProj);
        Assert.True(projected.Length >= 3);
        foreach (var v in projected)
        {
            Assert.InRange(v.X, -1.001f, 1.001f);
            Assert.InRange(v.Y, -1.001f, 1.001f);
        }

        var viewport = CellView.FullScreen().Polygons[0].Vertices;
        var onScreen = ScreenPolygonClip.Intersect(projected, viewport);
        Assert.True(onScreen.Length >= 3, "a doorway the eye looks through must cover the screen, not collapse to the void");
    }

    [Fact]
    public void Project_PortalEyeIsAlmostTouching_StaysVisibleOnScreen()
    {
        // Regression (2026-06-03 doorway "void"): the chase camera orbits to ~0.1 m from a
        // doorway portal. With RetailChaseCamera's 1.0 m near plane and the old w+z>=0 GL
        // near-clip — which, for a D3D CreatePerspectiveFieldOfView matrix, discards everything
        // within ~0.5 m of the eye — the whole doorway was clipped to empty, so the room behind
        // it was culled and rendered as a dark void (camera-orientation dependent; rotating away
        // fixed it). The eye-clip must be near-INDEPENDENT: a portal you're standing in must
        // still project (covering the screen) so the cell behind stays visible.
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1.0f, 5000f); // RetailChaseCamera
        var viewProj = view * proj;

        // A 2 m x 2 m doorway 0.1 m in front of the eye, facing it.
        var doorway = new[]
        {
            new Vector3(-1f, -1f, -0.1f), new Vector3(1f, -1f, -0.1f),
            new Vector3(1f,  1f, -0.1f),  new Vector3(-1f, 1f, -0.1f),
        };

        var projected = PortalProjection.ProjectToNdc(doorway, Matrix4x4.Identity, viewProj);
        Assert.True(projected.Length >= 3,
            "a doorway 0.1 m from the eye must still project (was clipped to empty -> void)");

        var viewport = CellView.FullScreen().Polygons[0].Vertices;
        var onScreen = ScreenPolygonClip.Intersect(projected, viewport);
        Assert.True(onScreen.Length >= 3,
            "the cell behind a doorway you're standing in must stay visible (the void bug)");
    }

    // ---------------------------------------------------------------------------
    // Faithful homogeneous (w-space) portal clip — port of retail PView::GetClip +
    // PrimD3DRender::xformStart + ACRender::polyClipFinish (decomp 432344 / 424310 /
    // 702749). The early divide + fixed side-plane clamp (ProjectToNdc) collapsed
    // grazing/near portals to zero-area edge slivers (-> the flap) and near doorways
    // to empty (-> the void/fallback). The faithful pipeline keeps homogeneous clip
    // coords (ProjectToClip — eye-plane clip only, no divide) and runs Sutherland-
    // Hodgman against the view region with w-aware edge tests, dividing the survivors
    // only after they are bounded to the region (ClipToRegion). 2026-06-06.
    // ---------------------------------------------------------------------------

    private static Vector2[] FullScreenCcw() => new[]
    {
        new Vector2(-1f, -1f), new Vector2(1f, -1f), new Vector2(1f, 1f), new Vector2(-1f, 1f),
    };

    [Fact]
    public void ProjectToClip_QuadInFront_KeepsVertsWithPositiveW()
    {
        var poly = new[]
        {
            new Vector3(-1, -1, -5), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, -5),
        };
        var clip = PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj());
        Assert.True(clip.Length >= 3);
        foreach (var v in clip)
            Assert.True(v.W > 0f, $"an in-front portal vertex must keep w>0 (homogeneous), got w={v.W}");
    }

    [Fact]
    public void ProjectToClip_QuadFullyBehind_ReturnsEmpty()
    {
        var poly = new[]
        {
            new Vector3(-1, -1, 5), new Vector3(1, -1, 5), new Vector3(1, 1, 5), new Vector3(-1, 1, 5),
        };
        Assert.True(PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj()).Length < 3);
    }

    [Fact]
    public void ClipToRegion_OnScreenQuad_ReturnsBoundedNdc()
    {
        // A small 2x2 quad at z=-5 is fully on-screen; clipping against the full screen
        // returns it bounded to [-1,1].
        var poly = new[]
        {
            new Vector3(-1, -1, -5), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, -5),
        };
        var clip = PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj());
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3);
        foreach (var v in ndc) { Assert.InRange(v.X, -1.001f, 1.001f); Assert.InRange(v.Y, -1.001f, 1.001f); }
    }

