OXCE supports only 320x200.
Unless you make your own fork of OXCE, 640x400 is not possible.
Thanks Meridian and Yankee for the wonderful openxcom mod.
Thanks Dioxine and all contributors for the wonderful xpiratez mod.
Sorry that my programming skill is not up to Meridian/Yankee standard. But if someone is interested, I can try to modify Geoscape and Battlescape for 1280X800 resolution. Please inform me via email drago888@gmail.com.
However, will only do it for PC version. (Preferably someone can teach me how to create an exe without need to install SDK and redistributable - eg how Meridian do it). If need to do for Linux, please inform me too (Only able to test on Ubuntu).
Also, need guidance on how to test - eg currently, some UFOpaedia from xpiratez uses the TFTD ufopaedia. I did not modify that portion previously (but modified after my last playthrough) as don't know how to test previously
Also, need help from talented artist to convert those small pictures (Or any of the HD images that can be improved) to high resolution. (as using ESRGAN for very small images, the results is very bad. (eg if you look at the xpiratez weapons in ufopaedia. The image is really ugly as there is only so much that the program can recreate from very small, low resolution images)
Thank You