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);
 }