Implement "Battletoads" 3-phase dot crawl

https://forums.nesdev.org/viewtopic.php?p=30625#p30625

Author: Gumball2415

Core/BaseVideoFilter.cpp

@@ -52,11 +52,17 @@ bool BaseVideoFilter::IsOddFrame()
 	return _isOddFrame;
 }
 
+uint8_t BaseVideoFilter::GetFieldPhase()
+{
+	return _fieldPhase;
+}
+
 void BaseVideoFilter::SendFrame(uint16_t *ppuOutputBuffer, uint32_t frameNumber)
 {
 	_frameLock.Acquire();
 	_overscan = _console->GetSettings()->GetOverscanDimensions();
-	_isOddFrame = frameNumber % 2;
+	_isOddFrame = frameNumber & 0x01;
+	_fieldPhase = frameNumber % 3;
 	UpdateBufferSize();
 	OnBeforeApplyFilter();
 	ApplyFilter(ppuOutputBuffer);

Core/BaseVideoFilter.h

@@ -15,6 +15,8 @@ class BaseVideoFilter
 	SimpleLock _frameLock;
 	OverscanDimensions _overscan;
 	bool _isOddFrame;
+	// https://forums.nesdev.org/viewtopic.php?p=30625#p30625
+	uint8_t _fieldPhase;
 
 	void UpdateBufferSize();
 
@@ -23,13 +25,14 @@ class BaseVideoFilter
 
 	virtual void ApplyFilter(uint16_t *ppuOutputBuffer) = 0;
 	virtual void OnBeforeApplyFilter();
-	bool IsOddFrame();
+	uint8_t GetFieldPhase();
 
 public:
 	BaseVideoFilter(shared_ptr<Console> console);
 	virtual ~BaseVideoFilter();
 
 	uint32_t* GetOutputBuffer();
+	bool IsOddFrame();
 	void SendFrame(uint16_t *ppuOutputBuffer, uint32_t frameNumber);
 	void TakeScreenshot(string romName, VideoFilterType filterType);
 	void TakeScreenshot(VideoFilterType filterType, string filename, std::stringstream *stream = nullptr, bool rawScreenshot = false);

Core/BisqwitNtscFilter.cpp

@@ -50,8 +50,7 @@ BisqwitNtscFilter::BisqwitNtscFilter(shared_ptr<Console> console, int resDivider
 			} else {
 				outputBuffer += GetOverscan().GetScreenWidth() * 64 / _resDivider / _resDivider * (120 - GetOverscan().Top);
 			}
-
-			DecodeFrame(120, 239 - GetOverscan().Bottom, _ppuOutputBuffer, outputBuffer, (IsOddFrame() ? 8 : 0) + 327360); 
+			DecodeFrame(120, 239 - GetOverscan().Bottom, _ppuOutputBuffer, outputBuffer, (_console->IsDotBypassed() ? (GetFieldPhase() * 4) : (IsOddFrame() ? 0 : 8)) + 327360);
 
 			_workDone = true;
 		}
@@ -71,7 +70,7 @@ void BisqwitNtscFilter::ApplyFilter(uint16_t *ppuOutputBuffer)
 
 	_workDone = false;
 	_waitWork.Signal();
-	DecodeFrame(GetOverscan().Top, 120, ppuOutputBuffer, GetOutputBuffer(), (IsOddFrame() ? 8 : 0) + GetOverscan().Top*341*8);
+	DecodeFrame(GetOverscan().Top, 120, ppuOutputBuffer, GetOutputBuffer(), (_console->IsDotBypassed() ? (GetFieldPhase() * 4) : (IsOddFrame() ? 0 : 8)) + GetOverscan().Top * 341 * 8);
 	while(!_workDone) {}
 }
 

Core/Console.cpp

@@ -568,6 +568,11 @@ uint32_t Console::GetFrameCount()
 	return _ppu ? _ppu->GetFrameCount() : 0;
 }
 
+bool Console::IsDotBypassed()
+{
+	return _ppu ? _ppu->IsDotBypassed() : false;
+}
+
 NesModel Console::GetModel()
 {
 	return _model;

