Block Parameters

The parameters for block subdivision can be specified through the block parameter panel in the inspector.


Parameter panel for block subdivision

Several parameters are available for the user to control the resulting street shapes. These parameters are described below.

Block Parameters can be individually set for each block.

Attributes can be mapped to Default, User, Object or to a map layer. See Mapping Attributes for details.

General Parameters

These parameters are available to all subdivision types.

Parameter Function
shapeCreation If true, shapes are created from the street network.
type The subdivision algorithm to use, as illustrated below.

Recursive Subdivision creates rectangular lots by repeatedly splitting the block.


Offset Subdivision creates lots only within a given distance from the street edges of the block.


Skeleton Subdivision creates street-aligned lots that always have access.


No Subdivision doesn't perform subdivision on this Shape.

alignment This parameter is only used if the initial shapes are uneven. This sets the alignment of the lot over the terrain. There are four numeric options, as illustrated below.

Valid values are [0,1,2,3]

0. Uneven. The lots follow the terrain, given uneven heights. .

1. Minimum. The lots lie at their lowest point of the terrain that they cover.

2. Maximum. The highest point of the lot is used.

3. Average. The average height of the lot is used.

seedAn integer seed to control randomness.

Specific Parameters

Recursive Subdivision

The recursive subdivison technique is the default. It subdivides the block into rectangular lots of various sizes.

Parameter Function
lotAreaMin
lotAreaMax
The lower and upper bounds of the area of lots obtained after subdivision.

Given in absolute area units

Subdivision obtained for smaller lotAreaMin and lotAreaMax values.

Subdivision obtained for larger lotAreaMin and lotAreaMax values.

Subdivision obtained when the difference between lotAreaMax and lotAreaMin is small.

Subdivision obtained when the difference between lotAreaMax and lotAreaMin is large.

lotWidthMin The minimum width of the side of a lot. Subdivision stops if the length of any of the sides of any of the resulting lots is less than this value. If this value is high, the area of resulting lots might be larger than the area specified by lotAreaMax.

Given in absolute length units

Subdivision obtained for a smaller lotWidthMin value.

Subdivision obtained for a larger lotWidthMin value.

irregularity The relative deviation of the split line from the middle point of the center of the 'oriented bounding box' (OBB). If this value is 0.0, the split line will be pivoted at the middle point of the OBB of the parent lot. A higher value results in the split line being further away from the middle point, and generally, in a higher difference in the areas of the two children nodes.

Given in range [0.0,1.0]

Subdivision obtained for an Irregularity value close to 0.

Subdivision obtained for an Irregularity value close to 0.5.

forceStreetAccess The factor indicating the preference for lots with street access. A higher value results in more lots having street access.

Given in range [0.0,1.0]

Subdivision obtained for a forceStreetAccess value close to 0.

Subdivision obtained for a forceStreetAccess value close to 1.0.

cornerWidthWidth of the interior side of the created corners. If this value is 0.0 no corners are created. The maximum value for this attribute is automatically computed to avoid self-intersections.

Given in absolute length units

Subdivision obtained for a smaller cornerWidth value.

Subdivision obtained for a larger cornerWidth value.

cornerAngleMaxCorner angle threshold. If the angle at the vertex of a block contour is less than this value, a corner lot is inserted. A larger value results in a more relaxed criterion for inserting corners, and thus in more corners being created. If this value is 0.0 no corners are created.

Given in degrees

Subdivision obtained for a smaller cornerAngle value.

Subdivision obtained for a larger cornerAngle value.

Offset Subdivision

A block which uses offset subdivision is offset to create a fixed width strip along the street edges, which is then subdivided into lots.

Parameter Function
offsetWidth The perpendicular distance from the block contour to the inwards offset polygon. Intuitively, this value corresponds to the depth of the lots that are created when offset subdivision is used.
  • If this value is close to 0.0, OBB subdivision is used.
  • If this value is high enough so that the offset polygon is collapsed, OBB subdivision is used.

Given in absolute length units

Subdivision obtained for a smaller offsetWidth value.

Subdivision obtained for a larger offsetWidth value.

subdivisionRecursiveAfter the offset routine, there is an option to also run the recursive subdivision on the result. This is controlled by the same set of parameters as the recursive subdivision scheme.
lotAreaMin
lotAreaMax
lotWidthMin
irregularity, forceStreetAccess,
cornerWidth,
cornerAngleMax
See above.

