diff --git a/src/AcDream.App/Rendering/PortalProjection.cs b/src/AcDream.App/Rendering/PortalProjection.cs
new file mode 100644
index 0000000..852d9a1
--- /dev/null
+++ b/src/AcDream.App/Rendering/PortalProjection.cs
@@ -0,0 +1,74 @@
+// PortalProjection.cs
+//
+// Phase A8.F: project a cell-local portal polygon to NDC screen space, clipping
+// against the GL near plane (w + z >= 0, i.e. z_ndc >= -1) so a portal straddling
+// the camera does not invert under the perspective divide. At the near plane w is
+// bounded away from zero, so the divide is safe — no eye-singularity blow-up.
+// Homogeneous form of the near-plane sidedness in retail PView::GetClip /
+// ConstructView(CBldPortal) (decomp:432344 / 433832).
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace AcDream.App.Rendering;
+
+public static class PortalProjection
+{
+    /// <summary>Project a cell-local polygon to NDC. Returns CCW NDC xy verts, or
+    /// fewer than 3 verts when the polygon is entirely behind the camera / degenerate.</summary>
+    public static Vector2[] ProjectToNdc(IReadOnlyList<Vector3> localPoly, Matrix4x4 cellToWorld, Matrix4x4 viewProj)
+    {
+        if (localPoly == null || localPoly.Count < 3) return System.Array.Empty<Vector2>();
+
+        Matrix4x4 m = cellToWorld * viewProj;
+
+        // To clip space (keep w).
+        var clip = new List<Vector4>(localPoly.Count);
+        foreach (var lp in localPoly)
+            clip.Add(Vector4.Transform(new Vector4(lp, 1f), m));
+
+        // Clip against the GL near plane (keep where w + z >= 0).
+        clip = ClipAgainstNearPlane(clip);
+        if (clip.Count < 3) return System.Array.Empty<Vector2>();
+
+        // Perspective divide → NDC xy.
+        var ndc = new Vector2[clip.Count];
+        for (int i = 0; i < clip.Count; i++)
+        {
+            float w = clip[i].W;
+            ndc[i] = new Vector2(clip[i].X / w, clip[i].Y / w);
+        }
+        return ndc;
+    }
+
+    // Sutherland-Hodgman against the GL near plane: keep where (w + z) >= 0  (z >= -w, i.e. z_ndc >= -1).
+    private static List<Vector4> ClipAgainstNearPlane(List<Vector4> poly)
+    {
+        var result = new List<Vector4>(poly.Count + 1);
+        for (int i = 0; i < poly.Count; i++)
+        {
+            Vector4 cur = poly[i];
+            Vector4 prev = poly[(i + poly.Count - 1) % poly.Count];
+            float dCur = cur.W + cur.Z;
+            float dPrev = prev.W + prev.Z;
+            bool curIn = dCur >= 0f;
+            bool prevIn = dPrev >= 0f;
+
+            if (curIn)
+            {
+                if (!prevIn) result.Add(Lerp(prev, cur, dPrev, dCur));
+                result.Add(cur);
+            }
+            else if (prevIn)
+            {
+                result.Add(Lerp(prev, cur, dPrev, dCur));
+            }
+        }
+        return result;
+    }
+
+    private static Vector4 Lerp(Vector4 p, Vector4 q, float dp, float dq)
+    {
+        float t = dp / (dp - dq);
+        return p + t * (q - p);
+    }
+}
diff --git a/tests/AcDream.App.Tests/Rendering/PortalProjectionTests.cs b/tests/AcDream.App.Tests/Rendering/PortalProjectionTests.cs
new file mode 100644
index 0000000..eeb6c6e
--- /dev/null
+++ b/tests/AcDream.App.Tests/Rendering/PortalProjectionTests.cs
@@ -0,0 +1,87 @@
+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);
+        }
+    }
+}