Blit: Fix scaled output handling

Fixes #594

SourceS/sdl2_to_1_2_backports.h

@@ -215,6 +215,21 @@ SDL_FreePalette(SDL_Palette *palette)
 	SDL_free(palette);
 }
 
+inline bool SDL_HasColorKey(SDL_Surface *surface)
+{
+	return (surface->flags & (SDL_SRCCOLORKEY | SDL_RLEACCELOK)) != 0;
+}
+
+inline int SDL_GetColorKey(SDL_Surface *surface, Uint32 *colorkey)
+{
+	if (!SDL_HasColorKey(surface)) {
+		SDL_SetError("Surface doesn't have a colorkey");
+		return -1;
+	}
+	*colorkey = surface->format->colorkey;
+	return 0;
+}
+
 //= Pixel formats
 
 #define SDL_PIXELFORMAT_INDEX8 1
@@ -259,7 +274,8 @@ inline void SDLBackport_PixelformatToMask(int pixelformat, Uint32 *flags, Uint32
  */
 inline bool SDLBackport_PixelFormatFormatEq(const SDL_PixelFormat *a, const SDL_PixelFormat *b)
 {
-	return a->BitsPerPixel == b->BitsPerPixel && (a->palette != nullptr) == (b->palette != nullptr);
+	return a->BitsPerPixel == b->BitsPerPixel && (a->palette != nullptr) == (b->palette != nullptr)
+	    && a->Rmask == b->Rmask && a->Gmask == b->Gmask && a->Bmask == b->Bmask;
 }
 
 /**

SourceS/sdl_compat.h

@@ -14,6 +14,7 @@
 #define SDLC_KEYSTATE_LEFT SDL_SCANCODE_LEFT
 #define SDLC_KEYSTATE_RIGHT SDL_SCANCODE_RIGHT
 #else
+#include "sdl2_to_1_2_backports.h"
 #define SDLC_KEYSTATE_LEFTCTRL SDLK_LCTRL
 #define SDLC_KEYSTATE_RIGHTCTRL SDLK_RCTRL
 #define SDLC_KEYSTATE_LEFTSHIFT SDLK_LSHIFT
@@ -26,6 +27,14 @@
 #define SDLC_KEYSTATE_RIGHT SDLK_RIGHT
 #endif
 
+inline bool SDLC_PixelFormatEq(const SDL_PixelFormat *a, const SDL_PixelFormat *b) {
+#ifndef USE_SDL1
+	return a->format == b->format;
+#else
+	return SDLBackport_PixelFormatFormatEq(a, b);
+#endif
+}
+
 inline const Uint8 *SDLC_GetKeyState()
 {
 #ifndef USE_SDL1

SourceX/dx.cpp

@@ -173,40 +173,59 @@ void CreatePalette()
 
 void BltFast(SDL_Rect *src_rect, SDL_Rect *dst_rect)
 {
-	if (OutputRequiresScaling()) {
-		ScaleOutputRect(dst_rect);
-		// Convert from 8-bit to 32-bit
-		SDL_Surface *tmp = SDL_ConvertSurface(pal_surface, GetOutputSurface()->format, 0);
-		if (SDL_BlitScaled(tmp, src_rect, GetOutputSurface(), dst_rect) <= -1) {
-			SDL_FreeSurface(tmp);
-			ErrSdl();
-		}
-		SDL_FreeSurface(tmp);
-	} else {
-		// Convert from 8-bit to 32-bit
-		if (SDL_BlitSurface(pal_surface, src_rect, GetOutputSurface(), dst_rect) <= -1) {
-			ErrSdl();
-		}
-	}
+	Blit(pal_surface, src_rect, dst_rect);
 }
 
 void Blit(SDL_Surface *src, SDL_Rect *src_rect, SDL_Rect *dst_rect)
 {
-	if (OutputRequiresScaling()) {
-		ScaleOutputRect(dst_rect);
-		// Convert from 8-bit to 32-bit
-		SDL_Surface *tmp = SDL_ConvertSurface(src, GetOutputSurface()->format, 0);
-		if (SDL_BlitScaled(tmp, src_rect, GetOutputSurface(), dst_rect) <= -1) {
-			SDL_FreeSurface(tmp);
+	SDL_Surface *dst = GetOutputSurface();
+	if (!OutputRequiresScaling()) {
+		if (SDL_BlitSurface(src, src_rect, dst, dst_rect) < 0)
 			ErrSdl();
-		}
-		SDL_FreeSurface(tmp);
-	} else {
-		// Convert from 8-bit to 32-bit
-		if (SDL_BlitSurface(src, src_rect, GetOutputSurface(), dst_rect) <= -1) {
+		return;
+	}
+
+	ScaleOutputRect(dst_rect);
+
+	// Same pixel format: We can call BlitScaled directly.
+	if (SDLC_PixelFormatEq(src->format, dst->format)) {
+		if (SDL_BlitScaled(src, src_rect, dst, dst_rect) < 0)
+			ErrSdl();
+		return;
+	}
+
+	// If the surface has a color key, we must stretch first and can then call BlitSurface.
+	if (SDL_HasColorKey(src)) {
+		SDL_Surface *stretched = SDL_CreateRGBSurface(SDL_SWSURFACE, dst_rect->w, dst_rect->h, src->format->BitsPerPixel,
+		    src->format->Rmask, src->format->Gmask, src->format->BitsPerPixel, src->format->Amask);
+		Uint32 colorkey;
+		SDL_GetColorKey(src, &colorkey);
+		SDLC_SetColorKey(stretched, colorkey);
+#ifndef USE_SDL1
+		if (SDL_ISPIXELFORMAT_INDEXED(src->format->format))
+			SDL_SetSurfacePalette(stretched, src->format->palette);
+#else
+		if (src->format->palette != nullptr)
+			SDL_SetPalette(stretched, SDL_LOGPAL, src->format->palette->colors, 0, src->format->palette->ncolors);
+#endif
+		SDL_Rect stretched_rect = { 0, 0, dst_rect->w, dst_rect->h };
+		if (SDL_SoftStretch(src, src_rect, stretched, &stretched_rect) < 0
+		    || SDL_BlitSurface(stretched, &stretched_rect, dst, dst_rect) < 0) {
+			SDL_FreeSurface(stretched);
 			ErrSdl();
 		}
+		SDL_FreeSurface(stretched);
+		return;
+	}
+
+	// A surface with a non-output pixel format but without a color key needs scaling.
+	// We can convert the format and then call BlitScaled.
+	SDL_Surface *converted = SDL_ConvertSurface(src, dst->format, 0);
+	if (SDL_BlitScaled(converted, src_rect, dst, dst_rect) < 0) {
+		SDL_FreeSurface(converted);
+		ErrSdl();
 	}
+	SDL_FreeSurface(converted);
 }
 
 /**