Core/Console.h

@@ -204,6 +204,8 @@ class Console : public std::enable_shared_from_this<Console>
 	VirtualFile GetPatchFile();
 	RomInfo GetRomInfo();
 	uint32_t GetFrameCount();
+	// https://forums.nesdev.org/viewtopic.php?p=30625#p30625
+	bool IsDotBypassed();
 	NesModel GetModel();
 
 	uint32_t GetLagCounter();

Core/NtscFilter.cpp

@@ -132,7 +132,7 @@ void NtscFilter::OnBeforeApplyFilter()
 
 void NtscFilter::ApplyFilter(uint16_t *ppuOutputBuffer)
 {
-	nes_ntsc_blit(&_ntscData, ppuOutputBuffer, _ntsc_border, PPU::ScreenWidth, IsOddFrame() ? 0 : 1, PPU::ScreenWidth, 240, _ntscBuffer, NES_NTSC_OUT_WIDTH(PPU::ScreenWidth)*4);
+	nes_ntsc_blit(&_ntscData, ppuOutputBuffer, _ntsc_border, PPU::ScreenWidth, (_console->IsDotBypassed() ? GetFieldPhase() : IsOddFrame()), PPU::ScreenWidth, 240, _ntscBuffer, NES_NTSC_OUT_WIDTH(PPU::ScreenWidth)*4);
 	GenerateArgbFrame(_ntscBuffer);
 }
 

Core/PPU.cpp

@@ -105,6 +105,8 @@ void PPU::Reset()
 	memset(_oamDecayCycles, 0, sizeof(_oamDecayCycles));
 	_enableOamDecay = _settings->CheckFlag(EmulationFlags::EnableOamDecay);
 
+	_isDotBypassed = false;
+
 	UpdateMinimumDrawCycles();
 }
 
@@ -997,14 +999,21 @@ void PPU::ProcessScanline()
 			LoadTileInfo();
 		}
 	} else if(_cycle == 337 || _cycle == 339) {
-		if(IsRenderingEnabled()) {
-			ReadVram(GetNameTableAddr());
-
-			if(_scanline == -1 && _cycle == 339 && (_frameCount & 0x01) && _nesModel == NesModel::NTSC && _settings->GetPpuModel() == PpuModel::Ppu2C02) {
-				//This behavior is NTSC-specific - PAL frames are always the same number of cycles
-				//"With rendering enabled, each odd PPU frame is one PPU clock shorter than normal" (skip from 339 to 0, going over 340)
+		if(_scanline == -1 && _cycle == 339 && (_frameCount & 0x01) && _nesModel == NesModel::NTSC && _settings->GetPpuModel() == PpuModel::Ppu2C02) {
+			if (IsRenderingEnabled()) {
+			//This behavior is NTSC-specific - PAL frames are always the same number of cycles
+			//"With rendering enabled, each odd PPU frame is one PPU clock shorter than normal" (skip from 339 to 0, going over 340)
 				_cycle = 340;
+				_isDotBypassed = false;
 			}
+			else {
+				// "By keeping rendering disabled until after the time when the clock is skipped on odd frames,
+				// you can get a different color dot crawl pattern than normal."
+				_isDotBypassed = true;
+			}
+		}
+		if (IsRenderingEnabled()) {
+			ReadVram(GetNameTableAddr());
 		}
 	}
 }

Core/PPU.h

@@ -109,6 +109,9 @@ class PPU : public IMemoryHandler, public Snapshotable
 		bool _enableOamDecay;
 		bool _corruptOamRow[32];
 
+		// https://forums.nesdev.org/viewtopic.php?p=30625#p30625
+		bool _isDotBypassed;
+
 		void UpdateStatusFlag();
 
 		void SetControlRegister(uint8_t value);
@@ -217,6 +220,11 @@ class PPU : public IMemoryHandler, public Snapshotable
 			return _frameCount;
 		}
 
+		bool IsDotBypassed()
+		{
+			return _isDotBypassed;
+		}
+
 		uint32_t GetFrameCycle()
 		{
 			return ((_scanline + 1) * 341) + _cycle;