# Syntax Reference¶

## Primitive Data Types¶

### Scalars¶

representing distances, angles, etc. as floating-point numbers, which can be sampled from various distributions

### Vectors¶

representing positions and offsets in space, constructed from coordinates with the syntax X @ Y (inspired by Smalltalk). By convention, coordinates are in meters, although the semantics of Scenic does not depend on this. More significantly, the vector syntax is specialized for 2-dimensional space. The 2D assumption dramatically simplifies much of Scenic’s syntax (particularly that dealing with orientations, as we will see below), while still being adequate for a variety of applications. However, it is important to note that the fundamental ideas of Scenic are not specific to 2D, and it would be easy to extend our implementation of the language to support 3D space.

representing orientations in space. Conveniently, in 2D these can be expressed using a single angle (rather than Euler angles or a quaternion). Scenic represents headings in radians, measured anticlockwise from North, so that a heading of 0 is due North and a heading of π/2 is due West. We use the convention that the heading of a local coordinate system is the heading of its y-axis, so that, for example, -2 @ 3 means 2 meters left and 3 ahead.

### Vector Fields¶

associating an orientation (i.e. a heading) to each point in space. For example, a vector field could represent the shortest paths to a destination, or the nominal traffic direction on a road

### Regions¶

representing sets of points in space. Scenic provides a variety of ways to define Regions: rectangles, circular sectors, line segments, polygons, occupancy grids, and explicit lists of points. Regions can have an associated vector field giving points in the region preferred orientations. For example, a Region representing a lane of traffic could have a preferred orientation aligned with the lane, so that we can easily talk about distances along the lane, even if it curves. Another possible use of preferred orientations is to give the surface of an object normal vectors, so that other objects placed on the surface face outward by default.

## Position Specifiers¶

### offset along direction by vector¶

Positions the object at the given coordinates, in a local coordinate system centered at ego and oriented along the given direction (which, if a vector field, is evaluated at ego to obtain a heading)

### (left | right) of vector [by scalar]¶

Depends on heading and width. Without the optional by scalar, positions the object immediately to the left/right of the given position; i.e., so that the midpoint of the object’s right/left edge is at that position. If by scalar is used, the object is placed further to the left/right by the given distance.

### (ahead of | behind) vector [by scalar]¶

As above, except placing the object ahead of or behind the given position (so that the midpoint of the object’s back/front edge is at that position); thereby depending on heading and height.

### beyond vector by vector [from vector]¶

Positions the object at coordinates given by the second vector, in a local coordinate system centered at the first vector and oriented along the line of sight from the ego. For example, beyond taxi by 0 @ 3 means 3 meters directly behind the taxi as viewed by the camera.

### (in | on) region¶

Positions the object uniformly at random in the given Region. If the Region has a preferred orientation (a vector field), also optionally specifies heading to be equal to that orientation at the object’s position.

### (left | right) of (OrientedPoint | Object) [by scalar]¶

Positions the object to the left/right of the given OrientedPoint, depending on the object’s width. Also optionally specifies heading to be the same as that of the OrientedPoint. If the OrientedPoint is in fact an Object, the object being constructed is positioned to the left/right of its left/right edge.

### following vectorField [from vector ] for scalar¶

Positions the object at a point obtained by following the given vector field for the given distance starting from ego (or the position optionally provided with from vector ). Optionally specifies heading to be the heading of the vector field at the resulting point. Uses a forward Euler approximation of the continuous vector field

### apparently facing heading [from vector]¶

Orients the object so that it has the given heading with respect to the line of sight from ego (or from the position given by the optional from vector). For example, apparently facing 90 deg orients the object so that the camera views its left side head-on

## Scalar Operators¶

### angle [from vector ] to vector¶

The heading to the given position from ego (or the position provided with the optional from vector ). For example, if angle to taxi is zero, then taxi is due North of ego

## Boolean Operators¶

### (Point | OrientedPoint) can see (vector | Object)¶

Whether or not a position or Objectis visible from a Point or OrientedPoint. Visible regions are defined as follows: a Point can see out to a certain distance, and an OrientedPoint restricts this to the circular sector along its heading with a certain angle. A position is then visible if it lies in the visible region, and an Object is visible if its bounding box intersects the visible region. Note that Scenic’s visibility model does not take into account occlusion, although this would be straightforward to add

### (vector | Object) in region¶

Whether a position or Object lies in the region; for the latter, the Object’s bounding box must be contained in the region. This allows us to use the predicate in two ways

### scalar deg¶

The given heading, interpreted as being in degrees. For example 90 deg evaluates to π/2

### direction relative to direction¶

The first direction, interpreted as an offset relative to the second direction. For example, -5 deg relative to 90 deg is simply 85 deg. If either direction is a vector field, then this operator yields an expression depending on the position property of the object being specified

## Vector Operators¶

### vector (relative to | offset by) vector¶

The first vector, interpreted as an offset relative to the second vector (or vice versa). For example, 5@5 relative to 100@200 is 105@205. Note that this polymorphic operator has a specialized version for instances of OrientedPoint, defined below (so for example -3@0 relative to taxi will not use this vector version, even though the Object taxi can be coerced to a vector)

### vector offset along direction by vector¶

The second vector, interpreted in a local coordinate system centered at the first vector and oriented along the given direction (which, if a vector field, is evaluated at the first vector to obtain a heading)

### vector relative to OrientedPoint¶

The given vector, interpreted in the local coordinate system of the OrientedPoint. So for example 1 @ 2 relative to ego is 1 meter to the right and 2 meters ahead of ego

## Statements¶

### import module¶

Imports a Scenic or Python module. This statement behaves as in Python, but when importing a Scenic module M it also imports any objects created and requirements imposed in M. Scenic also supports the form from module import identifier, … , which as in Python imports the module plus one or more identifiers from its namespace

### param identifier = value, …¶

Defines global parameters of the scenario. These have no semantics in Scenic, simply having their values included as part of the generated scene, but provide a general-purpose way to encode arbitrary global information. If multiple param statements define parameters with the same name, the last statement takes precedence, except that Scenic world models imported using the model statement do not override existing values for global parameters. This allows models to define default values for parameters which can be overridden by particular scenarios. Global parameters can also be overridden at the command line using the --param option.

### require boolean¶

Defines a hard requirement, requiring that the given condition hold in all instantiations of the scenario. As noted above, this is equivalent to an observe statement in other probabilistic programming languages

### mutate identifier, … [by number ]¶

Enables mutation of the given list of objects, adding Gaussian noise with the given standard deviation (default 1) to their position and heading properties. If no objects are specified, mutation applies to every Object already created