RDP has capability[1] to send draw commands directly.
From memory it has had this since the early days. At least I used RDP against NT4 servers over a single ISDN link and it was very smooth. I can't believe it could have done that by streaming bitmaps ala VNC.
[1]: https://docs.microsoft.com/en-us/openspecs/windows_protocols...
Found an RDesktop implementation from 2001[1] which implements the draw commands. So yeah, old stuff.
Good toolkits like WPF and UWP implement their own drawing by utilizing Direct3D. These two are using DX 9.0c and D3D 11.0 respectively, and both are supported in RDP protocol.
If you connect to a remote PC and run dxdiag.exe there, you’ll see 3D acceleration is supported, you’ll also see the GPU driver got replaced with completely different one, it’s no longer Intel, Nvidia or AMD, but the one from MS.
This has interesting consequences, e.g. all D3D APIs which move data from system RAM or VRAM or back now become network calls, the system RAM is on the server, VRAM is on the client.
P.S. Modern web browsers use Direct2D and DirectWrite to render stuff on Windows, there’s D3D 11 underneath, i.e. RDP remote rendering is fully supported for browsers and Electron apps.
However, as far as the actual implementation goes, RDP is closer to X11 than VNC. VNC is basically a remote frame buffer, akin to streaming video. This makes it very cross platform friendly, but also IMO feels very slow, even on a local network. RDP is closer to X11 in that it sends display directives (AFAIK, please correct me if I'm wrong) so that it can draw them on the client as they would be drawn on the server. Think sending a "draw button" command rather than sending an image of pixels that are how the button would be rendered. This, combined with compression, helps make RDP feel very fast compared to VNC, and can be used very well even over the internet, unlike X11. I connect to RDP servers in Azure all the time and sometimes forget I'm not working locally, while with VNC you're always aware that it's not as performant.
I really wish there was something like RDP as a server for Linux. (OP seems to be a client AFAICT?) Given Microsoft's recent investment in Linux, maybe this will happen soon?