Group #

A group is a shape that contains multiple shapes, rendered under <g> tag. For example, one can write,

t.s1 = Circle {}
t.s2 = Rectangle {}
t.s3 = Text {
    string: "Hello"
}
t.g = Group {
    shapes : [t.s1, t.s2, t.s3]
}
t.s4 = Circle {}
t.g2 = Group {
    shapes : [t.s4, t.g]
}

and get something like,

<svg ...>
  <g>
    <circle ... />
    <g>
      <circle ... />
      <rect ... />
      <text .../>
    </g>
  </g>
</svg>

Properties #

Constructing a Group #

A Group shape only contains one parameter, shapes, which is an unordered list of paths to previously-defined shapes. In the example above, shape t.g2 is a group that contains shape t.s4 (a Circle) and shape t.g (another Group). As illustrated, Group shapes can be nested.

We prohibit inline shape declarations within groups. In other words,

-- this is bad
bad_group = Group {
    shapes: [ Circle {}, Rectangle {} ]
}

does not compile. All shapes within a group must be declared previously and referred to by their paths.

Layering Semantics #

Inherently, shapes within a group are to stay together and be drawn together (meaning rendered on consecutive layers). In other words, if Group g only contains s1 and s2, then they must be drawn in the order of either s1, s2 or s2, s1, consecutively.

Hence, the following layering should not be allowed (and will hence generate a warning),

s1 = // some shape
s2 = // some shape
g = Group {
  shapes: [s1, s2]
}
s3 = // some shape

layer s1 above s2
layer s3 above s2
layer s3 below s1

because the layer directives enforce the layer ordering of bottom, s2, s3, s1, top which breaks apart group g containing s1, s2.

To achieve this goal, Group shapes have the following layering semantics:

• Layering directives on a group applies also to all members of the group.
• Layering directives on a group member applies to the entire group too.

Constraints and Objectives #

The Group shape has a bounding box, taken as the smallest axis-aligned rectangle that contains the bounding boxes of all its member shapes. With this bounding box defined, some constraints and objectives (such as contains, disjoint, and overlapping) can work as well.

For example,

s1 = // some shape
s2 = // some shape

g = Group {
    shapes: [s1, s2]
}

s3 = // some shape

ensure contains(g, s3)

will ensure that group g (and hence both members s1 and s2) is contained within s3.