    [Fact]
    public void ClipToRegion_FullyOffScreen_ReturnsEmpty_NotSliver()
    {
        // The flap's root: a portal entirely off one screen edge. The old early-divide +
        // side-plane clamp collapsed it to a zero-area sliver pinned to x=1.0 (proj=3) that
        // propagated a degenerate region one hop and then died; the faithful clip returns
        // EMPTY so the flood stops cleanly. Quad at z=-5, x in [3,5] -> ndc x ~[1.1,1.8], off right.
        var poly = new[]
        {
            new Vector3(3, -1, -5), new Vector3(5, -1, -5), new Vector3(5, 1, -5), new Vector3(3, 1, -5),
        };
        var clip = PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj());
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length < 3, $"fully off-screen portal must clip to empty, got {ndc.Length} verts (sliver)");
    }

    [Fact]
    public void ClipToRegion_PartlyOffScreen_ClipsToBoundedNonEmpty()
    {
        // A quad straddling the right edge -> clipped to the on-screen part, bounded, non-empty.
        var poly = new[]
        {
            new Vector3(0, -1, -5), new Vector3(4, -1, -5), new Vector3(4, 1, -5), new Vector3(0, 1, -5),
        };
        var clip = PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj());
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3, "a partly-on-screen portal must produce a non-empty clipped region");
        foreach (var v in ndc) { Assert.InRange(v.X, -1.001f, 1.001f); Assert.InRange(v.Y, -1.001f, 1.001f); }
    }

    [Fact]
    public void ClipToRegion_DoorwayEyeLooksThrough_CoversScreen_WithoutFallback()
    {
        // The void frame: chase eye 0.28 m from a 2x2 m doorway, looking through it. The doorway
        // subtends the whole screen. The faithful clip keeps the homogeneous verts (no early-divide
        // blow-up) and clips to the screen quad -> covers the screen (non-empty). This is what makes
        // the EyeInsidePortalOpening *fallback* unnecessary for an in-front doorway.
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1.0f, 5000f);
        var viewProj = view * proj;
        var doorway = new[]
        {
            new Vector3(-1f, -1f, -0.28f), new Vector3(1f, -1f, -0.28f),
            new Vector3(1f,  1f, -0.28f),  new Vector3(-1f, 1f, -0.28f),
        };
        var clip = PortalProjection.ProjectToClip(doorway, Matrix4x4.Identity, viewProj);
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3, "a doorway the eye looks through must cover the screen, not collapse to the void");
        foreach (var v in ndc) { Assert.InRange(v.X, -1.001f, 1.001f); Assert.InRange(v.Y, -1.001f, 1.001f); }
    }

    [Fact]
    public void ClipToRegion_StraddlingEye_OnScreenBounded_NoBlowup()
    {
        // A portal spanning from behind (z=+2) to in front (z=-5). The faithful clip keeps the
        // in-front part and bounds it to the screen — no perspective-inversion blow-up, non-empty.
        var poly = new[]
        {
            new Vector3(-1, -1, 2), new Vector3(1, -1, -5), new Vector3(1, 1, -5), new Vector3(-1, 1, 2),
        };
        var clip = PortalProjection.ProjectToClip(poly, Matrix4x4.Identity, ViewProj());
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3);
        foreach (var v in ndc) { Assert.InRange(v.X, -1.001f, 1.001f); Assert.InRange(v.Y, -1.001f, 1.001f); }
    }

    private static float AbsArea(Vector2[] p)
    {
        if (p == null || p.Length < 3) return 0f;
        float a2 = 0f;
        for (int i = 0; i < p.Length; i++) { var u = p[i]; var w = p[(i + 1) % p.Length]; a2 += u.X * w.Y - w.X * u.Y; }
        return MathF.Abs(a2) * 0.5f;
    }

