c++ SDL2在不含X的Raspberry Pi上？

我希望开发一些使用SDL2在7”RPi触摸屏上显示图形的代码，但我不想安装完整的桌面操作系统。我已经安装了Raspbian Buster Lite。一些简单的测试代码在我尝试运行时出现错误：

user@rpi4:~/01_hello_SDL $ ./hw
Window could not be created! SDL_Error: Could not initialize EGL
user@rpi4:~/01_hello_SDL $ sudo ./hw
error: XDG_RUNTIME_DIR not set in the environment.
Window could not be created! SDL_Error: Could not initialize EGL

我正在尝试创建窗口

SDL_CreateWindow( "SDL Tutorial", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL )

我找到了一个post，它引用了有关如何在不使用X的情况下构建SDL2的说明，但我希望有人能教我更多关于SDL如何在各种环境中找到显示器的信息，以及它是否有可能做我想做的事情。
几年前，我曾使用SDL 1.2在Beaglebone Black上运行Debian版本来制作全屏图形，但我似乎丢失了安装，也不记得它是如何设置的。我隐约记得fbdev的一些问题，它是非加速图形，但当时这并不重要（虽然我现在想得到加速图形，但这并不重要）。
示例代码：

/*This source code copyrighted by Lazy Foo' Productions (2004-2019)
and may not be redistributed without written permission.*/

//Using SDL and standard IO
#include <SDL.h>
#include <stdio.h>

//Screen dimension constants
const int SCREEN_WIDTH = 800;
const int SCREEN_HEIGHT = 480;

int main( int argc, char* args[] )
{
    //The window we'll be rendering to
    SDL_Window* window = NULL;
    
    //The surface contained by the window
    SDL_Surface* screenSurface = NULL;

    //Initialize SDL
    if( SDL_Init( SDL_INIT_VIDEO ) < 0 )
    {
        printf( "SDL could not initialize! SDL_Error: %s\n", SDL_GetError() );
    }
    else
    {
        //Create window
        window = SDL_CreateWindow( "SDL Tutorial", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_FULLSCREEN | SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL );
        if( window == NULL )
        {
            printf( "Window could not be created! SDL_Error: %s\n", SDL_GetError() );
        }
        else
        {
            //Get window surface
            screenSurface = SDL_GetWindowSurface( window );

            //Fill the surface white
            SDL_FillRect( screenSurface, NULL, SDL_MapRGB( screenSurface->format, 0xFF, 0xFF, 0xFF ) );
            
            //Update the surface
            SDL_UpdateWindowSurface( window );

            //Wait two seconds
            SDL_Delay( 2000 );
        }
    }

    //Destroy window
    SDL_DestroyWindow( window );

    //Quit SDL subsystems
    SDL_Quit();

    return 0;
}

好了，让它在我的树莓派3与2019-07-10-raspbian-buster-lite.img工作，都与默认的Broadcom blobs和KMS/DRM后端：
1.安装SDL 2建置相依性：

# install everything Debian uses to build SDL
sudo apt build-dep libsdl2

# needed for the KMSDRM backend:
sudo apt install libdrm-dev libgbm-dev

1.从Git获取最新的稳定SDL source tarball或标签（release-2.0.10），并将其提取到类似~/sdl-src的位置
1.运行SDL的configure脚本：

cd ~/sdl-src
./configure --enable-video-kmsdrm

下面是我的configure总结：

SDL2 Configure Summary:
Building Shared Libraries
Building Static Libraries
Enabled modules : atomic audio video render events joystick haptic sensor power filesystem threads timers file loadso cpuinfo assembly
Assembly Math   :
Audio drivers   : disk dummy oss alsa(dynamic) pulse(dynamic) sndio(dynamic)
Video drivers   : dummy rpi x11(dynamic) kmsdrm(dynamic) opengl opengl_es1 opengl_es2 vulkan wayland(dynamic)
X11 libraries   : xcursor xdbe xinerama xinput2 xinput2_multitouch xrandr xscrnsaver xshape xvidmode
Input drivers   : linuxev linuxkd
Using libsamplerate : YES
Using libudev       : YES
Using dbus          : YES
Using ime           : YES
Using ibus          : YES
Using fcitx         : YES

请注意Video drivers列表中的rpi和kmsdrm(dynamic)条目：

Video drivers   : dummy rpi x11(dynamic) kmsdrm(dynamic) opengl opengl_es1 opengl_es2 vulkan wayland(dynamic)
                        ^^^              ^^^^^^^^^^^^^^^

1.建置并安装SDL;在我的Rpi 3上花了大约4.5分钟：

make -j4 && sudo make install

1.构建测试程序：

g++ main.cpp `pkg-config --cflags --libs sdl2`

1.（可选）如果要使用KMSDRM后端而不是默认的OpenGL ES blob，请启用“Full KMS”驱动程序：

$ sudo raspi-config
select '7 Advanced Options'
select 'A7 GL Driver'
select 'G3 GL (Full KMS)'
reboot

