Open settings.ini in your Treasure Fleet directory using a text editor. Scroll down to find the list of resolutions. We want to place our new resolution at the end of the list, after 1920x1080, which is resolution6.

Paste the following after the resolution6 block (which starts with resolution6=1920x1080):

Note the line background7=gfx/background2560.png. This specifies the background image when playing. Changing this isn't strictly necessary and can be set to background7=gfx/background1920.png if you don't want to bother with an extra file (and don't mind it looking a little bit messy/confusing).

Things look messy without a fitting background image so I quickly edited the 1920x1080 version with Photoshop's content aware fill function. Save the image below (make sure to open the full sized image and not just save the thumbnail) and drop it into the gfx folder inside the game's root directory.

That's pretty much it! This method could be used to make other resolutions, even a 4K resolution but the text would be tiny at that resolution and so you would need to change the font size (which necessitates altering the size of buttons and such). Nevertheless, I thought I'd note down some things about the settings.ini file which might be useful to others who'd like to make a resolution.


Apart from fullscreen, these appear to control the font size (and font itself) used throughout the game.


A few things about the resolution blocks:
The resolutionX line (where X is an index number for the list) controls the resolution.
mapviewportrectX controls the map viewport size (the main view of the game). As far as I can tell the first two numbers are the Y and X coordinates to start drawing the viewport and the latter two are how many pixels to draw; in other words, how big the rectangle is.
minimaplocationX is where to draw the minimap, but it doesn't seem to be able to be made bigger, only moved.
The other lines control various button and panel locations which can usually be inferred from their naming.

The rest of the file is a whole lot of UI stuff if anyone wants to go really crazy and resize lots of things. I just wanted to get the game working in my native resolution so I've left them alone.

Finally, I'm not responsible for you breaking anything but if you do, it's easy enough to fix most things by opening user.ini in the game folder and changing the resolution line to 0. For everything else, verify your files with Steam.