    [Fact]
    public void ClipToRegion_SubjectFullyInsideRegion_ReturnsSubjectNotRegion()
    {
        // Regression for Build_AppliesReciprocalOtherPortalClip: a NARROW subject fully inside a WIDE
        // region must return the narrow (subject ∩ region = subject), NOT the wide region. The builder's
        // reciprocal clip is exactly this shape (reciprocal opening ∩ near-side region).
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(1.2f, 1.0f, 0.1f, 1000f);
        var vp = view * proj;
        var narrow = new[] { new Vector3(-0.3f, -0.9f, -3f), new Vector3(0.3f, -0.9f, -3f), new Vector3(0.3f, 0.9f, -3f), new Vector3(-0.3f, 0.9f, -3f) };
        var wide = new[] { new Vector3(-0.9f, -0.9f, -3f), new Vector3(0.9f, -0.9f, -3f), new Vector3(0.9f, 0.9f, -3f), new Vector3(-0.9f, 0.9f, -3f) };

        var narrowClip = PortalProjection.ProjectToClip(narrow, Matrix4x4.Identity, vp);
        // Build the region exactly as the builder does (clip wide against the full screen → CCW region).
        var wideRegion = PortalProjection.ClipToRegion(PortalProjection.ProjectToClip(wide, Matrix4x4.Identity, vp), FullScreenCcw());

        var clipped = PortalProjection.ClipToRegion(narrowClip, wideRegion);
        float narrowArea = AbsArea(PortalProjection.ClipToRegion(narrowClip, FullScreenCcw()));
        float wideArea = AbsArea(wideRegion);
        float clippedArea = AbsArea(clipped);
        Assert.True(clippedArea <= narrowArea + 1e-3f,
            $"subject∩region must be the narrow subject (area {narrowArea}), not the wide region (area {wideArea}); got {clippedArea}");
    }

    [Fact]
    public void ClipToRegion_AgainstSubRegion_TightensToIntersection()
    {
        // The region clip is the propagation step: clipping a wide on-screen portal against a
        // narrower view region must yield the intersection (the narrow region), not the wide portal.
        var wide = new[]
        {
            new Vector3(-2, -2, -5), new Vector3(2, -2, -5), new Vector3(2, 2, -5), new Vector3(-2, 2, -5),
        };
        var clip = PortalProjection.ProjectToClip(wide, Matrix4x4.Identity, ViewProj());
        var narrow = new[]
        {
            new Vector2(-0.3f, -0.3f), new Vector2(0.3f, -0.3f), new Vector2(0.3f, 0.3f), new Vector2(-0.3f, 0.3f),
        };
        var ndc = PortalProjection.ClipToRegion(clip, narrow);
        Assert.True(ndc.Length >= 3);
        foreach (var v in ndc) { Assert.InRange(v.X, -0.301f, 0.301f); Assert.InRange(v.Y, -0.301f, 0.301f); }
    }

    // ---------------------------------------------------------------------------
    // The W=0 knife-edge port (2026-06-11) — retail ACRender::polyClipFinish part 1
    // (0x006b6d00, pc:702749; pseudocode at docs/research/2026-06-11-polyclipfinish-
    // w0-clip-pseudocode.md). The eye-plane clip is at w >= 0 EXACTLY: boundary
    // intersections land at w == 0 (homogeneous directions), so a portal the eye is
    // CROSSING (stair openings on a spiral climb, the tower deck) yields the correct
    // unbounded half-region that the bounded view-region clip then cuts to the
    // screen. The previous EyePlaneW = 1e-4 made the boundary verts finite ~1e4-NDC
    // points and the resulting regions sat at the merge/dedup degeneracy threshold —
    // the climb-strobe class that the (now deleted) EyeInsidePortalOpening rescue
    // compensated for.
    // ---------------------------------------------------------------------------

    [Fact]
    public void ProjectToClip_EyeCrossingPortal_BoundaryVertsLandAtWZero()
    {
        // A horizontal floor opening 5 mm BELOW the eye, spanning from 1.5 m ahead to
        // 0.5 m behind — the spiral-climb crossing frame. The two edges crossing the
        // eye plane must emit intersections at exactly w == 0 (retail polyClipFinish
        // t = w0/(w0-w1)), not at an epsilon offset.
        var opening = new[]
        {
            new Vector3(-1f, -0.005f, -1.5f), new Vector3(1f, -0.005f, -1.5f),
            new Vector3(1f, -0.005f, 0.5f),   new Vector3(-1f, -0.005f, 0.5f),
        };
        var clip = PortalProjection.ProjectToClip(opening, Matrix4x4.Identity, ViewProj());
        Assert.True(clip.Length >= 3, "an eye-crossing portal must keep its forward half");
        int atZero = 0;
        foreach (var v in clip)
        {
            Assert.True(v.W >= 0f, $"no survivor may sit behind the eye plane, got w={v.W}");
            if (v.W == 0f) atZero++;
        }
        Assert.True(atZero >= 2, $"the two eye-plane crossings must land at exactly w==0, got {atZero}");
    }

