optimized display update region

DisplaySDL.cpp

@@ -36,6 +36,11 @@ DisplaySDL::~DisplaySDL() {
 }
 
 void DisplaySDL::updateWindow() const {
+  if(miTopDirtyScanline > miBottomDirtyScanline) {
+    // No changes since the last update
+    return;
+  }
+
   SDL_Surface *pSurface = SDL_GetWindowSurface(mpWindow);
   SDL_LockSurface(pSurface);
 
@@ -48,10 +53,15 @@ void DisplaySDL::updateWindow() const {
   lx = -1;
   ly = -1;
   for(sy = 0; sy < pSurface->h; sy++) {
+    // Map SDL surface coordinates to Chocochip8 Framebuffer coordinates
+    fy = sy * (double(chocochip8::gcHeight) / pSurface->h);
+    if(fy < miTopDirtyScanline || miBottomDirtyScanline < fy) {
+      // Skip scanlines that haven't changed since the last update
+      continue;
+    }
+
     for(sx = 0; sx < pSurface->w; sx++) {
-      // Map SDL surface coordinates to Chocochip8 Framebuffer coordinates
       fx = sx * (double(chocochip8::gcWidth) / pSurface->w);
-      fy = sy * (double(chocochip8::gcHeight) / pSurface->h);
 
       // Reuse color if this screen pixel maps to the same ChocoChip8 pixel as the last
       if(fx != lx || fy != ly) {
@@ -72,6 +82,8 @@ void DisplaySDL::updateWindow() const {
     }
   }
 
+  miTopDirtyScanline = chocochip8::gcHeight - 1;
+  miBottomDirtyScanline = 0;
   SDL_UnlockSurface(pSurface);
   SDL_UpdateWindowSurface(mpWindow);
 }

Interpreter.cpp

@@ -87,9 +87,7 @@ void Interpreter::tick() {
   case 0x0000: // 0NNN - call machine language routine
     switch(inst) {
     case 0x00E0: // clear display
-      for(auto &scanline : *mrDisplay.mpFramebuffer) {
-        scanline.reset();
-      }
+      mrDisplay.clear();
       break;
     case 0x00EE: // return from subroutine
       mvSpecialReg[SR_PC] = mCallStack.top();
@@ -292,7 +290,7 @@ void Interpreter::executeDraw(uint8_t x, uint8_t y, uint8_t n) {
 
     // we draw one scanline per sprite row in high-res mode, but twice in low-res mode
     for(int j = 0; j < (mIsHighResMode ? 1 : 2); j++) {
-      Scanline &targetScanline = mrDisplay.mpFramebuffer->at(y + i * (mIsHighResMode ? 1 : 2) + j);
+      Scanline &targetScanline = mrDisplay.getModifyableScanline(y + i * (mIsHighResMode ? 1 : 2) + j);
       if((targetScanline & spriteScanline) != 0) {
         collisionCount += 1;
       }

Peripherals.hpp

@@ -20,11 +20,32 @@ namespace chocochip8 {
 
   class Display {
   public:
-    friend class Interpreter;
-    Display(): mpFramebuffer{std::make_unique<Framebuffer>()} {}
+    Display():
+      mpFramebuffer{std::make_unique<Framebuffer>()},
+      miTopDirtyScanline{0},
+      miBottomDirtyScanline{gcHeight - 1} {}
+
     virtual ~Display() = default;
+
+    virtual void clear() {
+      for(auto &scanline : *mpFramebuffer) {
+        scanline.reset();
+      }
+      miTopDirtyScanline = 0;
+      miBottomDirtyScanline = gcHeight - 1;
+    }
+
+    virtual Scanline& getModifyableScanline(size_t y) {
+      Scanline& res = mpFramebuffer->at(y);
+      miTopDirtyScanline = std::min(miTopDirtyScanline, y);
+      miBottomDirtyScanline = std::max(miBottomDirtyScanline, y);
+      return res;
+    }
+
   protected:
     std::unique_ptr<Framebuffer> mpFramebuffer;
+    mutable size_t miTopDirtyScanline;
+    mutable size_t miBottomDirtyScanline;
   };
 
   class Buzzer {

main.cpp

@@ -25,7 +25,7 @@ int main(int argc, char* argv[]) {
   BuzzerSDL buzzer(440);
   DisplaySDL display(1280, 640);
   TestKeypad keypad;
-  chocochip8::Interpreter chip8(90, display, buzzer, keypad);
+  chocochip8::Interpreter chip8(1000, display, buzzer, keypad);
 
   auto rom = std::vector<char>();
   auto romfile = std::ifstream(argv[1] != NULL ? argv[1] : "/dev/stdin", std::ios_base::in | std::ios_base::binary);
@@ -38,6 +38,8 @@ int main(int argc, char* argv[]) {
 
   SDL_Event event;
   bool done = false;
+  Uint64 start, end;
+  start = SDL_GetTicks64();
   while(!done) {
     while(SDL_PollEvent(&event)) {
       if(event.type == SDL_QUIT) {
@@ -46,7 +48,11 @@ int main(int argc, char* argv[]) {
     }
     chip8.tick();
     display.updateWindow();
-    SDL_Delay(1000.0 / 60);
+    SDL_Delay(1);
+    end = SDL_GetTicks64();
+    if(end - start > 3) 
+      std::cout << "Frame took too long: " << end - start << "\n";
+    start = end;
   }
   
   buzzer.off();