Wsrtjtyk

Let's say I run a Linux and I have no desktop environment installed. I boot up my system and all I have is my shell.

Is it possible to compile a program that uses the OpenGL libraries or directly uses the GPU driver to draw to the screen?

As far as I could understand I would always need some kind of desktop environment that would provide me a window that I can draw on. To keep it
simple let's say I just want to draw a simple 2d shape like a triangle in the middle of the screen for example.

And if that's possible how can I do it and where can I read more about the topic? If I am able to draw directly over my terminal does this mean that I would be able to run my app on a system that has a desktop environment and still be able to see my triangle?

Is it possible to compile a program that uses the OpenGL libraries or directly uses the GPU driver to draw to the screen?

Yes. With the EGL API this has been formalized and works most well with NVidia GPUs and their proprietary drivers. NVidia has it described on their dev blog here https://devblogs.nvidia.com/egl-eye-opengl-visualization-without-x-server/

Essentially the steps are:

Create a OpenGL context for a PBuffer

#include <EGL/egl.h>



  static const EGLint configAttribs = {

          EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,

          EGL_BLUE_SIZE, 8,

          EGL_GREEN_SIZE, 8,

          EGL_RED_SIZE, 8,

          EGL_DEPTH_SIZE, 8,

          EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,

          EGL_NONE

  };    



  static const int pbufferWidth = 9;

  static const int pbufferHeight = 9;



  static const EGLint pbufferAttribs = {

        EGL_WIDTH, pbufferWidth,

        EGL_HEIGHT, pbufferHeight,

        EGL_NONE,

  };



int main(int argc, char *argv)

{

  // 1. Initialize EGL

  EGLDisplay eglDpy = eglGetDisplay(EGL_DEFAULT_DISPLAY);



  EGLint major, minor;



  eglInitialize(eglDpy, &major, &minor);



  // 2. Select an appropriate configuration

  EGLint numConfigs;

  EGLConfig eglCfg;



  eglChooseConfig(eglDpy, configAttribs, &eglCfg, 1, &numConfigs);



  // 3. Create a surface

  EGLSurface eglSurf = eglCreatePbufferSurface(eglDpy, eglCfg, 

                                               pbufferAttribs);



  // 4. Bind the API

  eglBindAPI(EGL_OPENGL_API);



  // 5. Create a context and make it current

  EGLContext eglCtx = eglCreateContext(eglDpy, eglCfg, EGL_NO_CONTEXT, 

                                       NULL);



  eglMakeCurrent(eglDpy, eglSurf, eglSurf, eglCtx);



  // from now on use your OpenGL context



  // 6. Terminate EGL when finished

  eglTerminate(eglDpy);

  return 0;

}

and then go about the rest as per usual. Or you can even ditch the PBuffer completely and just use OpenGL manages resources, i.e. render to framebuffer objects. For that end you can omit creating the surface and just make the context current.

Here's an example for using EGL without display, no EGL surface, with OpenGL managed framebuffer.

#include <GL/glew.h>

#include <GL/glut.h>

#include <EGL/egl.h>



#include <unistd.h>

#include <stdlib.h>

#include <assert.h>

#include <sys/mman.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>



#include <math.h>

#include <stdio.h>



using namespace std;



namespace render

{

    int width, height;

    float aspect;



    void init();

    void display();



    int const fbo_width = 512;

    int const fbo_height = 512;



    GLuint fb, color, depth;



    void *dumpbuf;

    int dumpbuf_fd;

};



static const EGLint configAttribs = {

    EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,

    EGL_BLUE_SIZE, 8,

    EGL_GREEN_SIZE, 8,

    EGL_RED_SIZE, 8,

    EGL_DEPTH_SIZE, 8,

    EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,

    EGL_NONE

};   



int main(int argc, char *argv)

{

    // 1. Initialize EGL

    EGLDisplay eglDpy = eglGetDisplay(EGL_DEFAULT_DISPLAY);



    EGLint major, minor;    

    eglInitialize(eglDpy, &major, &minor);



    // 2. Select an appropriate configuration

    EGLint numConfigs;