    [Fact]
    public void ClipToRegion_EyeCrossingFloorOpening_YieldsHalfRegionNotSliver()
    {
        // Same crossing frame: the visible set through an opening the eye is inside is
        // the half-screen below the opening's plane horizon — NOT the degenerate sliver
        // the epsilon clip produced. Full screen area is 4.0; the half-region must hold
        // a substantial part of it.
        var opening = new[]
        {
            new Vector3(-1f, -0.005f, -1.5f), new Vector3(1f, -0.005f, -1.5f),
            new Vector3(1f, -0.005f, 0.5f),   new Vector3(-1f, -0.005f, 0.5f),
        };
        var clip = PortalProjection.ProjectToClip(opening, Matrix4x4.Identity, ViewProj());
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3, "the crossing frame must produce a region, not empty (the climb strobe)");
        foreach (var v in ndc)
        {
            Assert.True(float.IsFinite(v.X) && float.IsFinite(v.Y), $"region verts must be finite, got ({v.X},{v.Y})");
            Assert.InRange(v.X, -1.001f, 1.001f);
            Assert.InRange(v.Y, -1.001f, 1.001f);
        }
        float area = AbsArea(ndc);
        Assert.True(area > 1.5f,
            $"the region must approximate the lower half-screen (area ~2.0 of 4.0), got {area} (sliver = the strobe bug)");
    }

    [Fact]
    public void EyeInPortalPlane_GazeAlongPlane_DegenerateViewPropagates()
    {
        // The spiral-climb knife edge: the eye sits IN a horizontal portal's plane with
        // the gaze ALONG the plane (climbing stairs through the opening). The opening is
        // visibly edge-on ON screen: ProjectToClip + ClipToRegion yield a zero-area
        // collinear region — and retail PROPAGATES it (ClipPortals forwards any count!=0
        // clip; no area gate), keeping the cell behind in the draw list. CellView.Add
        // must therefore ACCEPT the collinear polygon (the "L:" segment key) instead of
        // rejecting it as degenerate — rejection dropped the whole chain for the frame.
        var view = Matrix4x4.CreateLookAt(Vector3.Zero, new Vector3(0, 0, -1), Vector3.UnitY);
        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1.0f, 5000f);
        var vp = view * proj;
        // Horizontal opening in the y=0 plane (contains the eye), ahead of the camera.
        var opening = new[]
        {
            new Vector3(-1f, 0f, -1f), new Vector3(1f, 0f, -1f),
            new Vector3(1f, 0f, -4f),  new Vector3(-1f, 0f, -4f),
        };
        var clip = PortalProjection.ProjectToClip(opening, Matrix4x4.Identity, vp);
        Assert.True(clip.Length >= 3, "the in-plane opening's forward part must survive the W clip");
        var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
        Assert.True(ndc.Length >= 3, "the edge-on opening must yield its (zero-area) collinear region");

        var cellView = new CellView();
        Assert.True(cellView.Add(new ViewPolygon(ndc)),
            "a zero-area collinear view must be ACCEPTED (retail propagates degenerate views; " +
            "rejecting it drops the cell chain at the knife edge)");
        // Re-emission of the same degenerate view dedups (finite segment-key space = convergence).
        Assert.False(cellView.Add(new ViewPolygon(ndc)),
            "a re-emitted degenerate view must dedup via its segment key");
    }

    [Fact]
    public void ClipToRegion_NeverReturnsNonFiniteVerts()
    {
        // The measure-zero guard: whatever survives the bounded region clip must divide
        // to finite NDC. Exercise with a portal whose vertices sit ON the eye plane
        // (w == 0 inputs) plus one in front — degenerate input, must yield empty or finite.
        var degenerate = new[]
        {
            new Vector3(-1f, 0f, 0f), new Vector3(1f, 0f, 0f), new Vector3(0f, 1f, -2f),
        };
        var clip = PortalProjection.ProjectToClip(degenerate, Matrix4x4.Identity, ViewProj());
        if (clip.Length >= 3)
        {
            var ndc = PortalProjection.ClipToRegion(clip, FullScreenCcw());
            foreach (var v in ndc)
                Assert.True(float.IsFinite(v.X) && float.IsFinite(v.Y),
                    $"non-finite NDC vert leaked from the divide: ({v.X},{v.Y})");
        }
    }
}