From c35b18751db55c9fe1fd7e6003674dc83bc9ee02 Mon Sep 17 00:00:00 2001
From: mjbudd77 <mjbudd77@gmail.com>
Date: Mon, 13 Dec 2021 21:16:53 -0500
Subject: [PATCH] Added a triple buffer for video frame data to ensure clean
 transfer of that data between emulation and Qt GUI threads.

---
 src/drivers/Qt/ConsoleViewerGL.cpp  | 12 +++++++++---
 src/drivers/Qt/ConsoleViewerSDL.cpp | 12 +++++++++---
 src/drivers/Qt/ConsoleWindow.cpp    |  4 ++--
 src/drivers/Qt/nes_shm.h            |  4 +++-
 src/drivers/Qt/sdl-video.cpp        | 12 +++++++++---
 5 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/src/drivers/Qt/ConsoleViewerGL.cpp b/src/drivers/Qt/ConsoleViewerGL.cpp
index 079e5521..c473895a 100644
--- a/src/drivers/Qt/ConsoleViewerGL.cpp
+++ b/src/drivers/Qt/ConsoleViewerGL.cpp
@@ -464,16 +464,22 @@ double ConsoleViewGL_t::getAspectRatio(void)
 
 void ConsoleViewGL_t::transfer2LocalBuffer(void)
 {
-	int i=0, hq = 0;
+	int i=0, hq = 0, bufIdx;
 	int numPixels = nes_shm->video.ncol * nes_shm->video.nrow;
 	unsigned int cpSize = numPixels * 4;
  	uint8_t *src, *dest;
 
+	bufIdx = nes_shm->pixBufIdx-1;
+
+	if ( bufIdx < 0 )
+	{
+		bufIdx = NES_VIDEO_BUFLEN-1;
+	}
 	if ( cpSize > localBufSize )
 	{
 		cpSize = localBufSize;
 	}
-	src  = (uint8_t*)nes_shm->pixbuf;
+	src  = (uint8_t*)nes_shm->pixbuf[bufIdx];
 	dest = (uint8_t*)localBuf;
 
 	hq = (nes_shm->video.preScaler == 1) || (nes_shm->video.preScaler == 4); // hq2x and hq3x
@@ -492,7 +498,7 @@ void ConsoleViewGL_t::transfer2LocalBuffer(void)
 	}
 	else
 	{
-		memcpy( localBuf, nes_shm->pixbuf, cpSize );
+		memcpy( localBuf, src, cpSize );
 	}
 }
 
diff --git a/src/drivers/Qt/ConsoleViewerSDL.cpp b/src/drivers/Qt/ConsoleViewerSDL.cpp
index 08d3c6c9..04216592 100644
--- a/src/drivers/Qt/ConsoleViewerSDL.cpp
+++ b/src/drivers/Qt/ConsoleViewerSDL.cpp
@@ -206,16 +206,22 @@ double ConsoleViewSDL_t::getAspectRatio(void)
 
 void ConsoleViewSDL_t::transfer2LocalBuffer(void)
 {
-	int i=0, hq = 0;
+	int i=0, hq = 0, bufIdx;
 	int numPixels = nes_shm->video.ncol * nes_shm->video.nrow;
 	unsigned int cpSize = numPixels * 4;
  	uint8_t *src, *dest;
 
+	bufIdx = nes_shm->pixBufIdx-1;
+
+	if ( bufIdx < 0 )
+	{
+		bufIdx = NES_VIDEO_BUFLEN-1;
+	}
 	if ( cpSize > localBufSize )
 	{
 		cpSize = localBufSize;
 	}
-	src  = (uint8_t*)nes_shm->pixbuf;
+	src  = (uint8_t*)nes_shm->pixbuf[bufIdx];
 	dest = (uint8_t*)localBuf;
 