    EGLConfig eglCfg;



    eglChooseConfig(eglDpy, configAttribs, &eglCfg, 1, &numConfigs);



    // 3. Bind the API

    eglBindAPI(EGL_OPENGL_API);



    // 3. Create a context and make it current

    EGLContext eglCtx = eglCreateContext(eglDpy, eglCfg, EGL_NO_CONTEXT, 

    NULL);



    eglMakeCurrent(eglDpy, EGL_NO_SURFACE, EGL_NO_SURFACE, eglCtx);



    glewInit();

    // from now on use your OpenGL context

    render::init();

    render::display();



    // 4. Terminate EGL when finished

    eglTerminate(eglDpy);

    return 0;

}



void CHECK_FRAMEBUFFER_STATUS()

{                                                         

    GLenum status;

    status = glCheckFramebufferStatus(GL_DRAW_FRAMEBUFFER); 

    switch(status) {

    case GL_FRAMEBUFFER_COMPLETE:

        break;



    case GL_FRAMEBUFFER_UNSUPPORTED:

    /* choose different formats */

        break;



    default:

        /* programming error; will fail on all hardware */

        throw "Framebuffer Error";

    }

}



namespace render

{

    float const light_dir={1,1,1,0};

    float const light_color={1,0.95,0.9,1};



    void init()

    {

        glGenFramebuffers(1, &fb);

        glGenTextures(1, &color);

        glGenRenderbuffers(1, &depth);



        glBindFramebuffer(GL_FRAMEBUFFER, fb);



        glBindTexture(GL_TEXTURE_2D, color);

        glTexImage2D(   GL_TEXTURE_2D, 

                0, 

                GL_RGB8, 

                fbo_width, fbo_height,

                0, 

                GL_RGBA, 

                GL_UNSIGNED_BYTE, 

                NULL);



        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color, 0);



        glBindRenderbuffer(GL_RENDERBUFFER, depth);

        glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, fbo_width, fbo_height);

        glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth);



        GLint red_bits, green_bits, blue_bits, alpha_bits;



        glGetIntegerv(GL_RED_BITS,   &red_bits);

        glGetIntegerv(GL_GREEN_BITS, &green_bits);

        glGetIntegerv(GL_BLUE_BITS,  &blue_bits);

        glGetIntegerv(GL_ALPHA_BITS, &alpha_bits);



        fprintf(stderr, "FBO format R%dG%dB%dA%dn",

            (int)red_bits,

            (int)green_bits,

            (int)blue_bits,

            (int)alpha_bits );



        CHECK_FRAMEBUFFER_STATUS();



        dumpbuf_fd = open("/tmp/fbodump.rgb", O_CREAT|O_SYNC|O_RDWR, S_IRUSR|S_IWUSR);

        assert(-1 != dumpbuf_fd);

        dumpbuf = malloc(fbo_width*fbo_height*3);

        assert(dumpbuf);

    }



    void render()

    {

        static float a=0, b=0, c=0;



        glBindTexture(GL_TEXTURE_2D, 0);

        glEnable(GL_TEXTURE_2D);

        glBindFramebuffer(GL_FRAMEBUFFER, fb);



        glViewport(0,0,fbo_width, fbo_height);



        glClearColor(0,0,0,0);

        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );



        glMatrixMode(GL_PROJECTION);

        glLoadIdentity();

        glOrtho(-1, 1, -1, 1, -1, 1);



        glMatrixMode(GL_MODELVIEW);

        glLoadIdentity();



        glBegin(GL_TRIANGLES);

        glColor3f(1,0,0);

        glVertex3f(1,0,0);



        glColor3f(0,1,0);

        glVertex3f(0,1,0);



        glColor3f(0,0,1);

        glVertex3f(0,0,1);

        glEnd();



        glReadBuffer(GL_COLOR_ATTACHMENT0);

       glReadPixels(0,0,fbo_width,fbo_height,GL_RGB,GL_UNSIGNED_BYTE,dumpbuf);

        lseek(dumpbuf_fd, SEEK_SET, 0);

        write(dumpbuf_fd, dumpbuf, fbo_width*fbo_height*3);

    }

}