Flag::Stereo in an ordinary window, on a Linux desktop that mixes 3D and 2D windows

[danielcamposramos]

Hi Vladimír, and thank you for Magnum.

Sparky Stereo OS is a Linux desktop where 3D windows and 2D windows share the same screen: https://github.com/Sparky-OS/sparky-stereo-os
It is a stereo 3D edition of SparkyLinux, on Debian testing with KDE Plasma, and I make it.
A stereo program runs in an ordinary window next to a browser or an editor, and the compositor shows that window in 3D and the rest of the desktop in 2D.
The same desktop drives a 3D TV over HDMI frame packing, any monitor as red and cyan anaglyph, and a phone in a cheap viewer as the PC's display (photos: Sparky-OS/flat-headset#5).
It runs on ordinary GPUs through Mesa: no professional card and no full screen mode needed.

A Magnum program needs nothing new for this.
GLConfiguration::Flag::Stereo, with one eye drawn into BackLeft and the other into BackRight, is the whole interface, and Magnum has had it for years.

What I ran: a GlfwApplication with three flat cubes and two parallel cameras with off-axis frusta, on native Wayland, with Corrade and Magnum master built the way your package/debian builds them, on an AMD GPU and on llvmpipe.
In both eyes each cube landed where the projection predicts: all 24 silhouette edges within one pixel, and no vertical disparity.
Without the flag, the same program opens an ordinary 2D window.

What had to change was below Magnum.
On Wayland, OpenGL goes through EGL, which has no stereo config, so GLFW refused the flag.
Our forks of GLFW (https://github.com/Sparky-OS/glfw) and Mesa (https://gitlab.freedesktop.org/danielcamposramos/mesa) now give a stereo window two views through EGL_EXT_multiview_window, and the compositor takes it from there.

Two limits today.
Sdl2Application has no stereo flag, so an SDL program stays 2D unless it asks SDL for SDL_GL_STEREO itself.
The X11 route (GLX on Xwayland) still squeezes the picture; we are fixing it now, so this note is about Wayland only.

Thank you for an engine where stereo was already there to use.
I wrote this with help from Claude (Anthropic), and I checked the results myself.

[mosra]

Flag::Stereo is added for Sdl2Application in b639ce0.