Skeleton Subdivision

Skeleton subdivision attempts to subdivide a block such that every lot has access to the street. The sides of the lots are perpendicular to the roads they are adjacent to.

Parameter Function
lotWidthMin The ideal length of street front that each lot should possesses. This is increased or decreased by several other processes. A low lot width relative to the block size may create many narrow lots.

Given in absolute length units

Subdivision obtained for a lotWidthMin value of 15.

Subdivision obtained for a lotWidthMin value of 30.

simplify Amount of simplification that occurs. A high value creates irregular lots with fewer vertices.

Given in range [0.0,1.0]

Subdivision obtained for a smaller simplify value.

Subdivision obtained for a larger simplify value.

cornerAlignmentSkeleton subdivided lots face their nearest streets. At the corner of two streets, one will normally take priority. The corner alignment determines how this priority is assigned, either by Street length or by Street width.


Street width: the widest street takes priority. If the streets have similar average widths, the street length is used instead.


Street length: the longest street takes priority.

lotAreaMinAfter subdivision, lots with a small area are repeatedly combined with their neighbours until they are larger than this minimum. This reduces the number of smaller lots, but may create lots of more irregular shape.

Given in absolute area units

irregularityAs this parameter increases, it introduces a stochastic element into the lot width and lot edge direction.

Given in range [0.0,1.0]


Subdivision obtained for a smaller irregularity value.


Subdivision obtained for a larger irregularity value.

shallowLotFracLimit for merging triangular lots.


Subdivision obtained for a smaller shallowLotFrac value.


Subdivision obtained for a larger shallowLotFrac value.

No Subdivision

This simple subdivision technique subdivides the block into a single lot of the same shape. There is an option to remove the lot's corners.

Parameter Function
cornerWidth,
cornerAngleMax
See above.

Advanced

Description of Algorithms

Recursive OBB algorithm

The recursive OBB algorithm computes a split line at each step. If the two lots resulting from the split meet the user-specified constraints, the algorithm recurses on them. To determine the pivot point and direction of the split line, the minimum-area oriented bounding box (OBB) of the lot is computed. By default, the pivot point is set to the midpoint of the largest edge of the OBB, and the split line direction is set to the direction of the smallest edge of the OBB. The split line pivot and direction can be modified by three criteria:

Successive steps of the recursive OBB algorithm

Offset algorithm

The offset subdivision algorithm computes the inwards offset of the block contour and subdivides into lots the stripe between the block contour and its offset. The inwards offset is computed with CGAL. A set of sample points is computed along the offset. Consecutive points are separated by a distance computed as a function of the user-specified lot areas. Lines orthogonal to the offset at the sample points and passing through the sample points are used to split the stripe between the block contour and the offset.

Skeleton subdivision algorithm

The skeleton subdivision algorithm uses the straight skeleton (below, top left) to identify the center lines of the block. Given a set of skeleton faces, we identify those whose street edges are adjacent and of a similar curvature (below, top right). These faces are then grouped together. For each corner, the alignment priority (see cornerAlignment) determines how we assign the corner sections of these face-groups (below, bottom left). Finally each of the face-groups are sliced in a direction perpendicular to their street edges to create lots, and small lots are merged together until they are larger than lotAreaMin.

Successive steps of the skeleton subdivision algorithm

Consistent Indexing

As a result of the recursive nature of the subdivision algorithm and the different criteria dictated by shape attributes, the ordering of the lots resulting from subdivision might significantly vary after an editing operation. This is particularly inconvenient if models have been generated inside the lots.

To improve the consistency in the lot indexing among two consecutive subdivisions, the algorithm computes the relative position of each lot for each one of the two subdivisions, using a metric based on generalized barycentric coordinates. Pairs of lots that are the closest to each other in this barycentric space, are assigned the same index.

The same approach is also used to improve the consistency of the seed of each lot. As a result, two lots that are relatively in the same position of the block at two different subdivision configurations, have higher chances of sharing the same seed and attributes. The figure below shows a subdivision together with the shapes generated from a grammar that assigns one of 15 possible random colors to each lot. Due to the consistency logic above, the colors of lots that have similar relative positions inside the block are preserved, even though the topology and geometry of the subdivisions are different as a result of an editing operation.

Subdivision for initial block.

Subdivision for block after interactive editing.

Auto-generated street width attributes

For each resulting lot, an array of street width object attributes is generated.



Above: A typical lot selected in the viewport. Below: The generated street width object attributes in the inspector.

The first edge of a lot is the edge with maximal street width.

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