1.运行测试程序：

$ ./a.out 
Testing video drivers...
The path /dev/dri/ cannot be opened or is not available
The path /dev/dri/ cannot be opened or is not available
SDL_VIDEODRIVER available: x11 wayland KMSDRM RPI dummy
SDL_VIDEODRIVER usable   : RPI
The path /dev/dri/ cannot be opened or is not available
The path /dev/dri/ cannot be opened or is not available
SDL_VIDEODRIVER selected : RPI
SDL_RENDER_DRIVER available: opengl opengles2 opengles software
SDL_RENDER_DRIVER selected : opengles2

可以使用环境变量覆盖默认的视频/渲染驱动程序选择：

SDL_VIDEODRIVER=KMSDRM SDL_RENDER_DRIVER=software ./a.out

我不得不用envars来帮助SDL加载KMSDRM后端：

# no envvars, fails:
$ ./a.out 
Testing video drivers...
SDL_VIDEODRIVER available: x11 wayland KMSDRM RPI dummy
SDL_VIDEODRIVER usable   : KMSDRM
SDL_VIDEODRIVER selected : KMSDRM
SDL_CreateWindow(): Could not initialize OpenGL / GLES library

# with envvars, succeeds:
$ SDL_VIDEO_EGL_DRIVER=libEGL.so SDL_VIDEO_GL_DRIVER=libGLESv2.so ./a.out
Testing video drivers...
SDL_VIDEODRIVER available: x11 wayland KMSDRM RPI dummy
SDL_VIDEODRIVER usable   : KMSDRM
SDL_VIDEODRIVER selected : KMSDRM
SDL_RENDER_DRIVER available: opengl opengles2 opengles software
SDL_RENDER_DRIVER selected : opengl

下面是我一直在使用的测试程序：

// g++ main.cpp `pkg-config --cflags --libs sdl2`
#include <SDL.h>
#include <iostream>
#include <vector>

int main( int argc, char** argv )
{
    SDL_Init( 0 );

    std::cout << "Testing video drivers..." << '\n';
    std::vector< bool > drivers( SDL_GetNumVideoDrivers() );
    for( int i = 0; i < drivers.size(); ++i )
    {
        drivers[ i ] = ( 0 == SDL_VideoInit( SDL_GetVideoDriver( i ) ) );
        SDL_VideoQuit();
    }

    std::cout << "SDL_VIDEODRIVER available:";
    for( int i = 0; i < drivers.size(); ++i )
    {
        std::cout << " " << SDL_GetVideoDriver( i );
    }
    std::cout << '\n';

    std::cout << "SDL_VIDEODRIVER usable   :";
    for( int i = 0; i < drivers.size(); ++i )
    {
        if( !drivers[ i ] ) continue;
        std::cout << " " << SDL_GetVideoDriver( i );
    }
    std::cout << '\n';

    if( SDL_Init( SDL_INIT_EVERYTHING ) < 0 )
    {
        std::cerr << "SDL_Init(): " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }
    std::cout << "SDL_VIDEODRIVER selected : " << SDL_GetCurrentVideoDriver() << '\n';

    SDL_Window* window = SDL_CreateWindow
        (
        "SDL2",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
        640, 480,
        SDL_WINDOW_SHOWN
        );
    if( nullptr == window )
    {
        std::cerr << "SDL_CreateWindow(): " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }

    std::cout << "SDL_RENDER_DRIVER available:";
    for( int i = 0; i < SDL_GetNumRenderDrivers(); ++i )
    {
        SDL_RendererInfo info;
        SDL_GetRenderDriverInfo( i, &info );
        std::cout << " " << info.name;
    }
    std::cout << '\n';

    SDL_Renderer* renderer = SDL_CreateRenderer( window, -1, SDL_RENDERER_ACCELERATED );
    if( nullptr == renderer )
    {
        std::cerr << "SDL_CreateRenderer(): " << SDL_GetError() << '\n';
        return EXIT_FAILURE;
    }
    SDL_RendererInfo info;
    SDL_GetRendererInfo( renderer, &info );
    std::cout << "SDL_RENDER_DRIVER selected : " << info.name << '\n';

    bool running = true;
    unsigned char i = 0;
    while( running )
    {
        SDL_Event ev;
        while( SDL_PollEvent( &ev ) )
        {
            if( ( ev.type == SDL_QUIT ) ||
                ( ev.type == SDL_KEYDOWN && ev.key.keysym.sym == SDLK_ESCAPE ) )
            {
                running = false;
            }
        }

        SDL_SetRenderDrawColor( renderer, i, i, i, SDL_ALPHA_OPAQUE );
        SDL_RenderClear( renderer );
        SDL_RenderPresent( renderer );
        i++;
    }

    SDL_DestroyRenderer( renderer );
    SDL_DestroyWindow( window );
    SDL_Quit();
    return 0;
}

c++ SDL2在不含X的Raspberry Pi上？

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