High DPI Support

[thejustinwalsh]

Requesting High DPI support.

Looks like all the work in the hashlink SDL plugin is already done to support passing the SDL_WINDOW_ALLOW_HIGHDPI flag at window creation time. Not sure what the required support would look like when using DirectX.

Would love to see the feature abstracted through heaps and any advice on best implementation is welcome as I can submit a PR to support it.

[thejustinwalsh]

Notes:
On iOS you must do two things to get a high DPI rendering context.

1. NSHighResolutionCapable = YES in Info.plist
2. Pass in the SDL_WINDOW_ALLOW_HIGHDPI flag when creating the window by changing

   heaps/hxd/Window.hl.hx

   Line 42 in 170bb54

   window = new sdl.Window(title, width, height);

   
   to (formatted to not wrap)

window = new sdl.Window(title, width, height, 
  sdl.Window.SDL_WINDOWPOS_CENTERED, sdl.Window.SDL_WINDOWPOS_CENTERED,
  sdl.Window.SDL_WINDOW_SHOWN | 
  sdl.Window.SDL_WINDOW_RESIZABLE | 
  sdl.Window.SDL_WINDOW_ALLOW_HIGHDPI
);

From what I can tell if you Omit the NSHighResolutionCapable flag or set it to NO in your Info.plist the additional SDL_WINDOW_ALLOW_HIGHDPI seems to have no effect. Although I am not certain at this time of how this flag affects all platforms. Also worth noting, the calls to hxd.System.width and hxd.System.height will return the size of the device screen in points and you would need to know the DPI of your screen to apply the appropriate scale from points to pixels when running in High DPI mode.

[ncannasse]

In order to support this on SDL first, we would need to be able to query from SDL for the screen DPI, as described here https://nlguillemot.wordpress.com/2016/12/11/high-dpi-rendering/

This requires adding some new primitives to Hashlink/SDL support here:
https://github.com/HaxeFoundation/hashlink/tree/master/libs/sdl
and Haxe wrapper
https://github.com/HaxeFoundation/hashlink/tree/master/libs/sdl/sdl

Once this is done, I guess we could enable it with a define: -D highdpi and add the #if highdpi sections in the SDL Window support.

[AustinEast]

Looks like this issue can be fixed if you use the latest version of SDL (beyond the latest release on homebrew). This is a quick fix that has worked for me and others:

1. Grab a newer version of SDL and replace your current installation with it (this mirror works fine: https://github.com/spurious/SDL-mirror)
2. Rebuild hashlink (with the normal make -> make install instructions)
3. Rebuild your project and see the fixed display!