 	hq = (nes_shm->video.preScaler == 1) || (nes_shm->video.preScaler == 4); // hq2x and hq3x
@@ -234,7 +240,7 @@ void ConsoleViewSDL_t::transfer2LocalBuffer(void)
 	}
 	else
 	{
-		memcpy( localBuf, nes_shm->pixbuf, cpSize );
+		memcpy( localBuf, src, cpSize );
 	}
 }
 
diff --git a/src/drivers/Qt/ConsoleWindow.cpp b/src/drivers/Qt/ConsoleWindow.cpp
index 274a93c0..50671376 100644
--- a/src/drivers/Qt/ConsoleWindow.cpp
+++ b/src/drivers/Qt/ConsoleWindow.cpp
@@ -4253,16 +4253,16 @@ void consoleWin_t::transferVideoBuffer(void)
 {
 	if ( nes_shm->blitUpdated )
 	{
+		nes_shm->blitUpdated = 0;
+
 		if ( viewport_SDL )
 		{
 			viewport_SDL->transfer2LocalBuffer();
-			nes_shm->blitUpdated = 0;
 			viewport_SDL->render();
 		}
 		else if ( viewport_GL )
 		{
 			viewport_GL->transfer2LocalBuffer();
-			nes_shm->blitUpdated = 0;
 			viewport_GL->update();
 		}
 	}
diff --git a/src/drivers/Qt/nes_shm.h b/src/drivers/Qt/nes_shm.h
index ce83a131..5497430e 100644
--- a/src/drivers/Qt/nes_shm.h
+++ b/src/drivers/Qt/nes_shm.h
@@ -11,6 +11,7 @@
 
 #define  GL_NES_WIDTH   256
 #define  GL_NES_HEIGHT  240
+#define  NES_VIDEO_BUFLEN   3
 #define  NES_AUDIO_BUFLEN   480000
 
 struct  nes_shm_t
@@ -35,7 +36,8 @@ struct  nes_shm_t
 	char  runEmulator;
 	char  blitUpdated;
 
-	uint32_t  pixbuf[1048576]; // 1024 x 1024
+	int   pixBufIdx;
+	uint32_t  pixbuf[NES_VIDEO_BUFLEN][1048576]; // 1024 x 1024
 	uint32_t  avibuf[1048576]; // 1024 x 1024
 
 	void clear_pixbuf(void)
diff --git a/src/drivers/Qt/sdl-video.cpp b/src/drivers/Qt/sdl-video.cpp
index bc08c1de..da4a3254 100644
--- a/src/drivers/Qt/sdl-video.cpp
+++ b/src/drivers/Qt/sdl-video.cpp
@@ -398,6 +398,9 @@ static void vsync_test(void)
 	int i, j, k, l;
 	int cycleLen, halfCycleLen;
 	static int ofs = 0;
+	uint32_t *pixbuf;
+
+	pixbuf = nes_shm->pixbuf[nes_shm->pixBufIdx];
 
 	cycleLen = nes_shm->video.ncol / 4;
 
@@ -412,11 +415,11 @@ static void vsync_test(void)
 
 			if ( l < halfCycleLen )
 			{
-				nes_shm->pixbuf[k] = 0xffffffff; k++;
+				pixbuf[k] = 0xFFFFFFFF; k++;
 			}
 			else
 			{
-				nes_shm->pixbuf[k] = 0x00000000; k++;
+				pixbuf[k] = 0x00000000; k++;
 			}
 		}
 	}
@@ -492,8 +495,11 @@ doBlitScreen(uint8_t *XBuf, uint8_t *dest)
 void
 BlitScreen(uint8 *XBuf)
 {
-	doBlitScreen(XBuf, (uint8_t*)nes_shm->pixbuf);
+	int i = nes_shm->pixBufIdx;
 
+	doBlitScreen(XBuf, (uint8_t*)nes_shm->pixbuf[i]);
+
+	nes_shm->pixBufIdx = (i+1) % NES_VIDEO_BUFLEN;
 	nes_shm->blit_count++;
 	nes_shm->blitUpdated = 1;
 }