From c636c014247e7c2f24bae724eca176ba8ae22ca3 Mon Sep 17 00:00:00 2001
From: alyosha-tas <alexei.f.k@gmail.com>
Date: Thu, 18 Nov 2021 08:54:13 -0500
Subject: [PATCH] SMS: fix backdrop colors

---
 .../Consoles/Sega/SMS/VDP.Mode4.cs                    | 11 ++++-------
 src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.cs  |  7 +++++++
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.Mode4.cs b/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.Mode4.cs
index b21c507125..2206401dc2 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.Mode4.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.Mode4.cs
@@ -309,28 +309,25 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 				OverscanFrameBuffer = new int[OverscanFrameHeight * OverscanFrameWidth];
 			}
 
-			// TODO: overscan color can change, this should be calculated per scanline, not done in bulk at the end
-			int overscan_color = Palette[BackdropColor];
-
 			// Top overscan
 			for (int y=0; y<overscanTop; y++)
 				for (int x = 0; x < OverscanFrameWidth; x++)
-					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = overscan_color;
+					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = Backdrop_SL[0];
 			
 			// Bottom overscan
 			for (int y = overscanTop + 192; y < OverscanFrameHeight; y++)
 				for (int x = 0; x < OverscanFrameWidth; x++)
-					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = overscan_color;
+					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = Backdrop_SL[FrameHeight-1];
 
 			// Left overscan
 			for (int y = overscanTop; y < overscanTop + 192; y++)
 				for (int x = 0; x < overscanLeft; x++)
-					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = overscan_color;
+					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = Backdrop_SL[y - overscanTop];
 
 			// Right overscan
 			for (int y = overscanTop; y < overscanTop + 192; y++)
 				for (int x = overscanLeft + 256; x < OverscanFrameWidth; x++)
-					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = overscan_color;
+					OverscanFrameBuffer[(y * OverscanFrameWidth) + x] = Backdrop_SL[y - overscanTop];
 
 			// Active display area
 			for (int y = 0; y < 192; y++)
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.cs b/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.cs
index 4911661aee..4e979b7c80 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Sega/SMS/VDP.cs
@@ -47,6 +47,7 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 		public int[] FrameBuffer = new int[256 * 192];
 		public int[] GameGearFrameBuffer = new int[160 * 144];
 		public int[] OverscanFrameBuffer = null;
+		public int[] Backdrop_SL = new int[256];
 
 		public bool Mode1Bit => (Registers[1] & 16) > 0;
 		public bool Mode2Bit => (Registers[0] & 2) > 0;
@@ -382,6 +383,11 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 
 		internal void RenderCurrentScanline(bool render)
 		{
+			if (ScanLine < FrameHeight)
+			{
+				Backdrop_SL[ScanLine] = Palette[(byte)(16 + (Registers[7] & 15))];
+			}
+			
 			// only mode 4 supports frameskip. deal with it
 			if (TmsMode == 4)
 			{
@@ -423,6 +429,7 @@ namespace BizHawk.Emulation.Cores.Sega.MasterSystem
 			ser.Sync(nameof(CRAM), ref CRAM, false);
 			ser.Sync(nameof(VRAM), ref VRAM, false);
 			ser.Sync(nameof(HCounter), ref HCounter);
+			ser.Sync(nameof(Backdrop_SL), ref Backdrop_SL, false);
 			ser.EndSection();
 
 			if (ser.IsReader)