From 08290ece169c781388c154baa223eb750390e839 Mon Sep 17 00:00:00 2001
From: gramps <gramps@llamachile.shop>
Date: Mon, 5 Oct 2026 16:13:39 -0700
Subject: [PATCH] Compositor: prefill only uncovered rows when backdrop not
 fully covered (Unit 4)

---
 Services/Compositor/SceneCompositor.cs | 49 ++++++++++++++++++++++++--
 1 file changed, 47 insertions(+), 2 deletions(-)

diff --git a/Services/Compositor/SceneCompositor.cs b/Services/Compositor/SceneCompositor.cs
index 5261f6b..33a9e51 100644
--- a/Services/Compositor/SceneCompositor.cs
+++ b/Services/Compositor/SceneCompositor.cs
@@ -92,8 +92,53 @@ public sealed class SceneCompositor
             var covered = liveCaptureFrame != null
                 && liveCaptureFrame.Width == cropW && liveCaptureFrame.Height == cropH;
             if (!covered)
-                for (var i = 3; i < buffer.Length; i += 4)
-                    buffer[i] = 255; // opaque black base
+            {
+                if (liveCaptureFrame != null)
+                {
+                    var (uScale, uOx, uOy) = StretchMath.UniformToFill(cropW, cropH, liveCaptureFrame.Width, liveCaptureFrame.Height);
+                    var drawnW = liveCaptureFrame.Width * uScale;
+                    var drawnH = liveCaptureFrame.Height * uScale;
+                    var ox = (int)Math.Floor(uOx);
+                    var oy = (int)Math.Floor(uOy);
+                    var oh = (int)Math.Ceiling(oy + drawnH);
+                    var ow = cropW;
+                    if (oy > 0)
+                    {
+                        var topCount = oy * ow;
+                        for (var i = 3; i < topCount * 4; i += 4)
+                            buffer[i] = 255;
+                    }
+                    if (oh < cropH)
+                    {
+                        var start = oh * ow;
+                        var bottomCount = (cropH - oh) * ow;
+                        for (var i = start * 4 + 3; i < (start + bottomCount) * 4; i += 4)
+                            buffer[i] = 255;
+                    }
+                    if (ox > 0 || (int)Math.Ceiling(uOx + drawnW) < cropW)
+                    {
+                        var x0s = (int)Math.Floor(uOx);
+                        var x1s = (int)Math.Ceiling(uOx + drawnW);
+                        for (var y = oy; y < oh && y < cropH; y++)
+                        {
+                            var di = (y * ow + 0) * 4;
+                            for (var x = 0; x < x0s && x < ow; x++)
+                            {
+                                buffer[di + x * 4 + 3] = 255;
+                            }
+                            for (var x = x1s; x < ow; x++)
+                            {
+                                buffer[di + x * 4 + 3] = 255;
+                            }
+                        }
+                    }
+                }
+                else
+                {
+                    for (var i = 3; i < buffer.Length; i += 4)
+                        buffer[i] = 255; // opaque black base
+                }
+            }
 
             if (liveCaptureFrame != null)
                 BlitContent(buffer, cropW, cropH, 0, 0, cropW, cropH, liveCaptureFrame, 1f, false, false);