SDL Detect High DPI/Retina Display

I'm wondering what the best way to detect a high DPI display is. Currently I'm trying to use

, however this has only returned errors on the two different computers I tested with (MacBook Pro running OS X 10.11.5 and iMac running macOS 10.12 Beta (16A238m)). For reference, my code is bellow.

float diagDPI = -1;

float horiDPI = -1;

float vertDPI = -1;



int dpiReturn = SDL_GetDisplayDPI (0, &diagDPI, &horiDPI, &vertDPI);

std::cout << "GetDisplayDPI() returned " << dpiReturn << std::endl;



if (dpiReturn != 0)

{



    std::cout << "Error: " << SDL_GetError () << std::endl;

}



std::cout << "DDPI: " << diagDPI << std::endl << "HDPI: " << horiDPI << std::endl << "VDPI: " << vertDPI << std::endl;

Unfortunately, this is only giving me something like this:

/* Output */

GetDisplayDPI() returned -1

Error: 

DDPI: -1

HDPI: -1

VDPI: -1

Not Retina

I also tried comparing the OpenGL drawable size with the SDL window size, but is returning 0s, too. That code is bellow, followed by the output.

int gl_w;

int gl_h;



SDL_GL_GetDrawableSize (window, &gl_w, &gl_h);

std::cout << "GL_W: " << gl_w << std::endl << "GL_H: " << gl_h << std::endl;





int sdl_w;

int sdl_h;



SDL_GetWindowSize (window, &sdl_w, &sdl_h);

std::cout << "SDL_W: " << sdl_w << std::endl << "SDL_H: " << sdl_h << std::endl;



/* Output */

GL_W: 1280

GL_H: 720

SDL_W: 0

SDL_H: 0

It's entirely possible that I'm doing something wrong here, or making these calls in the wrong place, but I think more likely is that I'm on the wrong track entirely. There's a hint to disallow high-dpi canvases, so there's probably a simple bool somewhere, or something that I'm missing. I have certainly looked through the wiki, and checked Google, but I can't really find any help for this. Any suggestions or feedback are welcome!

Thank you for your time!