C ++半透明窗口SDL

我希望有一个半透明的SDL背景（与子表面或图像无关），而不是具有黑色背景，实际上是透明的，但我绘制的其他东西却不是。 我目前的代码是Code :: Blocks的SDL项目的一个稍微修改的副本，类似于各种应用程序具有圆angular边界或除矩形以外的奇怪形状。

#ifdef __cplusplus #include <cstdlib> #else #include <stdlib.h> #endif #ifdef __APPLE__ #include <SDL/SDL.h> #else #include <SDL.h> #endif int main ( int argc, char** argv ) { putenv("SDL_VIDEO_WINDOW_POS"); putenv("SDL_VIDEO_CENTERED=1"); // initialize SDL video if ( SDL_Init( SDL_INIT_VIDEO ) < 0 ) { printf( "Unable to init SDL: %s\n", SDL_GetError() ); return 1; } // make sure SDL cleans up before exit atexit(SDL_Quit); // create a new window SDL_Surface* screen = SDL_SetVideoMode(640, 480, 16, SDL_HWSURFACE|SDL_DOUBLEBUF|SDL_NOFRAME); if ( !screen ) { printf("Unable to set 640x480 video: %s\n", SDL_GetError()); return 1; } // load an image SDL_Surface* bmp = SDL_LoadBMP("cb.bmp"); if (!bmp) { printf("Unable to load bitmap: %s\n", SDL_GetError()); return 1; } // centre the bitmap on screen SDL_Rect dstrect; dstrect.x = (screen->w - bmp->w) / 2; dstrect.y = (screen->h - bmp->h) / 2; // program main loop bool done = false; while (!done) { // message processing loop SDL_Event event; while (SDL_PollEvent(&event)) { // check for messages switch (event.type) { // exit if the window is closed case SDL_QUIT: done = true; break; // check for keypresses case SDL_KEYDOWN: { // exit if ESCAPE is pressed if (event.key.keysym.sym == SDLK_ESCAPE) done = true; break; } } // end switch } // end of message processing // DRAWING STARTS HERE // clear screen SDL_FillRect(screen, 0, SDL_MapRGB(screen->format, 0, 0, 0)); // draw bitmap SDL_BlitSurface(bmp, 0, screen, &dstrect); // DRAWING ENDS HERE // finally, update the screen :) SDL_Flip(screen); } // end main loop // free loaded bitmap SDL_FreeSurface(bmp); // all is well ;) printf("Exited cleanly\n"); return 0; }