Using SDL and openGL drawing primitives does not work - opengl

Question

I tried to do some basic stuff like a gameloop and drawing with OpenGL by following a tutorial. In that tutorial SDL 1.2 is used but I am using 2.0 so I had to replace some old functions with the new versions.
Unfortunately the lines I want to draw don't show up.
I already tried playing with the numbers so that the line is not outside the window but it did not work.
#include "SDL.h"
#include "SDL_opengl.h"
#include <iostream>
int main(int argc, char* args[])
{
//initialize SDL
SDL_Init(SDL_INIT_EVERYTHING);
//Set openGL memory usage
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BUFFER_SIZE, 32);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 16);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_Window *window;
window=SDL_CreateWindow("MyWindow",
SDL_WINDOWPOS_CENTERED,
SDL_WINDOWPOS_CENTERED,
600,400,
SDL_WINDOW_OPENGL);
SDL_Renderer *renderer;
renderer = SDL_CreateRenderer(window,-1,SDL_RENDERER_ACCELERATED);
SDL_SetRenderDrawColor(renderer,255,255,255,255);//RGB/ALPHA
glShadeModel(GL_SMOOTH);
//2D rendering
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//Disable depth checking
glDisable(GL_DEPTH_TEST);
std::cout<<"OpenGL is running\n";
std::cout<<"Main loop has started\n";
//Handle GameLoop
bool isRunning=true;
//Handel events
SDL_Event event;
//main gameloop
while(isRunning)
{
//Events
while(SDL_PollEvent(&event))
{
if(event.type==SDL_QUIT)
isRunning=false;
//if esc button is released
if(event.type==SDL_KEYUP&&event.key.keysym.sym==SDLK_ESCAPE)
isRunning=false;
if(event.type==SDL_KEYUP&&event.key.keysym.sym==SDLK_r)
SDL_SetRenderDrawColor(renderer,255,0,0,255);
//Logic for certain events
}
//Logic
//Rendering
SDL_RenderClear(renderer);
SDL_RenderPresent(renderer);
glPushMatrix();
glOrtho(0,600,400,0,-1,1); //Set matrix
glBegin(GL_LINES);//start drawing
glColor4ub(255,0,0,255);
glVertex2i(0,0);
glVertex2i(600,400);
glEnd();// end drawing
glPopMatrix();
SDL_GL_SwapWindow(window);
}
SDL_Quit();
return 0;
}

Answer 1

Don't mix SDL_Renderer code and OpenGL. There's no provision (yet, maybe ever) in SDL2 for resetting Renderer GL state (if it's using the GL backend) that you trample with raw GL code.

Answer 2

First, don't mix OpenGL and the SDL_Renderer stuff. Then you actually need to create an OpenGL context for your window before rendering can work. Use this after your SDL_CreateWindow call:
SDL_GLContext glcontext = SDL_GL_CreateContext(window);
At the end you need to free the context:
SDL_GL_DeleteContext(glcontext);