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Cities: Skylines After Dark Dev Diary looks at modding changes

Posted on 28 Aug 2015 by Arazmus.
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The CO team are back with another Cities: Skylines developer diary and this latest update covers the topic of modding in the upcoming After Dark expansion.

The diary looks at some of the new features to be aware of with the Asset Editor and user created modes, dynamic lighting, and the look of districts. There’s a lot to take in here if you’re a modder so read carefully.

Today, let’s have a look at the two biggest changes/new stuff happening on the Modding side of Cities: Skylines.

Let’s get started on how the day/night cycle affects the Asset editor and user-created models:

After we decided to add this feature alongside the After Dark expansion, our first reaction was to ponder what would that mean for the ton of assets already created on the Steam Workshop. We knew it would be impossible to completely automate the process to lit those assets during night time, but we wanted to minimize and ease the amount of manual work required by content creators.

Our first step was to turn all prop assets which are expected to emit light into assets emitting light. Street lamps/poles (see below) and so forth are heavily used in custom assets, therefore it made sense to take that road, as it would provide a base lighting for all assets already using them.


To allow more granularity to the lighting process of a building, we also added various light props such as wall-mounted and floors lights. Heck we even added a sub-category to the Common Props toolbar. Below is an example of how to properly mess up the night look of a residential building using wall-mounted lights, also features the new Light props toolbar.


In order to visualize the asset both at day and night time in the Asset Editor, we replaced the previously existing slider Light rotation to control the time of the day. The lights toggling/animation logic are simulated in Editor just as they would be in-game, so the preview of the building for a given day time is accurate.


texture and 120 means full contribution. We hereby freed the range 128 to 255 to allow windows to be lit randomly. A value of 192 means neutral, so if you want a building to be unlit at all times, the illumination map needs to be solid 192. The likelihood of a window to be lit at a given time and its intensity are randomized for values within 128 and 255, the closer the value is to a boundary the more likely it is to be lit and stronger.

As an example below is a residential house with its windows lit and diffuse, specular and illumination map composited next to each other.



In this case, the UV mapping is such that the windows use very little texture space but you can see them clearly at the bottom right of each texture. Notice the illumination map value of unlit areas is 192.

Also, the specular map, while irrelevant to the lighting is shown in the picture as it is worth noticing in most cases, if you have already created a specular map for your asset, you already have a base windows mask to derive from when creating the illumination since windows are usually reflective and you have already cranked up the specular mask color.

Next, let’s talk about a brand new feature which affects both content creators and players: District styles.

After shipping the free patch introducing the European theme and buildings, a rather high demand for these buildings to be made available in all themes rose from the forums. It was not done as such because of both technical and visual motives:
– memory usage would have gotten very tight for our user with minimum system requirements and we did not want to break the game for them.

– Buildings have specific requirements to tie-in with corner buildings and we would have needed to redo the entire stock buildings set to scale with the wall-to-wall and corner buildings feature. That obviously was not a viable option.
Coincidentally, another pretty highly requested visual feature was the ability to set a policy which would define the look of a district.

While we did not feel comfortable making this feature a policy as policies should have an impact on gameplay and not purely on visuals, we did add an option to select what we call “Styles” from a new Dropdown component in the District info panel.
We added a default building set with the European buildings and when enabled in the Content Manager, it will be available in the game dropdown.


Once a style is set to a building, it will start using/replacing buildings from that style matching the level and density requirements for spawning hereby you can have a European style district in your city. The screenshot below shows European buildings in the good old Los Santos map which is built on a tropical theme.


This is pretty cool but here is the good(er) news: content creators can also create and share their own styles on the Steam workshop. In the Content Manager, there is a new section called Styles and you can create a style to which you can add any building uploaded to the Steam Workshop


Provided you include buildings which covers all levels and densities used in the game, you will have a fully customized look for a given district \o/
We can’t wait to see what new amazing combinations you are going to come up with!

That’s it for today.

Thanks for your time reading this diary, it’s back to coding for me, there is still some work to be done :)

Technical Director @ Colossal Order

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