When several objects overlap partially, some objects appear in
front of other objects. This effect is called drawing
order or Z-order. Objects of layer N
are placed in front of objects of layers N-1 to zero. Moving
objects from one layer to another is one way of influencing the
drawing order.
If there are several objects within the same
layer, then these objects again have a drawing order. Each layer
has a spatial data structure called quadtree which allows you to
determine very quickly which objects are at which position. By
default, the quadtree is enabled and determines the drawing order
automatically in order to achieve optimal performance. In this
case, the drawing order cannot be influenced.
If you want to specify the drawing order of
objects within the same layer, you must first enable the Z-ordering
option of the layer, by using the following method:
setZOrdering(boolean enable)
When Z-ordering is enabled for the layer,
you can specify the drawing order of the objects within the layer:
setIndex(IlvGraphic object, int index)
Note that the index is always a continuous
range from 0 to N. This means that if you set the index of an
object, the index of other objects will shift by +1 or -1 to adjust
the index range. The current index can be retrieved by
getIndex(IlvGraphic object)
When Z-ordering is enabled, objects with a
higher index appear in front of objects of the same layer with a
lower index.
Note
The drawing order between different layers
takes precedence over the drawing order within each layer: An
object that is in a higher-numbered layer is drawn in front of
another object in a lower-numbered layer even if the Z-order index
of the first object is smaller than Z-order index of the second
object. Therefore, the Z-order index determines only the drawing
order of objects within the same layer.
Scenarios for experts
There are basically three scenarios:
- The quadtree is enabled and Z-ordering is disabled. This results in the highest performance. In particular the hit-test (determining which objects are at a given position) is optimally fast. However, it is not possible to influence the drawing order within each layer.
- The quadtree is enabled and Z-ordering is enabled. This is slightly slower, depending on how many objects overlap in average. The hit-test uses the quadtree and is still fast. It is possible to specify the drawing order completely.
- The quadtree is disabled. No matter whether Z-ordering is enabled or disabled, it results in the same speed, which is a large magnitude slower than when the quadtree is enabled. When the quadtree is disabled, it is also possible to specify the drawing order.
A test with 10000 objects showed that
enabling Z-ordering slows down the hit-test in average by a
factor of 1.2-5, but disabling the quadtree slows down the
hit-test by a factor of 20-70. These factors depend on the number
of objects, and the factors are negligible if you have only a
very few objects. Furthermore, if Z-ordering is enabled, the
slowdown is mainly influenced by the overlapping depth (the
number of objects that are overlapping cover exactly the same
location): the higher the overlapping depth, the larger the
slowdown of enabled Z-ordering.