Introduced two new options in SDL: --autoscale and --keepratio. Autoscale will try to find the best scaling options for the monitor resolution. If keepratio is on, the native NES ratio will not be changed. I set autoscale to a default option, so SDL people please test this.

src/drivers/sdl/config.cpp

@@ -151,6 +151,8 @@ InitConfig()
     config->addOption('f', "fullscreen", "SDL.Fullscreen", 0);
     config->addOption('b', "bpp", "SDL.BitsPerPixel", 32);
     config->addOption("doublebuf", "SDL.DoubleBuffering", 0);
+    config->addOption("autoscale", "SDL.AutoScale", 1);
+    config->addOption("keepaspect", "SDL.KeepAspect", 1);
     config->addOption("xscale", "SDL.XScale", 1.0);
     config->addOption("yscale", "SDL.YScale", 1.0);
     config->addOption("xstretch", "SDL.XStretch", 0);

src/drivers/sdl/sdl-video.cpp

@@ -110,6 +110,18 @@ KillVideo()
 
 static int s_sponge;
 
+inline double GetXScale(int xres)
+{
+    return ((double)xres) / NWIDTH;
+}
+inline double GetYScale(int yres)
+{
+    return ((double)yres) / s_tlines;
+    //if(_integerscalefs)
+    //    scale = (int)scale;
+}
+    
+
 /**
  * Attempts to initialize the graphical video display.  Returns 0 on
  * success, -1 on failure.
@@ -121,6 +133,7 @@ InitVideo(FCEUGI *gi)
     const SDL_VideoInfo *vinf;
     int error, flags = 0;
     int doublebuf, xstretch, ystretch, xres, yres;
+    
 
     FCEUI_printf("Initializing video...");
 
@@ -142,6 +155,7 @@ InitVideo(FCEUGI *gi)
     // check the starting, ending, and total scan lines
     FCEUI_GetCurrentVidSystem(&s_srendline, &s_erendline);
     s_tlines = s_erendline - s_srendline + 1;
+    
 
 
     // check for OpenGL and set the global flags
@@ -169,10 +183,38 @@ InitVideo(FCEUGI *gi)
     if(vinf->hw_available) {
         flags |= SDL_HWSURFACE;
     }
-
+    
+    double auto_xscale = 0;
+    double auto_yscale = 0;
+    int autoscale;
     // check if we are rendering fullscreen
     if(s_fullscreen) {
         flags |= SDL_FULLSCREEN;
+        g_config->getOption("SDL.AutoScale", &autoscale);
+        if(autoscale)
+        {
+            auto_xscale = GetXScale(xres);
+            auto_yscale = GetYScale(yres);
+            double native_ratio = ((double)NWIDTH) / s_tlines;
+            double screen_ratio = ((double)xres) / yres;
+            int keep_aspect;
+            g_config->getOption("SDL.KeepAspect", &keep_aspect);
+            // Try to choose resolution
+        
+            if (screen_ratio < native_ratio)
+            {
+                // The screen is narrower than the original. Maximizing width will not clip
+                auto_xscale = auto_yscale = GetXScale(xres);
+                if (keep_aspect) 
+                    auto_yscale = GetYScale(yres);
+            }
+            else
+            {
+                auto_yscale = auto_xscale = GetYScale(yres);
+                if (keep_aspect) 
+                    auto_xscale = GetXScale(xres);
+            }
+        }
     }
     
     if(noframe) {
@@ -199,9 +241,18 @@ InitVideo(FCEUGI *gi)
     if(s_fullscreen) {
         int desbpp;
         g_config->getOption("SDL.BitsPerPixel", &desbpp);
-
-        g_config->getOption("SDL.XScale", &s_exs);
-        g_config->getOption("SDL.YScale", &s_eys);
+        
+        if (autoscale)
+        {
+            s_exs = auto_xscale;
+            s_eys = auto_yscale;
+        }
+        else
+        {
+            g_config->getOption("SDL.XScale", &s_exs);
+            g_config->getOption("SDL.YScale", &s_eys);
+        }
+        
         g_config->getOption("SDL.SpecialFX", &s_eefx);
 
 #ifdef OPENGL

src/drivers/sdl/sdl.cpp

@@ -58,6 +58,8 @@ char *DriverUsage=
 --nothrottle x	Disables artificial speed throttling, if x is nonzero.\n\
 --frameskip x	Sets # of frames to skip per emulated frame.\n\
 --(x/y)res x, -(x/y) x	Sets horizontal/vertical resolution to x for full screen mode.\n\
+--autoscale x Enables autoscaling if x is nonzero \n\
+--keepratio x Keeps native NES ratio when autoscaling if x is nonzero \n\
 --(x/y)scale x	Multiplies width/height by x (Real numbers >0 with OpenGL, otherwise integers >0).\n\
 --(x/y)stretch x	Stretchess to fill surface on x/y axis (fullscreen, only with OpenGL).\n\
 --bpp x, -b x	Sets bits per pixel for SDL surface(and video mode in fs). 8, 16, 32.\n\