static int engine_init_display(struct engine* engine) {
     ...

     // enable draw lines and colors
     glEnableClientState(GL_VERTEX_ARRAY);// glEnableClientState(GL_VERTEX_ARRAY) allow to use glVertexPointer
     glEnableClientState(GL_COLOR_ARRAY); // glEnableClientState(GL_COLOR_ARRAY) allow to use glColorPointer

     return 0;
}

static void engine_draw_frame(struct engine* engine) {
     if (engine->display == NULL) {
          // No display.
          return;
     }

     // set color (white)
     glClearColor(255, 255 ,255, 0);

     // fill device with color
     glClear(GL_COLOR_BUFFER_BIT);

     // set line points and line colors
     static GLfloat colors[] = {
                              1.0f, 0.0f, 0.0f, 1.0f, // red color
                              0.0f, 1.0f, 0.0f, 1.0f, // green color
                              0.0f, 0.0f, 1.0f, 1.0f // blue color
     };
     static GLfloat points[] = {
                         0, 0, 0,
                         0.9, 0.9, 0,
                         0.9, 0, 0
     };

     // draw line points and line color
     glLineWidth(10);
     glVertexPointer(3 /*columns count in variable points*/, GL_FLOAT, 0, points);
     glColorPointer(4 /*columns count in variable colors*/ , GL_FLOAT, 0, colors);
     glDrawArrays(GL_LINE_STRIP, 0 /*first index to draw*/, 3 /*count of points to draw*/);

     // switch gl buffers
     eglSwapBuffers(engine->display, engine->surface);
}