Monday 29 August 2016

Future City: Update 2 - Get Yourself Connected

This time we look at creating interesting boundaries and useful connectivity information between all our city zones.
We want a city with interesting connectivity between zones


It is as much the interconnections as the spaces in an environment that make a place interesting. Now the zones are laid out we need to look at how they are connected. As we have a number of limitations in how we implement this I'm going to use a fairly basic connection model similar to one I have used before which was based on having a single connection pathway between adjacent areas, like each side of a room having at most one door. The algorithm had the following rules (expressed in terms of rooms and doors):
  1. For your starting 'room', specify a door on each side (as desired).
  2. Decide where to split based on door positions (i.e. don't split down a doorway).
  3. When you split an area into two, always add a door into the new wall.
  4. Recurse (go-to step 2) for both sides.
This produces a map where all the four outer doors are guaranteed to be connected, and you can reach every generated room.
Previous connectivity test, all rooms remain connected, various in-fill rooms.

When is a door not a door?

For our city, the door/wall terminology is going to be used to mean something slightly different:
  1. A door corresponds to a region of free travel between areas, be it pedestrian, or vehicular.  This can mean either both sharing the same flat height level at the boundary, or that they both guarantee to match some underlying landscape shape at the boundary.
  2. A wall corresponds to a barrier between areas, this can take several forms and may or may not have some sort of minimum barrier height.  This could be a wall, fence, steep drop, ditch, or just allow the back of a building to sit along that edge.
Barrier 'wall' (red) and open 'door' (green) segments along the side of an area.
This has an important effect on the splitting constraints; in that I want to allow splitting the open part in two by a boundary.  This will introduce more interesting topologies and should create greater connectivity than the 'single path to exit' algorithm above.
Splitting the area in two but with splitting of an opening allowed.
This does come at a cost though as the calculations needed to maintain connectivity are in-fact more complicated.  I decided to try and track access back to the four starting sides more thoroughly this time so that when splitting, we can work out if we have the option not to add a door, again increasing the connection graph variety.
Also, remember, we are splitting the city up into zones at the moment, these are fairly high-level, and quite large.  The blocked boundaries aren't going to be massive long walls, but opportunities for the next stages of subdivision to use them in different ways to partition the city up into lots of interesting spaces.  Similarly, a long gap may not imply a continuous connection, only that there be some form of connection along the length of each area.  Different areas may decide to achieve this in different ways, be-it steps, a ramp, a wall with an archway, and so-on.

No Structures

A quick aside about structure support in Apparance.  At the moment there are only a handful of fundamental data types you can pass around.  It is feasible to support arbitrary collections of these wrapped up into custom data types (structures) that can be passed around together more easily using single connection points and wires.  This would make propagating things like the access parameters and connectivity information much, much easier and also allow us to modify and add to this information without the huge amount of rewiring it would otherwise need.  Unfortunately, as it stands, I can stretch what I have and make a few design compromises enough that proper structure support isn't enough of a priority.  Something I have mused over a lot, definitely elegantly implementable, and definitely a future feature.  For now though we will pack out information into existing types.

To door or not to door?


The presence of an opening in an side is represented by two horizontal values, the position of the left edge and the position of the right edge.  The absence of an opening is represented by either it's left edge being past the right end of the side, or the right edge bring past the left end of the side.
Opening specified by position of left and right ends.  Fully blocked side specified by either both at the right end of the wall or both at the left end of the wall.
To pass this information around we'll pack it into a Vector3, one of the fundamental types that is used to represent a 3D vector.  It has three floating point values X, Y, and Z of which we use X and Y to be the left and right edges of the opening.  The Z value isn't used yet, but will be needed when we want to encode some height information into the opening.
Packing of edge connectivity into a Vector3
Each area (or room) consists of four sides, each potentially accessible via openings, and so we need to track and pass around four of these Vector3 access values.  As a convention, these are referred to as West, East, South, and North (or their initial), and in that order (lower X axis, upper X axis, lower Y axis, upper Y axis).
West, East, South, and North sides of an area.


We also need to track connectivity available via each side of an area as we sub-divide.  Since we only need to track whether we can reach each of the four outer sides when we only need four bool values, or four bits of data.  This is much simpler information and rather than passing round a value for each side we can pack the information needed into a single integer as a bit-field.
How we pack information about which side of our area can provide access to which side of the city into a 32 bit integer.  A bit value of '1' means yes, we can get to it, and '0' means no we can't.
We will need add a few new bit-wise operators to support the packing and unpacking logic.  These are bit shift, AND/OR, and maybe a bit test operator too.  We can then build packing/unpacking procedures from these.
Connectivity packing and unpacking procedures
NOTE: I've been playing with clearer labelling of some of the simpler operators.  They can now have either large text, or characters from the Symbol font.  This combined with blank IO names and title suppression produces some nice compact visual representations.  You'll see some more below.


We are now ready to implement this new functionality on top of the work we did last time splitting our city into zones. To start with I factored out the splitting process into own procedure, this helps manage the complexity of the already fairly complicated procedure that decides whether we should split and which way.
The newly factored out Split procedure in place

Do the splits

New split point, side calculation, and access logic are all in their own procedure now. To make the splitting logic simpler we abstract away the orientation of the split by 'normalising' the incoming area information and then 'de-normalising' it back afterwards.  This allows us to implement the split logic once and be able to use it for both horizontal and vertical splits.  The four sides are termed Left, Right, Bottom, and Top instead of the W.E.S.N. directions we use normally, to help distinguish normalised from de-normalised.
The split operation.  Mostly consisting of the logic to help us treat either orientation of split the same way.

Where to split

The split itself is broken into two parts, where to split, and then how to split.
First we need to work out where across the area are we going to split.  Since we are allowing splits across openings this is technically only limited by the need to ensure we don't create any area too small (min zone size design parameter).

Neat & tidy, tidy & neat

At this point it would be nice to also limit the minimum feature size we create during a split.  Since an arbitrary split could be any distance from an arbitrary opening edge then we can create very tiny openings and short walls.  It would be good to reduce this effect and we can achieve it in two ways.  One is to snap to some overall grid spacing, and second to snap to the edges of the openings in the two walls we are intersecting. The Split Point procedure handles all this for us.
Deciding on where to split the area and generating access information for either side of the split.
When splitting an area into two halves, the connectivity is going to be affected by whether there is an opening in the new wall.  To decide on this we need to work out where each new area is already connected to and use this as the potential new connectivity across the new divide.  By checking to see if each side of the divide includes the opening from the two sides being intersected and combining their connectivity with the connectivity of the opposite wall we get a connectivity value from each side of the split.

How to split

Deciding on the dimensions of an opening in the new wall is fairly straightforward.   We do however need to check whether we need to force an opening though to maintain connectivity between the city outer walls.
Side splitting and connectivity update.


This update was implemented in two stages, first the access generation (wall/door) for each area, then the connectivity calculation and sub-division side opening decision making.  Here is an example where you can clearly see areas that end up disconnected from each other due to no connectivity management.
Disconnected regions (highlighted) present before connectivity checks were implemented
Once the connectivity system was in place it all started working properly.  Here is a selection of city zone layouts with connectivity information diagnostics display enabled.
Examples of a fully connected city


Next we will start breaking down each of our zones into city blocks.  The process is similar to before, but each district type may have different requirements as the layout of each should differ to keep things interesting.
We will also starting thinking about where significant buildings are going to be placed since we will soon require a skyline silhouette to appear as we approach the lower detail end of our city.  Development is a process of adding detail, top-down, exactly analogous to approaching the city gradually from a distance.  Think about what you expect to see, and when.

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