[mrhyperpenguin]

>Can't it be done just with a shader ?

Yep. If you look at the "How?" and "Performance vs Quality" sections, you can see that you need to render the scene six times to get the surrounding environment, so all you would just need to do is make a shader for the projection[0]. (Rendering the scene six times is pretty common in graphics in order to generate light probes for dynamic lighting and global illumination.)

I suspect with a modern GPU implementation (Vulkan) and some other minor optimizations, this could run pretty easily in real-time.

[0] - http://tksharpless.net/vedutismo/Pannini/panini.pdf