Understanding Streets #1: Shapes, Blocks, Segments and Nodes

A street, consisting of an edge with two nodes on either end.

Just as in a real city, your CityEngine model will likely be structured largely around streets, whether you import them from shapefiles or another source like Open Street Maps, generate them with the Grow Streets tool or draw them manually with the street creation tools.  Getting your geometry in order is only the first step.  Though you may be tempted to get in there and start setting parameters in the Inspector, the  real power begins when you apply textures and other parameters using rule files.

With the Polygonal Street Creation tool (keyboard shortcut “G”), I have first drawn a single street (left), and a closed block with parcels or “lots” (right). When you draw a closed loop of streets, the lots are created automatically.

Street creation with the toolbar highlighted at center left and the Inspector tabs at upper right.

Use keyboard short “Q” to activate the select tool, then drag a select box around the single street. In the Inspector window, note there are three tabs for Shapes, Segment and Nodes.

  • The street is comprised of a straight line called an “edge” that represents the street centerline. You can isolate the centerline by clicking the Segment tab in the Inspector and disabling Shape Creation.
  • Similarly, you can isolate the nodes, the center points for the intersections on either end of the street by disabling Shape Creation under the Nodes tab. Nodes appear at intersections or at articulation points between edges.
  • Parameters for both Segments and Nodes appear and can be set under the Shapes tab, though what you see is “context sensitive” and will vary based on what is enabled/disabled.

Again, use keyboard short “Q” to activate the select tool, then drag a select box around the closed block.

  • The Inspector shows four tabs: Shapes, Block, Segment and Nodes.
  • Click the Block tab and experiment with the “type” settings under Block Parameters. You have the option of selecting between Recursive, Offset, Skeleton and No Subdivisions.

So, street networks are created using three different kinds of Shapes:  Segments, Nodes and Blocks. You can configure them as you like by selecing individual features and setting their parameters in the Inspector.  But this is only the very beginning. You will want to texture and otherwise control your streets, intersections and lots by assigning a rule file and a start rule to each.  We will take that up in the next installment.

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