Linux中的SDL2空透明窗口

Question

Here is some sample SDL2 code I tried to run on my Linux computer running Ubuntu 18.04 with KDE Plasma Desktop Environment (I have multiple desktop environments installed in case it is relevant):

#include<iostream>
#include<SDL2/SDL.h>

int main(int argc, char** argv)
{
    if(SDL_Init(SDL_INIT_VIDEO) != 0){
        std::cerr << "SDL_Init() Error: " << SDL_GetError() << std::endl;
        return 1;
    }

    SDL_Window* win = SDL_CreateWindow(
        "Hello world",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        640,480,
        0
    );

    if(win == nullptr){
        std::cerr << "SDL_CreateWindow() Error: " << SDL_GetError() << std::endl;
        return 1;  
    }

    //Create and init the renderer
    SDL_Renderer* ren = SDL_CreateRenderer(win, -1, 0);
    if(ren == nullptr){
        std::cerr << "SDL_CreateRenderer() Error: " << SDL_GetError() << std::endl;
        SDL_DestroyWindow(win);
        return 1;
    }

    //Render something
    SDL_RenderSetLogicalSize(ren,640,480);

    //Set colour of renderer
    SDL_SetRenderDrawColor(ren,255,0,0,255);

    //Clear the screen to the set colour
    SDL_RenderClear(ren);

    //Show all the has been done behind the scenes
    SDL_RenderPresent(ren);

    //Delay so that we can see what is on the screen
    SDL_Delay(5000);

    //Clean Up
    SDL_DestroyRenderer(ren);
    SDL_DestroyWindow(win);
    SDL_Quit();

    return 0;
}

The red window that is supposed to appear appears only once when I run the program for the first time. All subsequent executions produce an empty transparent window with whatever is in the background. The background image drags along with the window.

I have tried SDL_WINDOW_SHOWN flag in SDL_CreateWindow() as well as SDL_RENDER_ACCELERATED flag for SDL_CreateRenderer() .

The only way to produce the red screen again is to reboot the system.

I even compiled and ran this with an IDE (CodeLite) and I still got the same results.

This particular question on SO shows similar problems. But the OP isn't using Linux and the problem isn't exactly the same.

Other posts on this website mention event processing but I haven't gotten that far. If at all it is necessary, I would be grateful for some resources on it as the documentation doesn't explain much.

Update :This program runs fine on another computer running Lubuntu 18.10.

Answer 1

Replace the SDL_Delay() (which blocks all event processing like notifying X11/Wayland & your window manager that your process is still alive) with a loop that calls SDL_PumpEvents() somehow, either directly (like below) or indirectly via SDL_PollEvent() / SDL_WaitEvent() :

const Uint32 startMs = SDL_GetTicks();
while( SDL_GetTicks() - startMs < 5000 )
{
    SDL_PumpEvents();

    //Render something
    SDL_RenderSetLogicalSize(ren,640,480);

    //Set colour of renderer
    SDL_SetRenderDrawColor(ren,255,0,0,255);

    //Clear the screen to the set colour
    SDL_RenderClear(ren);

    //Show all the has been done behind the scenes
    SDL_RenderPresent(ren);
}

All together:

#include <iostream>
#include <SDL2/SDL.h>

int main( int argc, char** argv )
{
    if( SDL_Init( SDL_INIT_VIDEO ) != 0 )
    {
        std::cerr << "SDL_Init() Error: " << SDL_GetError() << std::endl;
        return 1;
    }

    SDL_Window* win = SDL_CreateWindow
        (
        "Hello world",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        640, 480,
        0
        );

    if( win == nullptr )
    {
        std::cerr << "SDL_CreateWindow() Error: " << SDL_GetError() << std::endl;
        return 1;
    }

    //Create and init the renderer
    SDL_Renderer* ren = SDL_CreateRenderer( win, -1, 0 );
    if( ren == nullptr )
    {
        std::cerr << "SDL_CreateRenderer() Error: " << SDL_GetError() << std::endl;
        SDL_DestroyWindow( win );
        return 1;
    }

    const Uint32 startMs = SDL_GetTicks();
    while( SDL_GetTicks() - startMs < 5000 )
    {
        SDL_PumpEvents();

        //Render something
        SDL_RenderSetLogicalSize( ren, 640, 480 );

        //Set colour of renderer
        SDL_SetRenderDrawColor( ren, 255, 0, 0, 255 );

        //Clear the screen to the set colour
        SDL_RenderClear( ren );

        //Show all the has been done behind the scenes
        SDL_RenderPresent( ren );
    }

    //Clean Up
    SDL_DestroyRenderer( ren );
    SDL_DestroyWindow( win );
    SDL_Quit();

    return 0;
}