Ooh, nice high quality art.

Is it possible to port some assets from other projects, e.g. 0 A.D.? What license is required?

Of course, we first of all want to say thanks to everybody participating and to kurt for setting up this platform  -- awesome opportunity to show our players more various assets in-game!

Everything you create for this challenge will be directly visible in-game with our next release (planned mid- to end-june so 20th fits very well).
If you ever pondered about all the great stuff that never gets used, rest assured :)

Should you have further design or tech questions not answered so far: we hope to be available as much as possible on #opengameart. If we're too busy, feel free to step by at our IRC channel or ask here in this thread. This also holds if you are interested in further contributions or in Unknown Horizons as player ;-)

Looking forward to a lot of progress during the next two weeks!

All the best,
Chris

clint:  we license all our content under CC-BY-SA 3.0. From what I can tell, nobody has looked into 0AD.

All models are rendered in four resp. eight directions, so no direct import into UH.

eoc: here's an example of 0AD's art. This one is marked CC-BY-SA; maybe all of 0AD's art is the same.

0AD's art is fully 3D, so the pieces you want would have to just be rendered in the correct directions, right?

Do you have guidelines on asset size? What scale are your tiles (pixels per meter perhaps)? What size tiles are acceptable?

The images in both pictures above are in-game size and the buildings there represent 2x2 tiles.

To incorporate real metrics, we would have needed to shrink everything to an ugly degree. At the moment, our ships are about thrice as large as normal units. so that's not really to be called realistic. All too odd relations however will be detected by our players. Metrics we started with as orientation can be checked in our wiki.

Guidelines for this challenge are to fit the same tile size (rendered) as the other given building variations do.  
This is necessary because our engine checks a certain directory and then picks a random graphics set for this building in there. This can only work if all versions of a building have the same tile size when rendered.

For completely new buildings to be added to the game,  we usually give a rough approximation of the size we planned our design with. There also have been cases where our artists convinced us of a different layout after they tried their luck, so this estimation is totally not set in stone (and can easily be modified later). An example of how we describe new buildings could be found here or here.

An overview of the px equivalents whenever we mention 'tiles' is linked above in our wiki if anybody is interested.
Additionally, the folder structure of a sample action set (a log house) is found here where you find the common 2x2 tile size rendered. To see the actual images, navigate through that folder.
2x2 tiles is the size of our residential buildings tent, hut and house which are the main topic of the More Variety challenge. :)

Oh, I see this topic is already half-month old, but I just found it (I came to OGA to read FLARE forum and have seen the "Challenge Winner" section on side panel with a building image and I have thought "it looks like Unknown Horizons building" and indeed :) )

I want to provide few notes about 0ad, maybe this will be useful for someone:
* all 0ad art distributed under the same license as Unknown Horizons art (CC-BY-SA 3.0, see [url=http://trac.wildfiregames.com/browser/ps/trunk/binaries/data/mods/public...);
* building meshes stored in this folder: http://trac.wildfiregames.com/browser/ps/trunk/binaries/data/mods/public... ;
* most of files are *.dae ([url=http://en.wikipedia.org/wiki/COLLADA]COLLADA[/url]) files, you can use for example Blender embedded importer/exporter to work with these files (see these topics for additional info: [url=http://www.wildfiregames.com/forum/index.php?showtopic=14025]Blender 3D[/url], [url=http://www.wildfiregames.com/forum/index.php?showtopic=14577]Blender support[/url]) ;
* part of files are *.pmd ([url=http://trac.wildfiregames.com/wiki/PMD_File_Format]PMD_File_Format[/url]), which is old Wildfire Games own format, you need to convert them into *.dae with [url=http://trac.wildfiregames.com/ticket/461]this script[/url] first ;

That was about license/technical details. Now about 'setting' (is this right word ? :) ):
* 0ad currently have 3 factions: Hellenes, Celts, Iberians and time window 500 BC - 1 BC
* Unknown Horizons have a single 'nation' and time window... hm, I can't find it, but probably something like 1500 AD - 1700 AD (looking on Anno games names)

As you can see here is big difference but maybe basic economical buildings from 0ad can be used for first [url=http://wiki.unknown-horizons.org/w/DD/Increments]Increments[/url] of Unknown Horizons.

fcxSanya, thanks! This info is incredibly valuable.